ENVIRONMENTAL BIOLOGY AND EVOLUTION
UNIT - I
Some Basic Ecological Concepts
The term habitat refers to the kind of place where an organism normally lives. It includes
the arrangement of food, water, shelter and space that is suitable to meet an organism's needs.
You can think of this as the "address" where an organism lives. In contrast, a niche is the
"occupation" of an organism. It defines the role of an organism in an ecosystem, such as a
"fish-eating wader" for a heron, or a "plant-juice-sipping summer buzzer" for a cicada. An
organism's niche may change during different life stages. For example, a tadpole typically lives
in the water and eats plant material, while the adult frog may catch insects from the shore.
The source of energy for all life on Earth is the sun. Green plants (and some bacteria) are
the only organisms that can directly capture the sun's energy and change it into a form that
other organisms can use. Through the process of photosynthesis, plants use sunlight to change
carbon dioxide and water into sugar and oxygen. The oxygen is given off into the air, where it
is available to other organisms including humans. Simple sugar molecules make energy
available to plants and, by forming the basic units of complex carbohydrates, contribute to
plant structure. Other organisms then eat the plants, or eat organisms that eat plants, and in
doing so indirectly gain the benefit of the sun's energy to run their bodies. The flow of sunlight
energy is therefore passed from producers (green plants) to primary consumers (animals that
eat plants, such as leafhoppers) to secondary consumers (animals that eat other animals, such as
birds); this sequence is known as a food chain. As energy is passed along the food chain, much
is used up at each level as it works to run each organism. This energy is given off as heat and
results in less energy being available at each stage along the food chain. It takes a lot of grass to
support one rabbit, and many rabbits to support one hawk. As a consequence, there are many,
many green plants on the Earth, fewer animals that eat plants, and even fewer animals that eat
animals; this is known as the energy pyramid. In the bosque, the cottonwoods and other plants
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trap the sunlight energy and provide it in a form usable by the entire collection of other
organisms found there. They provide the foundation for life along the river.
Although sunlight energy is used up as it is passed along the food chain, fortunately there
is an abundant supply of this energy. In contrast, the materials from which all living things are
made are limited in supply and must be used over and over. The primary building blocks of all
living things include only six materials: carbon, hydrogen, oxygen, nitrogen, phosphorus, and
sulfur. When an organism dies and decomposes, these materials are returned to the system and
are used again. The carbon that was once part of a dinosaur's tail may now be in the tomato that
you eat for dinner! If these compounds are removed from the cycle in some way, they may
become limited in supply. For example, if a tree dies but the wood does not readily decompose,
carbon and the nutrients are trapped in the wood and are not available to other organisms. This
appears to be happening in the bosque. Without the annual flooding that once inundated the
forest, wetting the fallen wood and increasing the rate of decomposition, undecomposed wood
is now building up and trapping nutrients. This affects the health of the entire ecosystem.
One very important cycle is the water cycle. Rain that falls on a hillside percolates down
into the ground water, or may flow aboveground into a lake or the ocean. Water in the lake or
ocean then evaporates, and drops join together into clouds, to eventually fall again as rain. Our
use of water greatly affects the water cycle. In New Mexico we remove water from the
underground aquifer (water present in the bedrock below ground) much faster than it is
replenished. Much of this water evaporates directly into the atmosphere while we use it, and
may then fall again somewhere else on the planet, thus reducing the amount of water available
locally.
We also impact the cycling of materials by introducing poisons. As materials are cycled
over and over, toxins build up. Concentrations of toxins increase along food chains, since a
predator eats many preys with the toxin, a process known as biomagnification. These
increasing concentrations of toxins often have devastating effects. Some well-known examples
include top-predator species such as bald eagles and peregrine falcons that nearly became
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extinct due to the effects of DDT or other chemicals. Awareness of these problems may go a
long way towards helping to keep our cycles clean.
Through the flow of energy and the cycling of materials, all living things are interrelated.
A mouse not only gets energy from the seed that it eats, but also gets materials that will help to
build more mouse tissue. The mouse breathes out carbon dioxide which is taken in by plants,
which in turn give off oxygen used by the mouse. The mouse also depends on plants for finding
shelter, and it provides food for a snake or owl. The components of the bosque are interrelated
with connections extending to the surrounding uplands as well. Some connections are obvious,
such as birds that fly between the bosque and uplands at different times of day or during
different seasons, moving materials from one place to another. Others are more subtle, such as
water flowing underground. But these connections make our actions even more important.
Pesticides applied to our fields may add toxic materials to the river, affecting not only the water
itself but also all the organisms that depend on the water.
Change is an integral part of the natural world. Changes may occur over geologic time,
such as the transition of the Rio Grande from a series of lakes to the river that we know today,
or they may occur over much shorter time periods, such as the transition of a seed to a tree and
finally to a fallen log. Change was once an integral part of the natural Rio Grande riparian
ecosystem, as the river wandered across the floodplain leaving behind its ever-changing mosaic
of vegetation. However, human-induced changes have much different effects on the ecosystem.
The rate at which we are causing changes on Earth is much greater than has been known
previously, and we do not yet know the ecological consequences of most of our actions. By
understanding the ecological systems in which we live, and how we interact with them, we can
begin to lessen our impact on Earth.
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Energy flow
From
Left: Energy flow diagram of a frog. The frog represents a node in an extended food web. The
energy ingested is utilized for metabolic processes and transformed into biomass. The energy
flow continues on its path if the frog is ingested by predators, parasites, or as a
decaying carcass in soil. This energy flow diagram illustrates how energy is lost as it fuels the
metabolic
process
that
transform
the
energy
and
nutrients
into
biomass.
Right: An expanded three link energy food chain (1. plants, 2. herbivores, 3. carnivores)
illustrating the relationship between food flow diagrams and energy transformity. The
transformity of energy becomes degraded, dispersed, and diminished from higher quality to
lesser quantity as the energy within a food chain flows from one trophic species into another.
Abbreviations: I=input, A=assimilation, R=respiration, NU=not utilized, P=production,
B=biomass.[1]
In ecology, energy
flow, also
called the calorific
flow,
refers
to the
flow
of energy through a food chain. In anecosystem, ecologists seek to quantify the relative
importance of different component species and feeding relationships.
A general energy flow scenario follows:
Solar energy is fixed by the photoautotrophs, called primary producers, like
green plants. Primary consumers absorb most of the stored energy in the plant through digestion,
and transform it into the form of energy they need, such as adenosine triphosphate(ATP),
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through respiration. A part of the energy received by primary consumers, herbivores, is
converted to body heat (an effect of respiration), which is radiated away and lost from the
system. The loss of energy through body heat is far greater inwarm-blooded animals, which must
eat much more frequently than those that are cold-blooded. Energy loss also occurs in the
expulsion of undigested food (egesta) by excretion or regurgitation.
Secondary
consumers, carnivores,
then
consume
the
primary
consumers,
although omnivores also consume primary producers. Energy that had been used by the primary
consumers for growth and storage is thus absorbed into the secondary consumers through the
process of digestion. As with primary consumers, secondary consumers convert this energy into
a more suitable form (ATP) during respiration. Again, some energy is lost from the system, since
energy which the primary consumers had used for respiration and regulation of body temperature
cannot be utilised by the secondary consumers.
Tertiary consumers, which may or may not be apex predators, then consume the
secondary consumers, with some energy passed on and some lost, as with the lower levels of the
food chain.
A final link in the food chain are decomposers which break down the organic matter of
the tertiary consumers (or whichever consumer is at the top of the chain) and
release nutrients into the soil. They also break down plants, herbivores and carnivores that were
not eaten by organisms higher on the food chain, as well as the undigested food that is excreted
by herbivores and carnivores. Saprotrophic bacteria and fungi are decomposers, and play a
pivotal role in the nitrogen and carbon cycles.
The energy is passed on from trophic level to trophic level and each time about 90% of
the energy is lost, with some being lost as heat into the environment (an effect of respiration) and
some being lost as incompletely digested food (egesta). Therefore, primary consumers get about
10% of the energy produced by autotrophs, while secondary consumers get 1% and tertiary
consumers get 0.1%. This means the top consumer of a food chain receives the least energy, as a
lot of the food chain's energy has been lost between trophic levels. This loss of energy at each
level limits typical food chains to only four to six links.
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Tropic structure levels
Trophic levels are the feeding position in a food chain such as primary producers,
herbivore, primary carnivore, etc. Green plants form the first trophic level, the producers.
Herbivores form the second trophic level, while carnivores form the third and even the fourth
trophic levels. In this section we will discuss what is meant by food chains, food webs and
ecological pyramids.
Food Chains.
The feeding of one organism upon another in a sequence of food transfers is known as a
food chain. Another definition is the chain of transfer of energy (which typically comes from the
sun) from one organism to another. A simple food chain is like the following:
plant -- aphids -- beetle -- chameleon -- hawk.
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In this food chain, the rose plant is the primary producer. The aphids are the primary
consumers because they suck the juice from the rose plant. The beetle is the primary carnivore
because it eats the aphids. The chameleon, a secondary carnivore, eats the beetle. The hawk is
the tertiary carnivore because it eats the secondary carnivore, the chameleon. The hawk
eventually dies and its remains are broken down by decay-causing bacteria and fungi.
Food Webs
In an ecosystem there are many different food chains and many of these are cross-linked
to form a food web. Ultimately all plants and animals in an ecosystem are part of this complex
food web.
Ecological Pyramids
Trophic levels and the energy flow from one level to the next, can be graphically depicted
using an ecological pyramid. Three types of ecological pyramids can usually be distinguished
namely:
Number pyramid.
It is easily understood that many grass plants are needed to feed fewer snails on which, in
turn, even fewer chickens would be able to feed. This in turn requires only a few people to eat
the chickens that ate the snails. The Number pyramid shows the number of organisms in each
trophic level and does not take into consideration the size of the organisms and over-emphasizes
the importance of small organisms. In a pyramid of numbers the higher up one moves, so each
consecutive layer or level contains fewer organisms than the level below it.
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The Number Pyramid
Biomass pyramid.
This pyramid indicates the total mass of the organisms in each trophic level. The size of
the organism is over-emphasized and it can happen that the mass of level 2 is greater than that of
level 1, because the productivity of level 1 is not taken into consideration. Thus an enormous
mass of grass is required to support a smaller mass of buck, which in turn would support a
smaller mass of lions.
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The Biomass Pyramid
Energy pyramid.
The Energy pyramid indicates the total amount of energy present in each trophic level. It
also shows the loss of energy from one trophic level to the next. An energy pyramid shows
clearly that the energy transfer from one trophic level to the next is accompanied by a decrease
due to waste and the conversion of potential energy into kinetic energy and heat energy. The
energy pyramid is more widely used than the others because comparisons can be made between
trophic levels of different ecosystem. It is, however, more difficult to compile an energy pyramid
than it is compile the other types of pyramids.
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The Energy Pyramid
ADAPTATIONS OF PELAGIC ORGANISMS
All organisms must be adapted to the unique environments in which they are found, and
to accomplish three major objectives with the greatest possible success: finding food, avoiding
predators, and reproducing. Some aspects of the pelagic environment pose special challenges that
have led to unique adaptations for the organisms that live there. First, the pelagic habitat is
‘three-dimensional’—organisms must be able to move (and preferably see) in all directions.
Second, there are no solid substrates to provide refuge. For these reasons, superior swimming
ability is an important means of survival for many large fishes such as tuna, helping them to
capture prey and avoid predators. Indeed, many pelagic fishes are migratory, covering vast
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distances of the open ocean in search of food. These types of fish are streamlined, with thick
bodies and heavy musculature. Various forms of camouflage are common in pelagic organisms.
Fishes in the epipelagic realm are generally countershaded, so their bodies show a color
gradient from dark on the dorsal surface to light on the ventral surface. This type of coloration
makes them difficult to see, both from above and below. Others are silvery, helping them to
reflect sunlight near the surface, and blend in with the surrounding waters. (Left) Red
ctenophore, difficult to see at depth. (middle) Deep-water squid of the genus Histioteuthis, also
red, and covered with photophores (right) the great white shark (Carcharadon carcharias)
exhibits counter-shading with a dark top side (dorsal surface) and a lighter stomach (ventral
surface). Mesopelagic organisms, which live in deeper, darker waters, use different forms of
camouflage. Being black or red in color can help make an organism difficult to spot. Recall from
the physics and chemistry section that red is the first color to attenuate in the water column, so if
you are a red animal, you are essentially invisible because there is no red light to bounce off of
you! For the same reason, being black absorbs all kinds of light, making you less visible as well.
Many mesopelagic fish and OCN 201L—Spring 2011 3 squids have photophores used for
counterillumination, which works in the same way as countershading .
Photophores are light producing organs capable of matching the intensity and color of the
light from the surface, helping to obscure the organism‟s silhouette. Above 400m the light from
the surface becomes too intense for counterilluminating organisms to match, so their vertical
distributions are limited to below 400m in the daytime. All these adaptations to avoid being seen
can have a negative impact on the ability to attract mates. Some mesopelagic fish have lights on
their tails called sternchasers, which advertise them to members of the same species and opposite
sex. Photophores are usually arranged in species-specific patterns, making the organism easy to
identify. Two mesopelagic fish of the class Myctophidae (commonly called lanternfish). (Top)
Sternchasers are visible on the top and bottom of the tail, which are bioluminescent and can be
flashed to confuse potential predators, as well as attract mates. (Bottom) A row of photophores is
visible along the ventral edge of this specimen, and can be used for counterillumination.
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Low light levels in the mesopelagic zone can make it difficult to spot prey. Many fish use
photophores mounted on the ends of fins or dangling projections that act as lures to bring prey in close
where the fish can attack them. Mesopelagic organisms can also have modified tubular eyes with
increased sensitivity to light and enhanced depth perception. Many have eyes 2-5 times larger than
normal, which are sometimes set at upward-looking angles, maximizing the ability to look for shadows
cast by other animals. In addition, some kinds of fish use photophores for „headlights;‟ Malacosteus uses
red headlamps to detect red prey while most animals at depth cannot see red light. This is partially due to
the fact that most animals have reduced eyes because light is minimal in the deep ocean.
Benthic Adaptation
Benthic fish, also known as groundfish, are those that stay on or very near the sea bottom,
whether in shallow or deep water. Unlike fish that spend their time swimming, benthic fish are
very dense and have negative buoyancy, allowing them to effortlessly lie on the bottom or bury
themselves. Other adaptations commonly seen in benthic fish include the lack of a swim bladder
and a flattened body shape. They are predominantly bottom feeders that eat detritus, or ambush
predators that lie in wait for their prey to come within striking distance.
There are many different types of benthic fish. Flatfish, including flounder, halibut,
plaice, sole and turbot, lie on the the sea floor or bury themselves in the sand. Asymmetrical
physical adaptations include having both eyes on one side of their head, and different
pigmentation on each side of their bodies; the side facing down is usually pale, while the side
facing up is camouflaged. Some species of flatfish are predators, feeding on smaller fish, while
others eat mainly invertebrates.
Rays and skates are also flat, but are bilaterally symmetrical, with their eyes on top of
their head. These cartilaginous fish are predators that bury themselves and wait for prey, feeding
largely on crustaceans, clams, oysters and snails.
Rattails are benthic fish that have a large head and mouth, and a body that rapidly tapers
down to a long, narrow tail. They live in the deep sea, and are scavengers, feeding chiefly on
invertebrates. Some species of rattails are brotulas, chimaeras and grenadiers.
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Sandy shore adaptation
The intertidal zone is covered part of the day by water and is part of the day exposed to
air. High tides bring nutrients and food with it. When the tide retreats, waste products, eggs and
larvae are taken. This causes changes for the organisms that live here. They have adapted to this
changing environment, as seen on rocky shores.
The burrowing must be rapid and powerful on high-energy sandy beaches. This is
because the animals must not be swept away by incoming waves and swash. They also need to
be high mobile and must be able to deal with the swash climate. In contrast with rocky shores,
desiccation is not an overriding concern, because the animals can retreat into the substratum or
below the water table. Intertidal filter-feeders cannot feed while the tide has retreated. Many
species of the meiofauna use vertical tidal migrations through the sand column. Other species
move up and down the beach with the tides. This is inadequate for the maintenance of
appropriate rhythmic behavior so responses to changing environmental factors are essential.
There is a difference between directional (such as light, slope of the beach, water currents) and
nondirectional (such as disturbance of the sand, changes in temperature, hydrostatic pressure)
stimuli. Directional stimuli act as orientational signs, while nondirectional stimuli act as
releasing factors. Because of the absence of attached macrophytes, the predominant feeding
types are filter-feeding and scavenging. Adaptations to respiration of animals in low-energy
sandy beaches are different from those on surf-swept beaches. Some adaptations are an increased
ventilation rate or increased efficiency, reduced metabolic rate or other ways of conserving
energy.
Many sheltered-shore animals are facultative anaerobes. This is an adaptation during
ebb tide. Other animals in oxygenated surf-swept beaches are essentially aerobic. The majority
of the intertidal animals have tolerance levels of natural variables that exceed those necessary for
survival in their particular habitats. Some species descent into the burrow to escape high
temperatures. Another solution isevaporative cooling by replacing water through entering the
burrow, plunging into the sea or absorption from the substratum.
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Another problem for intertidal animals is the time of reproduction. There is variation in
the number of eggs, the anatomy of the reproductive organs, the morphology of the egg cases,
times of breeding, mating behavior and developmental stages. Adaptations for this is to
reproduce at frequent times (iteroparous) or to reproduce just once in a year (semelparous). This
depends from species to species. Some species follow the lunar cycle to reproduce at the right
time. To ovoid predation, several behaviors are developed. The first one is to burrow very deep.
Another one is tidal migration, so the animals remain protected from predation. Other responses
are escaping movements or an impressive threat display by crabs by holding their chelae open
and aloft. According to circumstances, the behavior of the animals can be modified. This is
calledphenotypic plasticity.
Several
groups
of
vertebrates
make
use
of
sandy
beaches
for foraging, nesting and breeding. Turtles nest on the backshore of sandy beaches. Birds use the
beach for foraging, nesting and roosting. Seals use several areas of the beach for nesting,
molting, breeding and raising pups. Other terrestrial animals such as otters, baboons, raccoons,
lions. They descend onto the beach to forage.
Muddy shore adaptation
Muddy shores occur where the energy of coastal currents and wave action is
minimal, allowing fine particles of silt to settle to the bottom. The result is an accumulation of
mud on the shores of protected bays and mouths of coastal streams and rivers. Most muddy
beaches occur in estuarine areas. However, some muddy shore areas may be found in coastal
inlets and embayments where salinity is about the same as the adjacent sea. Plant Life & Growth
few plants have adapted to living on muddy shores. Their growth is restricted by turbidity which
reduces light penetration into the water and thereby inhibits photosynthesis. In addition, the lack
of solid structures to which algae may attach it and siltation which smothers plants effectively
prevents much plant colonization of muddy shores. While the lack of oxygen in mud makes life
for fauna in muddy shores difficult, the abundance of food as organic detritus provides nutrition
for a large number of detritus feeders.
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Muddy shores, with their finer sediment, have smaller interstitial spaces and these trap
organic matter. Smaller spaces means that drainage when the tide drops is less and so muddy
shores hold on to their water. Hence we see a really important difference between the two shores:
animals living on sandy shores will be subjected to greater desiccation. Mud accumulates in very
sheltered areas and may occur in patches on rocky shores, especially in the lower shore. Sand
may be deposited in similar places but will also get deposited on exposed, shallow shores.
With care the likely zonation that you can find on a sheltered sandy shore will have two
or three zones. Amongst the strandline will be sandhoppers like Talitrus. Ragworms can be
found in the upper shore, moving actively around in search of prey like the lugworm whose
castes should be visible on the surface. Other sedentary worm like the red threads Audouinia sp
colour the sand. Lower down diversity increases as it has less desiccation. On the surface the
crown of tentacles of the sand mason are visible. This sedentary worm produces a long tube
deep into the sand within which it lives. Cockles have a short siphon and so lie very close to the
surface and can get easily disturbed. Another bivalve is the Tellin shell and these maybe found
in quite exposed sandy shores as, with the long siphons, are able to survive deeper in the
sediment. At the extreme low water marks will be razor shells and burrowing echinoderms, the
sea potato.
`Exposed sandy shores will have even fewer species because of the disturbance by the
turbulent water. Donax is a bivalve able to survive this as it can quickly re-burrow and escape
predators. Mud, deposited in calm conditions, will be a flatter habitat (hence the term mudflat)
and water is unlikely to drain. This minimal desiccation negates much in the way of zonation on
the shore. However, the diversity of species is likely to be higher than sand.
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UNIT - II
Community
In ecology,
a community or biocoenosis is
an
assemblage
or association of
populations of two or more different species occupying the same geographical area and in a
particular time. The term community has a variety of uses. In its simplest form it refers to groups
of organisms in a specific place or time, for example, "the fish community of Lake Ontario
before industrialization".
Community ecology or synecology is the study of the interactions between species in
communities on many spatial and temporal scales, including the distribution, structure,
abundance, demography, and interactions between coexistingpopulations.[1] The primary focus of
community ecology is on the interactions between populations as determined by
specific genotypic and phenotypiccharacteristics.
Community
ecology
has
its
origin
in European plant sociology. Modern community ecology examines patterns such as variation
in species richness, equitability, productivity and food web structure (see community structure);
it
also
examines
processes
such
as
predator-prey population
dynamics, succession,
and community assembly.
On a deeper level the meaning and value of the community concept in ecology is up for
debate. Communities have traditionally been understood on a fine scale in terms of local
processes constructing (or destructing) an assemblage of species, such as the way climate change
is likely to affect the make-up of grass communities.[2] Recently this local community focus has
been criticised. Robert Ricklefs has argued that it is more useful to think of communities on a
regional scale, drawing on evolutionary taxonomy and biogeography,[1] where some species
or clades evolve and others go extinct.
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Interspecific interactions
Competition
Species can compete with each other for finite resources. It is considered to be an
important limiting factor of population size, biomass and species richness. Many types of
competition have been described, but proving the existence of these interactions is a matter of
debate. Direct competition has been observed between individuals, populations and species, but
there is little evidence that competition has been the driving force in the evolution of large
groups.
1. Interference competition: occurs when an individual of one species directly interferes
with an individual of another species. Examples include a lion chasing a hyena from a
kill, or a plant releasing allelopathic chemicals to impede the growth of a competing
species.
2. Exploitative competition: occurs via the consumption of resources. When an individual
of one species consumes a resource (e.g., food, shelter, sunlight, etc.), that resource is no
longer available to be consumed by a member of a second species. Exploitative
competition is thought to be more common in nature, but care must be taken to
distinguish it from apparent competition.
3. Apparent competition: occurs when two species share a predator. The populations of
both species can be depressed by predation without direct exploitative competition. [7]
Predation
Predation is hunting another species for food. This is a positive-negative (+ -) interaction in that
the predator species benefits while the prey species is harmed. Some predators kill their prey
before eating them (e.g., a hawk killing a mouse). Other predators are parasites that feed on prey
while alive (e.g., a vampire bat feeding on a cow). Herbivores feed on plants (e.g., a cow
grazing). Predation may affect the population size of predators and prey and the number of
species coexisting in a community.
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Mutualism
Mutualism is an interaction between species in which both benefit. Examples
include Rhizobium bacteria growing in nodules on the roots of legumes and insects pollinating
the flowers of angiosperms.
Commensalism
Commensalism is a type of relationship among organisms in which one organism benefits
while the other organism is neither benefited nor harmed. The organism that benefited is called
the commensal while the other organism that is neither benefited nor harmed is called the host.
For example, an epiphytic orchid attached to the tree for support benefits the orchid but neither
harms nor benefits the tree. The opposite of commensalism is amensalism, an interspecific
relationship in which a product of one organism has a negative effect on another organism.
Community Successions
Ecological succession is the observed process of change in the species structure of
an ecological community over time. The time scale can be decades (for example, after a
wildfire), or even millions of years after a mass extinction.
The community begins with relatively few pioneering plants and animals and develops
through increasing complexity until it becomes stable or self-perpetuating as a climax
community. The “engine” of succession, the cause of ecosystem change, is the impact of
established species upon their own environments. A consequence of living is the sometimes
subtle and sometimes overt alteration of one's own environment.
It is a phenomenon or process by which an ecological community undergoes more or less
orderly and predictable changes following a disturbance or the initial colonization of a new
habitat. Succession may be initiated either by formation of new, unoccupied habitat, such as
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from a lava flow or a severe landslide, or by some form of disturbance of a community, such as
from a fire, severe windthrow, or logging. Succession that begins in new habitats, uninfluenced
by pre-existing communities is called primary succession, whereas succession that follows
disruption of a pre-existing community is called secondary succession.
Succession was among the first theories advanced in ecology. The study of succession
remains at the core of ecological science. Ecological succession was first documented in the
Indiana Dunes of Northwest Indiana which led to efforts to preserve the Indiana Dunes. Exhibits
on ecological succession are displayed in the Hour Glass, a museum in Ogden Dunes.
Homeostasis
Homoeostasis is the property of a system in which variables are regulated so that internal
conditions remain stable and relatively constant. Examples of homeostasis include the regulation
of temperature and the balance between acidity and alkalinity (pH). It is a process that maintains
the stability of the human body's internal environment in response to changes in external
conditions.
The concept was described by French physiologist Claude Bernard in 1865 and the
word was coined by Walter Bradford Cannon in 1926. Although the term was originally used
to refer to processes within living organisms, it is frequently applied to automatic control
systems such as thermostats. Homeostasis requires a sensor to detect changes in the condition to
be regulated, an effector mechanism that can vary that condition; and a negative
feedback connection between the two.
All
living
organisms
depend
on
maintaining
a
complex
set
of
interacting metabolic chemical reactions. From the simplest unicellular organisms to the most
complex plants and animals, internal processes operate to keep the conditions within tight limits
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to allow these reactions to proceed. Homeostatic processes act at the level of the cell, the tissue,
and the organ, as well as for the organism as a whole.
Principal Homeostatic processes include the following:
•
"Warm-blooded" (endothermic) animals (mammals and birds) maintain a constant body
temperature, whereas ectothermic animals (almost all other animals) exhibit wide
body temperature variation. An advantage of temperature regulation is that it allows
an organism to function effectively in a broad range of environmental conditions. For
example, ectotherms tend to become sluggish at low temperatures, whereas a co-located
endotherm may be fully active. That thermal stability comes at a price, since an automatic
regulation system requires additional energy. If the temperature rises, the body loses heat by
sweating or panting, via the latent heat of evaporation. If it falls, this is counteracted by
increased metabolic action, by shivering, and—in fur- or feather-coated creatures—by
thickening the coat.
•
Regulation of the pH of the blood at 7.365 (a measure of alkalinity and acidity).
•
All animals also regulate their blood glucose concentration. Mammalsregulate their blood
glucose with insulin and glucagon. The human body maintains glucose levels constant most
of the day, even after a 24-hour fast. Even during long periods of fasting, glucose levels are
reduced only very slightly.[3] Insulin, secreted by the beta cells of the pancreas, effectively
transports glucose to the body's cells by instructing those cells to keep more of the glucose
for their own use (see Dynamic equilibrium). If the glucose inside the cells is high, the cells
will convert it to the insoluble glycogen to prevent the soluble glucose from interfering with
cellular metabolism. Ultimately this lowers blood glucose levels, and insulin helps to
preventhyperglycemia. When insulin is deficient
or cells become
resistant
to
it, diabetes occurs. Glucagon, secreted by the alpha cells of the pancreas, encourages cells to
20
break down stored glycogen or convert non-carbohydrate carbon sources to glucose
via gluconeogenesis, thus preventing hypoglycemia.
•
The kidneys are used to remove excess water and ions from the blood. These are then
expelled as urine. The kidneys perform a vital role in homeostatic regulation in mammals,
removing excess water, salt, and urea from the blood.
•
If the water content of the blood and lymph fluid falls, it is restored in the first instance
by extracting water from the cells. The throat and mouth become dry, so that the symptoms
of thirst motivate the animal to drink.
•
If the oxygen content of the blood falls, or the carbon-dioxide concentration increases,
blood flow is increased by more vigorous heart action and the speed and depth of breathing
increases.
•
Sleep timing depends upon a balance between homeostatic sleep propensity, the need for
sleep as a function of the amount of time elapsed since the last adequate sleep episode,
and circadian rhythms that determine the ideal timing of a correctly structured and
restorative sleep episode.
•
Personality traits are often conceptualized as a person specific setpoint level around
which mood states fluctuate in time.[5]
21
Control mechanisms
All homeostatic control mechanisms have at least three interdependent components for
the variable being regulated: Thereceptor is the sensing component that monitors and responds to
changes in the environment. When the receptor senses a stimulus, it sends information to a
"control center", the component that sets the range at which a variable is maintained. The control
center determines an appropriate response to the stimulus. The control center then sends signals
to an effector, which can be muscles, organs, or other structures that receive signals from the
control center. After receiving the signal, a change occurs to correct the deviation by depressing
it with negative feedback.
Negative feedback
Negative feedback mechanisms consist of reducing the output or activity of any organ or
system back to its normal range of functioning. A good example of this is regulating blood
pressure. Blood vessels can sense resistance of blood flow against the walls when blood pressure
increases. The blood vessels act as the receptors and they relay this message to the brain. The
brain then sends a message to the heart and blood vessels, both of which are the effectors. The
heart rate would decrease as the blood vessels increase in diameter (known as vasodilation). This
change would cause the blood pressure to fall back to its normal range. The opposite would
happen when blood pressure decreases, and would causevasoconstriction.
Another important example is seen when the body is deprived of food. The body would
then reset the metabolic set point to a lower than normal value. This would allow the body to
continue to function, at a slower rate, even though the body is starving. Therefore, people
depriving themselves of food while trying to lose weight would find it easy to shed weight
initially and much harder to lose more after. This is due to the body's readjusting itself to a lower
metabolic set-point to allow the body to survive with its low supply of energy. Exercise can
change this effect by increasing the metabolic demand.
22
Another good example of negative feedback mechanism is temperature control.
The hypothalamus, which monitors the body temperature, is capable of determining even the
slightest variation of normal body temperature (36.5 degrees Celsius). Response to such variation
could be stimulation of glands that produce sweat to reduce the temperature or signaling various
muscles to shiver to increase body temperature.
Both feedbacks are equally important for the healthy functioning of one's body.
Complications can arise if any of the two feedbacks are affected or altered in any way.
Homeostatic imbalance
Many diseases involve a disturbance of homeostasis. As the organism ages, the efficiency
in its control systems becomes reduced. The inefficiencies gradually result in an unstable internal
environment that increases the risk of illness, and leads to the physical changes associated with
aging. Certain homeostatic imbalances, such as high core temperature, a high concentration of
salt in the blood, or low concentration of oxygen, can generate homeostatic emotions (such as
warmth, thirst, or breathlessness), which motivate behavior aimed at restoring homeostasis (such
as removing a sweater, drinking or slowing down).
The concept of homeostasis is central to the topic of Ecological Stoichiometry. There, it
refers to the relationship between the chemical composition of an organism and the chemical
composition of the nutrients it consumes. Stoichiometric homeostasis helps explain nutrient
recycling and population dynamics. Throughout history, ecological succession was seen as
having a stable end-stage called the climax (see Frederic Clements), sometimes referred to as the
'potential biodiversity' of a site, shaped primarily by the local climate. This idea has been largely
abandoned by modern ecologists in favor of nonequilibrium ideas of how ecosystems function,
as most natural ecosystems experience disturbance at a rate that makes a "climax" community
unattainable.
Only on small, isolated habitats known as ecological islands can the phenomenon be
observed. One such case study is the island of Krakatoa after its major eruption in 1883: the
23
established stable homeostasis of the previous forest climax ecosystem was destroyed, and all
life was eliminated from the island. In the years after the eruption, Krakatoa went through a
sequence of ecological changes in which successive groups of new plant or animal species
followed one another, leading to increasing biodiversity and eventually culminating in a reestablished climax community.
This ecological succession on Krakatoa occurred in a number of stages; a sere is defined
as "a stage in a sequence of events by which succession occurs". The complete chain of seres
leading to a climax is called a prisere. In the case of Krakatoa, the island reached its climax
community, with eight hundred different recorded species, in 1983, one hundred years after the
eruption that cleared all life off the island. Evidence confirms that this number has been
homeostatic for some time, with the introduction of new species rapidly leading to elimination of
old ones. The evidence of Krakatoa, and other disturbed island ecosystems, has confirmed many
principles of Island Biogeography, mimicking general principles of ecological succession albeit
in a virtually closed system comprised almost exclusively of endemic species.
Biogeochemical cycle
In ecology, a biogeochemical cycle is a circuit or pathway by which a chemical element
or molecule moves through both biotic ("bio-") and abiotic ("geo-") compartments of
an ecosystem. In effect, the element is recycled, although in some such cycles there may be
places (called "sinks") where the element is accumulates for a long period of time.
All chemical elements occurring in organisms are part of biogeochemical cycles. In
addition to being a part of living organisms, these chemical elements also cycle through abiotic
factors of ecosystems, such as water (hydrosphere), land (lithosphere), and air (atmosphere); the
living factors of the planet can be referred to collectively as the biosphere. The biogeochemical
cycles provide a clear demonstration of one of the fundamental principles of biological systems:
24
The harmonious interactions between organisms and their environment, both biotically and
abiotically.
All the chemicals, nutrients, or elements used in ecosystems by living organisms—such
as carbon, nitrogen, oxygen, and phosphorus—operate on a closed system, which means that
these chemicals are recycled, instead of lost, as they would be in an open system. The energy of
an ecosystem occurs in an open system; the sun constantly gives the planet energy in the form
of light, which is eventually used and lost in the form of heat, throughout the trophic levels of
a food web.
Although components of the biogeochemical cycle are not completely lost, they can be
held for long periods of time in one place. This place is called a reservoir, which, for example,
includes such things as coal deposits that are storing carbon for a long period of time. When
chemicals are held for only short periods of time, they are being held in exchange
pools. Generally, reservoirs are abiotic factors while exchange pools are biotic factors. Examples
of exchange pools include plants and animals, which temporarily use carbon in their systems and
release it back into a particular reservoir. Carbon is held for a relatively short time in plants and
animals when compared to coal deposits. The amount of time that a chemical is held in one place
is called its residence time.
The most well-known and important biogeochemical cycles include the carbon cycle,
the nitrogen cycle, the oxygen cycle, the phosphorus cycle, and the water cycle. Biogeochemical
cycles always involve equilibrium states: A balance in the cycling of the element between
compartments. However, overall balance may involve compartments distributed on a global
scale.
Biogeochemical cycles of particular interest in ecology are:
•
Nitrogen cycle
•
Oxygen cycle
25
•
Carbon cycle
•
Phosphorus cycle
•
Sulfur cycle
•
Water cycle
•
Hydrogen cycle
The nitrogen cycle is a complicated biogeochemical cycle, and is only summarized here.
This cycle involves living components, water, land, and air. Nitrogen is a very important element
in that it is part of both proteins (present in the composition of the amino acids that make those
proteins) as well as nucleic acids, such as DNA and RNA (present in nitrogenous bases).
The largest reservoir of nitrogen is the atmosphere, in which about 78 percent of which
made up of nitrogen gas (N2). Nitrogen gas is “fixed,” in a process called nitrogen fixation.
Nitrogen fixation combines nitrogen with oxygen to create nitrates (NO3). Nitrates can then be
used byplants or animals (which eat plants, or eat animals that have eaten plants).
Nitrogen can be fixed either by lightning, industrial methods (such as for fertilizer), in
free nitrogen-fixing bacteria in the soil, as well as in nitrogen-fixing bacteria present
in roots of legumes (such as rhizobium). Nitrogen-fixing bacteria use certain enzymes that are
capable of fixing nitrogen gas into nitrates and include free bacteria in soil, symbiotic bacteria in
legumes, and also cyanobacteria, or blue-green algae, in water.
After being used by plants and animals, nitrogen is then disposed of in decay and wastes.
Decomposers and detritivores ingest the detritus from plants and animals and nitrogen is changed
into ammonia, or nitrogen with 3 hydrogen atoms (NH3). Ammonia is toxic and cannot be used
by plants or animals, but nitrite bacteria present in the soil can take ammonia and turn it into
nitrite—nitrogen with two oxygen atoms (NO2). Although nitrite is also unusable by most plants
and animals, nitrate bacteria change nitrites back into nitrates, usable by plants and animals.
26
Some
nitrates
are
also
converted
back
into
nitrogen
gas
through
the
process
of denitrification, which is the opposite of nitrogen-fixing; this process is also called
nitrification. Certain denitrifying bacteria are responsible for this.
Oxygen cycle
The oxygen
cycle is
the biogeochemical
cycle that
describes
the
movement
ofoxygen within and between its three main reservoirs: The atmosphere, the biosphere, and the
lithosphere (the crust and the uppermost layer of the mantle). The main driving factor of the
oxygen cycle is photosynthesis, which is responsible for the modern Earth's atmosphere and life
as it is today. If all photosynthesis were to cease, the Earth's atmosphere would be devoid of all
but trace amounts of oxygen within 5000 years. The oxygen cycle would no longer exist.
Reservoirs and fluxes
The vast amount of molecular oxygen is contained in rocks and minerals within the Earth
(99.5 percent). Only a small fraction has been released as free oxygen to the biosphere (0.01
percent) and atmosphere (0.49 percent). The main source of oxygen within the biosphere and
atmosphere is photosynthesis, which breaks down carbon dioxide and water to create sugars and
oxygen:
27
CO2 + H2O + energy → CH2O + O2.
An additional source of atmospheric oxygen comes from photolysis, whereby high
energy ultraviolet radiation breaks down atmospheric water and nitrite into component
molecules. The free H and N atoms escape into space leaving O2 in the atmosphere: 2H2O +
energy → 4H + O2.
The main way oxygen is lost from the atmosphere is via respiration and decay
mechanisms in which animal life consumes oxygen and releases carbon dioxide. Because
lithospheric minerals are reduced in oxygen, surface weathering of exposed rocks also consumes
oxygen. An example of surface weathering chemistry is formation of iron-oxides (rust), such as
those found in the red sands of Australia:
4FeO + 3O2 → 2Fe2O3.
Oxygen is also cycled between the biosphere and lithosphere. Marine organisms in the
biosphere create carbonate shell material (CaCO3) that is rich in molecular oxygen. When the
organism dies, its shell is deposited on the shallow sea floor and buried over time to
create limestone rock. Weathering processes initiated by organisms can also free oxygen from
the land mass. Plants and animals extract nutrient minerals from rocks and release oxygen in the
process.
The following tables offer estimates of oxygen cycle reservoir capacities and fluxes.
These numbers are based primarily on estimates from Walker (1980).
Ozone
The presence of atmospheric oxygen has led to the formation of ozone and the ozone
layer within the stratosphere. The ozone layer is extremely important to modern life, as it absorbs
harmful ultraviolet radiation:
O2 + uv energy → 2O
O + O2 + uv energy → O3
28
The absorbed solar energy also raises the temperature of the atmosphere within the ozone
layer, creating a thermal barrier that helps trap the atmosphere below (as opposed to bleeding out
into space).
Phosphorus and atmospheric oxygen
There is an interesting theory that phosphorus (P) in the ocean helps regulate the amount
of atmospheric oxygen. Phosphorus dissolved in the oceans is an essential nutrient to
photosynthetic life and one of the key limiting factors. Oceanic photosynthesis contributes
approximately 45 percent of the total free oxygen to the oxygen cycle (largely from algae). The
population growth of photosynthetic organisms is primarily limited by the availability of
dissolved phosphorus.
One side effect of mining and industrial activities is a dramatic increase in the amount of
phosphorus being discharged to the world's oceans. However, this increase in available
phosphorus has not resulted in a corresponding increase in oceanic photosynthesis.
An increase in photosynthesizer population results in increased oxygen levels in the
oceans. The elevated oxygen levels promote the growth of certain types of bacteria that compete
for uptake of dissolved phosphorus. This competition limits the amount of phosphorous available
to photosynthetic life, thus buffering their total population as well as the levels of O2.
Carbon cycle
The carbon cycle is the biogeochemical cycle by which carbon is exchanged between the
biosphere, lithosphere, hydrosphere, and atmosphere of the Earth. (Other bodies may have
carbon cycles, but little is known about them.)
All of these components are reservoirs of carbon. The cycle is usually discussed as four
main reservoirs of carbon interconnected by pathways of exchange. The reservoirs are the
atmosphere, terrestrial biosphere (usually includes freshwater systems), oceans, and sediments
29
(includes fossil fuels). The annual movements of carbon, the carbon exchanges between
reservoirs, occur because of various chemical, physical, geological, and biological processes.
The ocean contains the largest pool of carbon near the surface of the Earth, but most of that pool
is not involved with rapid exchange with the atmosphere. Major molecules of carbon are carbon
dioxide (CO2), carbon monoxide (CO), methane (CH4), calcium carbonate (CaCO3), and glucose
(in plant organic matter,C6H12O6), and many others, as well as many ions containing carbon.
The global carbon budget is the balance of the exchanges (incomes and losses) of
carbon between the carbon reservoirs or between one specific loop (e.g., atmosphere-biosphere)
of the carbon cycle. An examination of the carbon budget of a pool or reservoir can provide
information about whether the pool or reservoir is functioning as a source or sink for carbon
dioxide.
Phosphorous cycle
The phosphorus cycle is the biogeochemical cycle that describes the movement
of phosphorus through the lithosphere, hydrosphere, and biosphere. Unlike many other
biogeochemicals, the atmosphere does not play a significant role in the movements of
phosphorus, because phosphorus and phosphorus-based compounds are usually solids in the
typical ranges of temperature and pressure found on Earth.
Phosphorus in the environment
Phosphorus normally occurs in nature as part of a phosphate ion, consisting of a
phosphorus atom and
some
number
of
oxygen
atoms,
the
most
abundant
form
(called orthophosphate) having four oxygens: PO43-. Most phosphates are found as salts in ocean
sediments or in rocks. Over time, geologic processes can bring ocean sediments to land, and
weathering will carry terrestrial phosphates back to the ocean. Plants absorb phosphates from the
soil. The plants may then be consumed by herbivores, which in turn may be consumed by
carnivores. After death, the animal or plant decays, and the phosphates are returned to the soil.
Runoff may carry them back to the ocean, or they may be reincorporated into rock.
30
The primary biological importance of phosphates is as a component of nucleotides, which
serve as energy storage within cells (ATP) or, when linked together, form the nucleic
acids DNA and RNA. Phosphorus is also found in bones, whose strength is derived from calcium
phosphate, and in phospholipids (found in all biological membranes). Phosphates move quickly
through plants and animals; however, the processes that move them through the soil or ocean are
very slow, making the phosphorus cycle overall one of the slowest biogeochemical cycles.
Human influence
Phosphates may be extracted from the soil to be incorporated into artificial fertilizer.
However, fertilizers not absorbed by plants mostly enter the groundwater and collect in lakes and
ponds. The extra phosphates are a major contributor to the process called eutrophication, which
causes excessive growth of water plants and algae populations.
Sulfur Cycle
Sulfur is one of the constituents of many proteins, vitamins, and hormones. It recycles
like other biogeochemical cycles.
The essential steps of the sulfur cycle are:
•
Mineralization of organic sulfur to the inorganic form, hydrogen sulfide: (H 2S).
•
Oxidation of sulfide and elemental sulfur (S) and related compounds to sulfate (SO42-).
•
Reduction of sulfate to sulfide.
•
Microbial immobilization of the sulfur compounds and subsequent incorporation into
the organic form of sulfur
These are often termed as follows:
Assimilative sulfate reduction, in which sulfate (SO42-) is reduced to organic sulfhydryl
groups (R-SH) by plants, fungi, and variousprokaryotes. The oxidation states of sulfur are
+6 in sulfate and -2 in R-SH.
31
Desulfuration, in which organic molecules containing sulfur can be desulfurated,
producing hydrogen sulfide gas (H2S), oxidation state = -2.
Oxidation of hydrogen sulfide, produces elemental sulfur (So), oxidation state = 0. This
reaction is done by the photosynthetic green and purple sulfur bacteria and some
chemolithotrophs (organisms using inorganic compounds for ATP production).
Further oxidation of elemental sulfur by sulfur oxidizers produces sulfate.
Dissimilative sulfur reduction, in which elemental sulfur can be reduced to hydrogen
sulfide.
Dissimilative sulfate reduction, in which sulfate reducers generate hydrogen sulfide from
sulfate.
Human impact on the sulfur cycle is primarily in the production of sulfur dioxide (SO2)
from industry (such as burning coal) and the internal combustion engine. Sulfur dioxide can
precipitate onto surfaces where it can be oxidized to sulfate in the soil (it is also toxic to some
plants), reduced to sulfide in the atmosphere, or oxidized to sulfate in the atmosphere as sulfuric
acid, a principal component of acid rain.
Water cycle
The water cycle—technically known as the hydrologic cycle—is the continuous
circulation of water within the Earth's hydrosphere, and is driven by solar radiation. The
hydrosphere includes the atmosphere, land, surface water, and groundwater. As water moves
through the cycle, it changes state between liquid, solid, and gas phases. Water moves through
different reservoirs, including ocean, atmosphere, groundwater, rivers, and glaciers, by the
physical processes of evaporation (including plant transpiration), sublimation, precipitation,
infiltration, runoff, and subsurface flow.
32
Precipitation, which is the falling of water in any form to earth; infiltration, which is
the process in which water is absorbed into the soil (it may also flow off the surface called
surface run off); evaporation or transpiration, which occurs either when water is heated and
turns into water vapor or when plants use the water and give it off as water vapor, respectively;
and condensation, which occurs when water vapor cools and forms clouds. This process is then
repeated over again.
The total amount, or mass, of water in the water cycle remains essentially constant, as
does the amount of water in each reservoir of the water cycle. This means that rate of water
added to one reservoir must equal, on average, over time, the rate of water leaving the same
reservoir. The largest reservoir is the collection of oceans, accounting for 97 percent of the
Earth's water. The next largest quantity (2 percent) is stored in solid form in the ice caps
and glaciers. The water contained within all living organisms represents the smallest reservoir.
The volume of water in the freshwater reservoirs, particularly those that are available
for human use, are important water resources.
The residence time of a water molecule in a particular reservoir varies greatly from the
order of seconds to hours and days (as in evaporation of precipitation) to much longer time scales
of thousands of years. Groundwater can spend over 10,000 years underground before leaving,
and ocean water can be on the order of a thousand years old.
Hydrogen cycle
Hydrogen is a key component of many biogeochemical cycles, including the the water
cycle, carbon cycle, nitrogen cycle, and the sulfur cycle. Because hydrogen is a component of the
water molecule, the hydrogen cycle and water cycle are deeply linked. Plants also
recombine waterand carbon dioxide from the soil and atmosphere to form glucose in a process
known as photosynthesis. If the plant is consumed, the hydrogen molecules are transferred to the
grazing animal. The organic matter is then stored in soils as the plant or animal dies, and the
hydrogen molecules are released back into the atmosphere through oxidation.
33
UNIT – III
Natural resource
A natural resource is anything that people can use which comes from nature. People do
not make natural resources, but gather them from the earth. Examples of natural resources
are air,water, wood, oil, wind energy, hydro-electric energy, iron, and coal. Refined oil is not a
natural resource because people make it. We often say there are two sorts of natural resources:
renewable resources and non-renewable resources.
•
A renewable resource grows again and comes back again after we use it. For example,
soil, sunlight, water and woodare renewable resources.
•
A non-renewable resource is a resource that does not grow and come back, or a resource
that would take a very long time to come back. For example, coal is a non-renewable
resource. When we use coal, there is less coal afterward. One day, there will be no more of it
to make goods. The non-renewable resource can be used directly (for example, burning oil to
cook), or we can find a renewable resource to use (for example, using wind energy to
make electricity to cook). It is important to conserve (save) non-renewable resources,
because if we use them too quickly there will not be enough.
Most natural resources are limited. This means they will eventually run out. A perpetual
resource has a never-ending supply. Some examples of perpetual resources include solar
energy, tidal energy, and wind energy. Some of the things influencing supply of resources
include whether it is able to be recycled, and the availability of suitable substitutes for the
material. Non-renewable resources cannot be recycled. For example, oil, minerals, and other
non-renewable resources cannot be recycled.
Demand
The demand for resources can change with new technology, new needs, and
new economics (e.g. changes in cost of the resources). Some material can go completely out of
34
use, if people do not want it any more. Demand of natural resources is very high, but availability
is very low.
Avilability
All places have their own natural resources. When people do not have a certain resource
they need, they can either replace it with another resource, or trade with another country to get
the resource. Some resources are difficult to find, so people sometimes fight to have them (for
example, oil resources).
When people do not have some natural resources, their quality of life can get lower. So, we need
to protect our resources from pollution. For example, when they cannot get clean water, people
may become ill; if there is not enough wood, trees will be cut and the forest will disappear over
time (deforestation); if there are not enough fish in a sea, people can die of starvation. Some
examples of renewable resources are wood, solar energy, trees, wind, hydroelectric power, fish,
and sunlight. So humans should begin saving their natural resources. Or else, all will be lost and
it will be difficult for humans to survive.
Renewable Resources
Renewable resources are resources that are replenished by the environment over
relatively short periods of time. This type of resource is much more desirable to use because
often a resource renews so fast that it will have regenerated by the time you've used it up. Think
of this like the ice cube maker in your refrigerator. As you take some ice out, more ice gets
made. If you take a lot of ice out, it takes a little more time to refill the bin but not a very long
time at all. Even if you completely emptied the entire ice cube bin, it would probably only take a
few hours to 'renew' and refill that ice bin for you. Renewable resources in the natural
environment work the same way.
35
Solar energy is one such resource because the sun shines all the time. Imagine trying to
harness all of the sun's energy before it ran out! Wind energy is another renewable resource. You
can't stop the wind from blowing any more than you can stop the sun from shining, which makes
it easy to 'renew.'
Any plants that are grown for use in food and manufactured products are also renewable
resources. Trees used for timber, cotton used for clothes, and food crops, such as corn and wheat,
can all be replanted and regrown after the harvest is collected.
Animals are also considered a renewable resource because, like plants, you can breed
them to make more. Livestock, like cows, pigs and chickens, all fall into this category. Fish are
also considered renewable, but this one is a bit trickier because even though some fish are
actually farmed for production, much of what we eat comes from wild stocks in lakes and
oceans. These wild populations are in a delicate balance, and if that balance is upset by
overfishing, that population may die out.
Water is also sometimes considered a renewable resource. You can't really 'use up' water,
but you also can't make more of it. There is a limited supply of water on earth and it cycles
through the planet in various forms - as a liquid (our oceans), a solid (our polar ice caps and
glaciers) and a gas (as clouds and water vapor).
Liquid water can be used to generate hydroelectric power, which we get from water
flowing through dams. This is considered a renewable resource because we don't actually take
the water out of the system to get electricity. Like sunshine and wind, we simply sit back and let
the resource do all the work!
Geothermal energy is a renewable resource that provides heat from the earth - 'geo'
means 'earth' and 'thermal' means 'heat.' You know all of those volcanoes on Earth that spew hot
lava when they erupt? That lava has got to come from somewhere, right? It's actually sitting
underneath the earth's surface as incredibly hot rock and magma.
36
We find the most heat in places like plate boundaries because these are like large cracks
under Earth's surface where the heat can escape as well as places on Earth where the crust is
relatively thin. Old Faithful and other natural springs and geysers are the result of geothermal
energy, and that water can be hotter than 430°F!
Biofuels are renewable resources that are fuels made from living organisms - literally
biological fuels. Ethanol is a biofuel because it's derived from corn. Biodiesel is vehicle fuel
made from vegetable oil, and I bet you didn't know that people can actually run their cars on
used oil from restaurants! Firewood, animal dung and peat burned for heat and cooking purposes
are also biofuels because they come from living (or once-living) organisms.
Non-Renewable Resources
In contrast to renewable resources, non-renewable resources are resources that are not
easily replenished by the environment. Let's think about this in terms of that ice cube maker
again. Imagine that this time you don't have an automatic ice maker at home, you have to wait
for someone to bring it to you, and they only do this once a month.
If you used up all your ice quickly, it wouldn't regenerate in your refrigerator and you
would be out of ice until the next delivery comes. The same thing happens with non-renewable
resources on Earth, except the wait time is much longer than a month - usually more like
thousands or millions of years!
The fuels we use to heat our homes and drive our cars are non-renewable resources
because there is just no way that the Earth can regenerate them in a usable time frame. Minerals
are also considered non-renewable resources because not only do they take millions of years of
heat and pressure to form deep underground, but they're also found in a very limited quantity on
Earth. Not all non-renewable resources are usable only once, though.
37
Animal resources
In biomedical research, experimental animals have taken on enormous importance as
models for elucidating and predicting behavior, health, and disease or for information regarding
basic biologic processes. In most areas of research, there is an increasing recognition that
constant, dependable experimental conditions are essential in order to obtain reproducible and
reliable information. Most investigators are aware of the need for a research system with as few
variables as possible, but oftentimes the experimental animal is not considered.
In a living organism, there are two basic sources of variation — genetic and
environmental. Accurately defined, standardized, and properly housed laboratory animals are
needed in order to accomplish meaningful biomedical research. Use of animals harboring overt
or latent diseases, housed in crowded, unsanitary conditions, or maintained in an environment
which results in abnormal behavioral and physiological responses certainly compromises and
brings into question the validity of research accomplished.
Also, there is concern for the comfort and well-being of the experimental animals
themselves. It is unacceptable to subject them to needless suffering or deprivation. Scientific,
legal, and ethical considerations have prompted standards that are becoming increasingly
comprehensive and rigorous for the handling, care, and use of experimental animals.
A third factor involved in experimental animal care is that of fiscal responsibility. In the face of
current financial limitations on animal-based research, from an institutional perspective,
centralized management of the animal resources should result in more efficient and economical
use of personnel, equipment, and space while concurrently providing appropriate care and
housing of animals.
There is a growing recognition that the care of experimental animals is a shared
responsibility between the institution and individual investigators and instructors. Many
institutions have recognized that animal resources are specialized professionally, economically,
and organizationally and that it is preferable to have one centralized unit responsible for all
experimental and teaching animal resources.
38
By experience, it has been shown that such an arrangement is in the best interests of the
individual scientist, the host institution, and granting agencies sponsoring research. However,
centralized management of animal resources in no way is intended to limit the user’s freedom
and obligation to plan and conduct animal experiments and instructional activities in accordance
with accepted scientific practice.
Considering the above factors, the UGA College of Veterinary Medicine (CVM)
organized the Animal Resources (AR) unit for the central management of the experimental and
teaching animal resources. The AR unit is to provide two basic functions for the College:
•
The safe and effective procurement, quarantine, conditioning, housing, husbandry, and
veterinary medical care of all animals used in the research and teaching programs.
•
Provide technical assistance, advice, and consultation regarding use of experimental
animals.
•
In biomedical research, experimental animals have taken on enormous importance as
models for elucidating and predicting behavior, health, and disease or for information
regarding basic biologic processes. In most areas of research, there is an increasing
recognition that constant, dependable experimental conditions are essential in order to
obtain reproducible and reliable information. Most investigators are aware of the need for
a research system with as few variables as possible, but oftentimes the experimental
animal is not considered.
•
In a living organism, there are two basic sources of variation — genetic and
environmental. Accurately defined, standardized, and properly housed laboratory animals
are needed in order to accomplish meaningful biomedical research. Use of animals
harboring overt or latent diseases, housed in crowded, unsanitary conditions, or
maintained in an environment which results in abnormal behavioral and physiological
39
responses certainly compromises and brings into question the validity of research
accomplished.
•
Also, there is concern for the comfort and well-being of the experimental animals
themselves. It is unacceptable to subject them to needless suffering or deprivation.
Scientific, legal, and ethical considerations have prompted standards that are becoming
increasingly comprehensive and rigorous for the handling, care, and use of experimental
animals.
•
A third factor involved in experimental animal care is that of fiscal responsibility. In the
face of current financial limitations on animal-based research, from an institutional
perspective, centralized management of the animal resources should result in more
efficient and economical use of personnel, equipment, and space while concurrently
providing appropriate care and housing of animals.
•
There is a growing recognition that the care of experimental animals is a shared
responsibility between the institution and individual investigators and instructors. Many
institutions have recognized that animal resources are specialized professionally,
economically, and organizationally and that it is preferable to have one centralized unit
responsible for all experimental and teaching animal resources.
•
By experience, it has been shown that such an arrangement is in the best interests of the
individual scientist, the host institution, and granting agencies sponsoring research.
However, centralized management of animal resources in no way is intended to limit the
user’s freedom and obligation to plan and conduct animal experiments and instructional
activities in accordance with accepted scientific practice.
•
Considering the above factors, the UGA College of Veterinary Medicine (CVM)
organized the Animal Resources (AR) unit for the central management of the
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experimental and teaching animal resources. The AR unit is to provide two basic
functions for the College:
•
The safe and effective procurement, quarantine, conditioning, housing, husbandry, and
veterinary medical care of all animals used in the research and teaching programs.
•
Provide technical assistance, advice, and consultation regarding use of experimental
animals.
Conventional and non-conventional sources of renewable energy
Conventional : Energy that has been used from ancient times is known as conventional energy.
Coal, natural gas, oil, and firewood are examples of conventional energy sources. (or usual)
sources of energy (electricity) are coal, oil, wood, peat, uranium.
Non-conventional (or unusual) sources of energy include:
• Solar power
• Hydro-electric power (dams in rivers)
• Wind power
• Tidal power
• Ocean wave power
• Geothermal power (heat from deep under the ground)
• Ocean thermal power (the difference in heat between shallow and deep water)
• Biomass (burning of vegetation to stop it producing methane)
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• Biofuel (producing ethanol (petroleum) from plants.
We hope that all the conventional sources will become rare, endangered and extinct, as
they produce lots of carbon dioxide that adds to the greenhouse effect in the atmosphere
(uranium leaves different dangerous byproducts). And we similarly hope that all the nonconventional sources will become conventional, common, and every day, as they are all free,
green and emit no carbon dioxide (well, biomass does, but it prevents the production of methane
which is a greenhouse gas 21 times more dangerous that CO2).
Brief description of non-conventional energy resources:
Solar Energy:
Most of the renewable energy is ultimately “Solar energy” that is directly collected from sun
light. Energy is released by the Sun as electromagnetic waves. The energy reaching earth’s
atmosphere consists of about
• 8% UV radiation
• 46% visible light
• 46% infrared radiations
Solar energy storage is as per figure below:
Solar Energy can be used in two ways:
• Solar heating
• Solar electricity
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Solar Heating is to capture/concentrate sun’s energy for heating buildings and for
cooking/heating foodstuffs etc. solar electricity is mainly produced by using photovoltaic solar
cells which is made of semi conducting materials that directly converts sunlight into electricity.
Obviously the Sun doesn’t provide constant energy at any spot on the Earth, so it’s use is limited.
Therefore often Solar cells are used to charge batteries which are used either as secondary energy
source or for other applications of intermittent use such as night lightening or water pumping etc.
A solar power plant offers good option for electrification of disadvantageous locations such as
hilly regions, forests, deserts and islands where other resources are neither available nor
exploitable in techno economically viable manner.
Wind Energy:
The origin for Wind Energy is Sun. When sun ray falls on the earth, it’s surface gets
heated up and as a consequence unevenly winds are formed. Kinetic energy in the wind can be
used to run wind turbines but the output power depends upon the wind speed. Turbines generally
require a wind in the range of 20km/hr. In practice relatively few land areas have significantly
prevailing winds. Otherwise wind power is one of the most cost competitive renewable energy
today and this has been the most rapidly-growing means of electricity generation at the turn of
21st century and provides a complement to a large scale base load power stations. Its long term
technical potential is believed to be 5 times current global energy consumption or 40 times
current electricity demand.
Water Power
Energy in the water can be harnessed and used in the form of motive energy or
temperature difference. Since water is about 1000 times heavier than air, even a slow flowing
stream of water can yield great amount of energy.
There are many forms:
• Hydroelectric energy, a term usually reserved for hydroelectric dam
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• Tidal power, which captures energy from the tides in horizontal direction. Tides come in, raise
water levels in a basin, and tides roll out. The water is made to pass through turbine to get out of
the basin. Power generation through this method has a varying degree of success.
• Wave power, which uses energy in waves. The waves will usually make large pontoons go up
and down in the water. The wave power is also hard to tap.
Hydro electrical energy is therefore only viable option. However, even probably this
option is also not there with the developed nations for future energy production because most
major sites within these nations with potential for harnessing gravity in this way are already
being exploited or are unavailable for other reasons such as environmental consideration. On the
other side, large hydro potential of millions of megawatts is available with the developing
countries but major bottleneck in the way of development of these large hydro projects is that
each site calls for the huge investment.
Micro/Small hydro power
This is non-conventional and renewable source and is easy to tap. Quantitatively small
volume of water, with large falls and quantitatively not too large volumes of water, with small
fall, can be tapped. This force of flowing and falling water is used to run water turbines to
generate electricity.
Geothermal Energy
Geothermal energy is very clean source of power. It comes from radioactive decay in the
core of the Earth, which heats the Earth from inside out and thus energy/power can be extracted
owing to the temperarture difference between hot rock deep in the earth and relatively cool
surface air or water. This requires that the hot rock be relatively shallow, so it is site-specific and
can only be applied in geologically active areas.
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It can be used in two ways
• Geothermal heating
• Geothermal electricity
As stated above, geothermal energy from the core of the earth is closer to surface in some
area than in others. Where hot underground steam or water can be tapped and brought to the
surface it may be directly used to heat or cool buildings or indirectly used to generate electricity
by running steam turbines. Even otherwise, on most of the globe, the temperature of the crust
few feet below the surface is buffered at a constant 7-14 degree Celsius, so liquid can be preheated or pre-cooled in underground pipelines, providing free cooling in the summer and heating
in the winter by using heat pumps.
Biomass
Solid biomass
Plants use photosynthesis to store solar energy in the form of chemical energy.Theeasiest
way to release this energy is by burning the dried up plants. Solid biomass such as firewood or
combustible field crops including dried manure is usually burnt to heat water and to drive
turbines. Field crops may be grown specifically for combustion or may be for other purposes and
the processed plant waste then used for combustion.Most sort of biomass including sugarcane
residue, wheat chaff, corn cobs and other plant matter can be, and is, burnt quiet successfully.
Currently
biomass
contributes
15%
of
total
energy
supply
world
wide.
A drawback is that all these biomass needs to go through some of these steps: It needs to be
grown, collected, dried and fermeneted and burned. All of these steps require resources and an
infrastructure.
Bio-fuel
Bio-fuel is any fuel that derives from biomass- recently living organisms or their
metabolic byproducts, such as manure from cows. Typically bio-fuel is burnt to release it’s
stored chemical energy. Biomass, can be directly used as fuel or to produce liquid biofuel.
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Agriculturally produced biomass fuels, such as biodiesel, ethanol and bagasse(often byproduct of
sugarcane cultivation) can be burnt in internal combustion engines or boilers.
Biogas
Biogas can easily be produced from current waste streams,such as paper production,
sugarcane production, sewage, animal waste and so forth. The various waste streams have to be
slurred together and allowed to naturally ferment, producing 55% to 70% inflammable methane
gas. Biogas production has the capacity to provide us with about half of our energy needs, either
burned for electrical productions or piped into current gas line for use. This has to be done and
made a priority. The payback period of biogas is around 2-3 years, rather in case of community
and institutional Biogas plant is even less. Therefore biogas electrification at community level is
required to be implemented.
A. Introduction and definition of environmental pollution – We know that, a living organism
cannot live by itself. Organisms interact among themselves. Hence, all organisms, such as plants,
animals and human beings, as well as the physical surroundings with whom we interact, form a
part of our environment. All these constituents of the environment are dependent upon each
other. Thus, they maintain a balance in nature. As we are the only organisms try to modify the
environment to fulfill our needs; it is our responsibility to take necessary steps to control the
environmental imbalances.
The environmental imbalance gives rise to various environmental problems. Some of the
environmental problems are pollution, soil erosion leading to floods, salt deserts and sea recedes,
desertification, landslides, change of river directions, extinction of species, and vulnerable
ecosystem in place of more complex and stable ecosystems, depletion of natural resources, waste
accumulation, deforestation, thinning of ozone layer and global warming. The environmental
problems are visualized in terms of pollution, growth in population, development,
industrialization, unplanned urbanization etc. Rapid migration and increase in population in the
urban areas has also lead to traffic congestion, water shortages, solid waste, and air, water and
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noise pollution are common noticeable problems in almost all the urban areas since last few
years.
Environmental pollution is defined as the undesirable change in physical, chemical and
biological characteristics of our air, land and water. As a result of over-population, rapid
industrializations, and other human activities like agriculture and deforestation etc., earth became
loaded with diverse pollutants that were released as by-products. Pollutants are generally
grouped under two classes:
(a) Biodegradable pollutants – Biodegradable pollutants are broken down by the activity of
micro-organisms and enter into the biogeochemical cycles. Examples of such pollutants are
domestic waste products, urine and faucal matter, sewage, agricultural residue, paper, wood and
cloth etc.
(b) Non- Biodegradable pollutants – Non-biodegradable pollutants are stronger chemical
bondage, do not break down into simpler and harmless products. These include various
insecticides and other pesticides, mercury, lead, arsenic, aluminum, plastics, radioactive waste
etc.
B. Classification of Environmental Pollution – Pollution can be broadly classified according to
the components of environment that are polluted. Major of these are: Air pollution, Water
pollution, Soil pollution (land degradation) and Noise pollution. Details of these types of
pollutions are discussed below with their prevention measures.
(1) Air Pollution: Air is mainly a mixture of various gases such as oxygen, carbon dioxide,
nitrogen. These are present in a particular ratio. Whenever there is any imbalance in the ratio of
these gases, air pollution is caused. The sources of air pollution can be grouped as under.
(i) Natural; such as, forest fires, ash from smoking volcanoes, dust storm and decay of organic
matters.
(ii) Man-made due to population explosion, deforestation, urbanization and industrializations.
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Certain activities of human beings release several pollutants in air, such as carbon monoxide
(CO), sulfur dioxide (SO2), hydrocarbons (HC), oxides of nitrogen (NOx), lead, arsenic,
asbestos, radioactive matter, and dust. The major threat comes from burning of fossil fuels, such
as coal and petroleum products. Thermal power plants, automobiles and industries are major
sources of air pollution as well. Due to progress in atomic energy sector, there has been an
increase in radioactivity in the atmosphere. Mining activity adds to air pollution in the form of
particulate matter. Progress in agriculture due to use of fertilizers and pesticides has also
contributed towards air pollution. Indiscriminate cutting of trees and clearing of forests has led to
increase in the amount of carbon dioxide in atmosphere. Global warming is a consequence of
green house effect caused by increased level of carbon dioxide (CO2). Ozone (O3) depletion has
resulted in UV radiation striking our earth.
The gaseous composition of unpolluted air
The Gases
Parts
(vol)
Nitrogen
756,500
Oxygen
202,900
Water
31,200
Argon
9,000
Carbon Dioxide
305
Neon
17.4
48
per
million
Helium
5.0
Methane
0.97-1.16
Krypton
0.97
Nitrous oxide
0.49
Hydrogen
0.49
Xenon
0.08
Organic vapours
ca.0.02
Harmful Effects of air pollution –
(a) It affects respiratory system of living organisms and causes bronchitis, asthma, lung cancer,
pneumonia etc. Carbon monoxide (CO) emitted from motor vehicles and cigarette smoke affects
the central nervous system.
(b) Due to depletion of ozone layer, UV radiation reaches the earth. UV radiation causes skin
cancer, damage to eyes and immune system.
(c) Acid rain is also a result of air pollution. This is caused by presence of oxides of nitrogen and
sulfur in the air. These oxides dissolve in rain water to form nitric acid and sulfuric acid
respectively. Various monuments, buildings, and statues are damaged due to corrosion by acid
present in the rain. The soil also becomes acidic. The cumulative effect is the gradual
degradation of soil and a decline in forest and agricultural productivity.
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(d) The green house gases, such as carbon dioxide (CO2) and methane (CH4) trap the heat
radiated from earth. This leads to an increase in earth’s temperature.
(e) Some toxic metals and pesticides also cause air pollution.
(2) Water Pollution: Water is one of the prime necessities of life. With increasing number of
people depend on this resource; water has become a scarce commodity. Pollution makes even the
limited available water unfit for use. Water is said to be polluted when there is any physical,
biological or chemical change in water quality that adversely affects living organisms or makes
water unsuitable for use. Sources of water pollution are mainly factories, power plants, coal
mines and oil wells situated either close to water source or away from sources. They discharge
pollutants directly or indirectly into the water sources like river, lakes, water streams etc. The
harmful effects of water pollution are:
(a) Human beings become victims of various water borne diseases, such as typhoid, cholera,
dysentery, hepatitis, jaundice, etc.
(b) The presence of acids/alkalies in water destroys the microorganisms, thereby hindering the
self-purification process in the rivers or water bodies. Agriculture is affected badly due to
polluted water. Marine eco-systems are affected adversely.
(c) The sewage waste promotes growth of phytoplankton in water bodies; causing reduction of
dissolved oxygen.
(d) Poisonous industrial wastes present in water bodies affect the fish population and deprives us
of one of our sources of food. It also kills other animals living in fresh water.
(e) The quality of underground water is also affected due to toxicity and pollutant content of
surface water.
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(2.1) Water pollution by industries and its effects
A change in the chemical, physical, biological, and radiological quality of water that is
injurious to its uses. The term “water pollution” generally refers to human-induced changes to
water quality. Thus, the discharge of toxic chemicals from industries or the release of human or
livestock waste into a nearby water body is considered pollution.
The contamination of ground water of water bodies like rivers, lakes, wetlands, estuaries, and
oceans can threaten the health of humans and aquatic life. Sources of water pollution may be
divided into two categories. (i) Point-source pollution, in which contaminants are discharged
from a discrete location. Sewage outfalls and oil spills are examples of point-source pollution.
(ii) Non-point-source or diffuse pollution, referring to all of the other discharges that deliver
contaminants to water bodies. Acid rain and unconfined runoff from agricultural or urban areas
falls under this category.
The principal contaminants of water include toxic chemicals, nutrients, biodegradable
organics, and bacterial & viral pathogens. Water pollution can affect human health when
pollutants enter the body either via skin exposure or through the direct consumption of
contaminated drinking water and contaminated food. Prime pollutants, including DDT and
polychlorinated biphenyls (PCBs), persist in the natural environment and bioaccumulation
occurs in the tissues of aquatic organisms. These prolonged and persistent organic pollutants are
transferred up the food chain and they can reach levels of concern in fish species that are eaten
by humans. Moreover, bacteria and viral pathogens can pose a public health risk for those who
drink contaminated water or eat raw shellfish from polluted water bodies.
Contaminants have a significant impact on aquatic ecosystems. Enrichment of water
bodies with nutrients (principally nitrogen and phosphorus) can result in the growth of algae and
other aquatic plants that shade or clog streams. If wastewater containing biodegradable organic
matter is discharged into a stream with inadequate dissolved oxygen, the water downstream of
the point of discharge will become anaerobic and will be turbid and dark. Settleable solids will
be deposited on the streambed, and anaerobic decomposition will occur. Over the reach of stream
where the dissolved-oxygen concentration is zero, a zone of putrefaction will occur with the
production of hydrogen sulfide (H2S), ammonia (NH3), and other odorous gases. Because many
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fish species require a minimum of 4–5 mg of dissolved oxygen per liter of water, they will be
unable to survive in this portion of the stream.
Direct exposures to toxic chemicals are also a health concern for individual aquatic plants
and animals. Chemicals such as pesticides are frequently transported to lakes and rivers via
runoff, and they can have harmful effects on aquatic life. Toxic chemicals have been shown to
reduce the growth, survival, reproductive output, and disease resistance of exposed organisms.
These effects can have important consequences for the viability of aquatic populations and
communities.
Wastewater discharges are most commonly controlled through effluent standards and
discharge permits. Under this system, discharge permits are issued with limits on the quantity
and quality of effluents. Water-quality standards are sets of qualitative and quantitative criteria
designed to maintain or enhance the quality of receiving waters. Criteria can be developed and
implemented to protect aquatic life against acute and chronic effects and to safeguard humans
against deleterious health effects, including cancer.
(3) Soil pollution (Land degradation): Land pollution is due to
(i) Deforestation and
(ii) Dumping of solid wastes.
Deforestation increases soil erosion; thus valuable agricultural land is lost. Solid wastes
from household and industries also pollute land and enhance land degradation. Solid wastes
include things from household waste and of industrial wastes. They include ash, glass, peelings
of fruit and vegetables, paper, clothes, plastics, rubber, leather, brick, sand, metal, waste from
cattle shed, night soil and cow dung. Chemicals discharged into air, such as compounds of sulfur
and lead, eventually come to soil and pollute it. The heaps of solid waste destroy the natural
beauty and surroundings become dirty. Pigs, dogs, rats, flies, mosquitoes visit the dumped waste
and foul smell comes from the waste. The waste may block the flow of water in the drain, which
then becomes the breeding place for mosquitoes. Mosquitoes are carriers of parasites of malaria
52
and dengue. Consumption of polluted water causes many diseases, such as cholera, diarrhea and
dysentery.
(4) Noise pollution : High level noise is a disturbance to the human environment. Because of
urbanization, noise in all areas in a city has increased considerably. One of the most pervasive
sources of noise in our environment today is those associated with transportation. People reside
adjacent to highways, are subjected to high level of noise produced by trucks and vehicles pass
on the highways. Prolonged exposure to high level of noise is very much harmful to the health of
mankind.
In industry and in mines the main sources of noise pollution are blasting, movement of heavy
earth moving machines, drilling, crusher and coal handling plants etc. The critical value for the
development of hearing problems is at 80 decibels.
Chronic exposure to noise may cause noise-induced hearing loss. High noise levels can
contribute to cardiovascular effects. Moreover, noise can be a causal factor in workplace
accidents.
C. Fundamentals of prevention and control of air pollution:
As mentioned above, air pollutants can be gaseous or particulate matters. Different techniques
for controlling these pollutants are discussed below:
a. Methods of controlling gaseous pollutants –
1. Combustion – This technique is used when the pollutants are in the form of organic gases or
vapors. During flame combustion or catalytic process, these organic pollutants are converted into
water vapor and relatively less harmful products, such as CO2.
2. Absorption – In this technique, the gaseous effluents are passed through scrubbers or
absorbers. These contain a suitable liquid absorbent, which removes or modifies one or more of
the pollutants present in the gaseous effluents.
3. Adsorption – The gaseous effluents are passed through porous solid adsorbents kept in
suitable containers. The organic and inorganic constituents of the effluent gases are trapped at
the interface of the solid adsorbent by physical adsorbent.
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b. Methods to control particulate emissions –
1. Mechanical devices generally work on the basis of the following:
(i) Gravity: In this process, the particles settle down by gravitational force.
(ii) Sudden change in direction of the gas flow. This causes the particles to separate out due to
greater momentum.
2. Fabric Filters: The gases containing dust are passed through a porous medium. These porous
media may be woven or filled fabrics. The particles present in the gas are trapped and collected
in the filters. The gases freed from the particles are discharged.
3. Wet Scrubbers: Wet scrubbers are used in chemical, mining and metallurgical industries to
trap SO2, NH3, metal fumes, etc.
4. Electrostatic Precipitators: When a gas or an air stream containing aerosols in the form of dust,
fumes or mist, is passed between two electrodes, then, the aerosol particles get precipitated on
the electrode.
c. Other practices in controlling air pollution –Apart from the above, following practices also
help in controlling air pollution.
(i) Use of better designed equipment and smokeless fuels, hearths in industries and at home.
(ii) Automobiles should be properly maintained and adhere to recent emission-control standards.
(iii) More trees should be planted along road side and houses.
(iv) Renewable energy sources, such as wind, solar energy, ocean currents, should fulfill energy
needs.
(v) Tall chimneys should be installed for vertical dispersion of pollutants.
d. General air pollution control devices / equipments for industries – The commonly used
equipments / process for control of dust in various industries are (a) Mechanical dust collectors
in the form of dust cyclones; (b) Electrostatic precipitators – both dry and wet system; (c)
54
particulate scrubbers; (d) Water sprayer at dust generation points; (e) proper ventilation system
and (f) various monitoring devices to know the concentration of dust in general body of air.
The common equipments / process used for control of toxic / flue gases are the (a) process of
desulphurisation; (b) process of denitrification; (c) Gas conditioning etc. and (d) various
monitoring devices to know the efficacy of the systems used.
e. Steps, in general, to be taken for reduction of air pollution – To change our behavior in order
to reduce AIR POLLUTION at home as well as on the road, few following small steps taken by
us would lead to clean our Environment.
At Home:
1. Avoid using chemical pesticides or fertilizers in your yard and garden. Many fertilizers are a
source of nitrous oxide, a greenhouse gas that contributes to global warming. Try organic
products instead.
2. Compost your yard waste instead of burning it. Outdoor burning is not advisable, as it pollutes
air. Breathing this smoke is bad for you, your family and your neighbors. Plus, you can use the
compost in your garden.
3. If you use a wood stove or fireplace to heat your home, it would be better to consider
switching to another form of heat which does not generate smoke. It is always better to use
sweater or warm clothing than using fireplace.
4. Be energy efficient. Most traditional sources of energy burn fossil fuels, causing air pollution.
Keep your home well-maintained with weather-stripping, storm windows, and insulation.
Lowering your thermostat can also help – and for every two degrees Fahrenheit you lower it, you
save about two percent on your heating bill.
5. Plant trees and encourage other to plant trees as well. Trees absorb and store carbon dioxide
from the atmosphere, and filter out air pollution. During warmer days, trees provide cool air,
unnecessary use of energy on air conditioning is avoided, hence the air pollution.
6. Try to stop smoking; at home, at office or at outside. Tobacco smoking not only deteriorates
self’s health, it affects others health too.
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On the Road:
7. Keep your vehicle well maintained. A poorly maintained engine both creates more air
pollution and uses more fuel. Replace oil and air filters regularly, and keep your tires properly
inflated.
8. Drive less. Walking, bicycling, riding the bus, or working from home can save you money as
well as reducing air pollution.
9. Don’t idle your vehicle. If you stop for more than 30 seconds, except in traffic, turn off your
engine.
10. Don’t buy more car than you need. Four-wheel drive, all-wheel drive, engine size, vehicle
weight, and tire size all affect the amount of fuel your vehicle uses. The more fuel it uses the
more air pollution it causes.
D. Water pollution prevention and control:
Water is a key resource for our quality of life. It also provides natural habitats and ecosystems for plant and animal species. Access to clean water for drinking and sanitary purposes is
a precondition for human health and well-being. Clean unpolluted water is essential for our
ecosystems. Plants and animals in lakes, rivers and seas react to changes in their environment
caused by changes in chemical water quality and physical disturbance of their habitat.
Water pollution is a human-induced change in the chemical, physical, biological, and
radiological quality of water that is injurious to its existing, intended, or potential uses such as
boating, waterskiing, swimming, the consumption of fish, and the health of aquatic organisms
and ecosystems. Thus, the discharge of toxic chemicals from a pipe or the release of livestock
waste into a nearby water body is considered pollution. The contamination of ground water,
rivers, lakes, wetlands, estuaries, and oceans can threaten the health of humans and aquatic life.
Contaminants have a significant impact on aquatic ecosystems. for example, enrichment
of water bodies with nutrients (principally nitrogen and phosphorus) can result in the growth of
algae and other aquatic plants that shade or clog streams. Direct exposures to toxic chemicals
such as pesticides, is also a health concern for individual aquatic plants and animals. Without
56
healthy water for drinking, cooking, fishing, and farming, the human race would perish. Clean
water is also necessary for recreational interests such as swimming, boating, and water skiing.
a. Sources of Water Pollution – Sources of water pollution are generally divided into two
categories. The first is point-source pollution, in which contaminants are discharged from a
discrete location. Sewage outfalls and oil spills are examples of point-source pollution. The
second category is non-point-source or diffuses pollution, referring to all of the other discharges
that deliver contaminants to water bodies.
Numerous manufacturing plants pour off undiluted corrosives, poisons, and other noxious
byproducts to water streams. The construction industry discharges slurries of gypsum, cement,
abrasives, metals, and poisonous solvents. The mining industry also presents persistent water
pollution problems. In yet another instance of pollution, hot water discharged by factories and
power plants causes so-called ‘thermal pollution’ by increasing water temperatures. Such
increases change the level of oxygen dissolved in a body of water, thereby disrupting the water’s
ecological balance, killing off some plant and animal species while encouraging the overgrowth
of others. Towns and municipalities are also major sources of water pollution.
In many public water systems, pollution exceeds safe levels. One reason for this is that
much groundwater has been contaminated by wastes pumped underground for disposal or by
seepage from surface water. When contamination reaches underground water tables, it is difficult
to correct and spreads over wide areas. Discharge of untreated or only partially treated sewage
into the waterways threatens the health of their own and neighboring populations as well. Along
with domestic wastes, sewage carries industrial contaminants and a growing tonnage of paper
and plastic refuse. Although thorough sewage treatment would destroy most disease-causing
bacteria, the problem of the spread of viruses and viral illness remains. Additionally, most
sewage treatment does not remove phosphorus compounds, contributed principally by
detergents.
b. Dangers of Water Pollution – Virtually all water pollutants are hazardous to humans as well
as lesser species; sodium is implicated in cardiovascular disease, nitrates in blood disorders.
Mercury and lead can cause nervous disorders. Some contaminants are carcinogens. DDT is
57
toxic to humans and can alter chromosomes. Along many shores, shellfish can no longer be
taken because of contamination by DDT, sewage, or industrial wastes.
c. Prevention and Control of Water Pollution – Sewage should be treated before it is discharged
into the river or ocean. This is possible through modern techniques.
Sewage is first passed through a grinding mechanism. This is then passed through several
settling chambers and neutralized with lime. Up to this stage, the process is called primary
treatment. The sewage still contains a large number of pathogenic and non-pathogenic
organisms, and also sufficient quantity of organic matter. The neutralized effluents are sent to
UASB (up-flow anaerobic sludge blanket). It is a reactor. In this, the anaerobic bacteria degrade
the biodegradable material present in the waste water. This removes foul odor and releases
methane, which can be used elsewhere. In this system, the pollution load is reduced upto 85
percent. After this, water is sent to aeration tanks where it is mixed with air and bacteria.
Bacteria digest the organic waste material. This is called biological or secondary treatment. Even
after the treatment, water is not yet fit for drinking. The harmful microorganisms need to be
killed. The final step (tertiary treatment) is, therefore, a disinfection process, to remove final
traces of organics, bacteria, dissolved inorganic solids, etc. For tertiary treatment, methods, such
as chlorination, evaporation, and exchange absorption may be employed. These depend upon the
required quality of the final treatment.
Apart from the above, you should also adopt the following practices:
(i) Waste food material, paper, decaying vegetables and plastics should not be thrown into open
drains.
(ii) Effluents from distilleries, and solid wastes containing organic matter should be sent to
biogas plants for generation of energy.
(iii) Oil slicks should be skimmed off from the surface with suction device. Sawdust may be
spread over oil slicks to absorb the oil components.
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E. Soil erosion and its prevention:
Soil erosion by water, wind and tillage affects both agriculture and the natural
environment. Soil loss, and its associated impacts, is one of the most important (yet probably the
least well-known) of today’s environmental problems. It is mostly due to poor land use practices,
which include deforestation, overgrazing, unmanaged construction activity and road or trail
building.
Soil is a complex mixture of living and non-living materials. It provides anchorage and
sustenance to plants. Natural agents like water and wind, constantly tend to remove the top soil
and cause erosion. Rain falling upon the unprotected top soil, washes it down into the streams.
Due to the absence of plant covering, eroded soil cannot hold water. Water rushes into the rivers
and overflows as flood. Dust storm also causes soil erosion. The particles of top soil are picked
up in such quantities that they form clouds of dust. Human beings also cause soil erosion. The
growing human habitation and expansion of urban areas lead to removal of vegetation. Once
vegetation is removed, the naked soil gets exposed to wind and water. Improper tillage is another
cause of soil erosion. Farmers often loosen the top soil for removing weeds and preparing seed
beds. They also leave agricultural fields lying fallow for long time. These practices expose the
top soil to the wind and cause erosion.
Soil erosion is always a result of mankind’s unwise actions, such as overgrazing or
unsuitable cultivation practices. These leave the land unprotected and vulnerable. Accelerated
soil erosion by water or wind may affect both agricultural areas and the natural environment, and
is one of the most widespread of today’s environmental problems. Soil erosion is just one form
of soil degradation. Other kinds of soil degradation include salinisation, nutrient loss, and
compaction.
Prevention of soil erosion – Plants provide protective cover on the land and prevent soil erosion
for the reasons:
(a) plants slow down water as it flows over the land (runoff) and this allows much of the rain to
soak into the ground;
59
(b) plant roots hold the soil in position and prevent it from being washed away;
(c) plants break the impact of a raindrop before it hits the soil, thus reducing its ability to erode;
(d) plants in wetlands and on the banks of rivers are of particular importance as they slow down
the flow of the water and their roots bind the soil, thus preventing erosion.
Preventing soil erosion requires technical changes to adopt. Aspects of technical changes
include:
(i) use of contour ploughing and wind breaks;
(ii) leaving unploughed grass strips between ploughed land;
(iii) making sure that there are always plants growing on the soil, and that the soil is rich in
humus (decaying plant and animal remains). This organic matter is the “glue” that binds the soil
particles together and plays an important part in preventing erosion;
(iv) avoiding overgrazing and the over-use of crop lands;
(v) allowing indigenous plants to grow along the river banks instead of ploughing and planting
crops right up to the water’s edge;
(vi) encouraging biological diversity by planting several different types of plants together;
(vii) conservation of wetlands.
We can check soil erosion by adopting the following additional practices:
1. Intensive cropping and use of proper drainage canals.
2. Terracing on the sloping fields. This retards the speed of the flowing water.
3. Planting trees and sowing grasses.
4. Extensive aforestation practices to be carried out.
60
F. Mitigation of Noise pollution:
Reducing noise pollution by muffling the sounds at the source is one of the best methods
in industry and for urban living. Protective equipment is generally mandatory when noise levels
exceed 85 dB(A) in industry. Creation of green cover adjacent to municipal roads and in mines is
the way to mitigate noise pollution. It has been observed that noise level reduces by 10 decibels
per every 10m wide green belt development. Apart, redesigning industrial equipment, shock
mounting assemblies and physical barriers in the workplace are also for reduction and exposure
of unwanted industrial noise.
High way noise pollution can be mitigated by constructing noise barriers. Artificial noise
barriers are solid obstructions built between the highway and the residential areas along a
highway. They block major portion of noise produced by passing vehicles on a highway.
Effective noise barriers typically reduce noise levels by as much as half or more. The
construction of noise barrier may be built in the form of earth mounds, vertical wall along the
highways for creation of blockage of sound generated by heavy vehicles. Creation of greenbelt in
the space between the residences and highways also reduces the noise nuisance.
G. Conservation and protection of environment:
By now, all of us have realized how important it is to protect the environment for our
own survival. The term ‘conservation’ of environment relates to activities which can provide
individual or commercial benefits, but at the same time, prevent excessive use leading to
environmental damage. Conservation may be distinguished from preservation, which is
considered to be “maintaining of nature as it is, or might have been before the intervention of
either human beings or natural forces.” We know that natural resources are getting depleted and
environmental problems are increasing. It is, therefore, necessary to conserve and protect our
environment. Following practices help in protecting our environment.
1. Rotation of crops.
2. Judicious use of fertilisers, intensive cropping, proper drainage and irrigation.
61
3. Treatment of sewage, so that it does not pollute the rivers and other water bodies.
4. Composting organic solid waste for use as manure.
5. Planting trees in place of those removed for various purposes.
6. National parks and conservation forests should be established by the government.
7. Harvesting of rain water.
Some action points to protect or improve the environment –
(i) Dispose the waste after separating them into biodegradable and non-biodegradable waste
material.
(ii) Start a compost heap or use a compost bin. This can be used to recycle waste food and other
biodegradable materials.
(iii) Avoid unnecessary or wasteful packaging of products.
(iv) Reuse carry bags.
(v) Plant trees. They will help to absorb excess carbon dioxide.
(vi) Observe World Environment Day on 5th June.
(vii) Never put any left over chemicals, used oils down the drain, toilet or dump them on the
ground or in water or burn them in the garden. If you do so, it will cause pollution.
(viii) Don’t burn any waste, especially plastics, for the smoke may contain polluting gases.
(ix) Use unleaded petrol and alternate sources of energy, and keep the engine properly tuned and
serviced and the tyres inflated to the right pressure, so that vehicle runs efficiently.
(x) Avoid fast starts and sudden braking of automobiles.
(xi) Walk or cycle where it is safe to do so – walking is free; cycling can help to keep you fit.
62
(xii) Use public transport wherever you can, or form a car pool for everyday travel.
(xiii) Send your waste oil, old batteries and used tyres to a garage for recycling or safe disposal;
all these can cause serious pollution.
Biologial indicators
Bio indicators are species that can be used to monitor the health of an environment
or ecosystem. They are any biological species or group of species whose function, population, or
status can reveal the qualitative status of the environment. One example of a group of bioindicators is the copepods and other small water crustaceans that are present in many water
bodies. Such organisms can be monitored for changes (biochemical, physiological,
or behavioural) that may indicate a problem within their ecosystem. Bio indicators can tell us
about the cumulative effects of different pollutants in the ecosystem and about how long a
problem may have been present, which physical and chemical testing cannot.
A biological monitor, or biomonitor, can be defined as an organism that provides
quantitative information on the quality of the environment around it. Therefore, a good
biomonitor will indicate the presence of the pollutant and also attempt to provide additional
information about the amount and intensity of the exposure.
A bio indicator is an organism or biological response that reveals the presence of the
pollutants by the occurrence of typical symptoms or measurable responses, and is therefore
more qualitative. These organisms (or communities of organisms) deliver information on
alterations in the environment or the quantity of environmental pollutants by changing in one of
the following ways: physiologically, chemically or behaviourally. The information can be
deduced through the study of:
1. their content of certain elements or compounds
2. their morphological or cellular structure
63
3. metabolic-biochemical processes
4. behaviour, or
5. population structure(s).
The importance and relevance of biomonitors, rather than man-made equipment, is
justified by the statement: "There is no better indicator of the status of a species or a system than
a species or system itself. The use of a biomonitor is described as biological
monitoring (abbr. biomonitoring) and is the use of the properties of an organism to obtain
information on certain aspects of the biosphere. Biomonitoring of air pollutants can be passive or
active. Passive methods observe plants growing naturally within the area of interest. Active
methods detect the presence of air pollutants by placing test plants of known response
and genotype into the study area. Bioaccumulative indicators are frequently regarded as
biomonitors. Depending on the organism selected and their use, there are several types of bioindicators.
Plant indicators
The presence or absence of certain plant or other vegetative life in an ecosystem can
provide important clues about the health of the environment: environmental preservation. There
are several types of plant biomonitors, including mosses, lichens, tree bark, bark pockets, tree
rings, leaves, andfungi.
•
Lichens are organisms comprising both fungi and algae. They are found on rocks and tree
trunks, and they respond to environmental changes in forests, including changes in forest
structure – conservation biology, air quality, and climate. The disappearance of lichens in a
forest may indicate environmental stresses, such as high levels of sulfur dioxide, sulfur-based
pollutants, and nitrogen oxides.
64
•
The composition and total biomass of algal species in aquatic systems serves as an
important metric for organic water pollution and nutrient loading such as nitrogen and
phosphorus.
There are genetically engineered organisms, that that can respond to toxicity levels in
the environment; e.g., a type of genetically engineered grass that grows a different colour if there
are toxins in the soil.
Animal indicators and toxins
An increase or decrease in an animal population may indicate damage to the ecosystem
caused by pollution. For example, if pollution causes the depletion of important food sources,
animal species dependent upon these food sources will also be reduced in number: population
decline. Overpopulation can be the result of opportunistic species growth. In addition to
monitoring the size and number of certain species, other mechanisms of animal indication
include monitoring the concentration of toxins in animal tissues, or monitoring the rate at which
deformities arise in animal populations, or their behaviour either directly in the field or in a lab.
Microbial indicators and chemical pollutants
Microorganisms can be used as indicators of aquatic or terrestrial ecosystem health.
Found in large quantities, microorganisms are easier to sample than other organisms. Some
microorganisms will produce new proteins, called stress proteins, when exposed to contaminants
such as cadmium and benzene. These stress proteins can be used as an early warning system to
detect changes in levels of pollution.
Microbial Prospecting for oil and gas (MPOG) is often used to identify prospective areas
for oil and gas occurrences. In many cases oil and gas is known to seep toward the surface as
a hydrocarbon reservoir
will
usually
leak
or
have
leaked
towards
the
surface
through buoyancy forces overcoming sealing pressures. These hydrocarbons can alter the
chemical and microbial occurrences found in the near surface soils or can be picked up directly.
Techniques used for MPOG includeDNA analysis, simple bug counts after culturing a soil
65
sample in a hydrocarbon based medium or by looking at the consumption of hydrocarbon gases
in a culture cell.
Microalgae as bio-indicators for water quality
Microalgae have gained attention in the recent years due to several reasons because of
their greater sensitivity to pollutants than many other organisms. In addition they occur
abundantly in nature, they are an essential component in very many food webs, they are easy to
culture and to use in assays and there are few if any ethical issues involved in their use.
Euglena gracilis is a motile freshwater photosynthetic flagellate. Although Euglena is
rather tolerant to acidity, it responds rapidly and sensitively to environmental stresses such as
heavy metals or inorganic and organic compounds. Typical responses are the inhibition of
movement and the change of orientation parameters. Moreover, this organism is very easy to
handle and grows, making it a very useful tool for ecotoxicological assessments. One very useful
particularity of this organism is the gravitactic orientation, which is very sensitive to pollutants.
The gravireceptors are impaired by pollutants such as heavy metals and organic or
inorganic compounds. Therefore the presence of such substances is associated with random
movement of the cells in the water column. For short term tests, gravitactic orientation of E.
gracilis is very sensitive. Other species such as Paramecium biaurelia (see Paramecium aurelia)
also use gravitactic orientation.
ECOTOX
ECOTOX is an automatic bioassay device used to test the quality of water samples, by
the detection of toxic chemicals. It is small piece of hardware containing a miniaturized
microscope linked to a camera, an observation cuvette, pumps to mix the water samples with the
microalgae; everything being connected to a computer equipped with software. One of the
biggest advantages of this device is the automated measurements and analysis, which reduces the
risks of personal error. Moreover, it is easy to use, quite cheap and fast: only 10 min are
66
necessary to test a water sample and the corresponding control. Examples of use are the test of
seepage water or the determination of the efficiency of purification systems by testing treated
waste water before and after purification. The determination of the samples quality is derived
from analysis of several parameters related to the movement of the microalgae. All
measurements are made automatically with real time image analysis. First the orientation
behaviour of the cells is determined using two parameters: the percentage of cells moving
upwards giving the direction of the movement and the r-value indicating the precision of the
gravitactic orientation which varies from a random movement (r-value=0) to a single direction
(r-value=1).
Other important parameters are the velocity, the cell motility which represents the
percentage of cells moving faster than the minimum velocity and the cell compactness giving
information about the shape of the cell. All parameters are compared with a control sample of
unpolluted tap water and the percentage of inhibition is calculated. An inhibition indicates the
presence of a pollutant. Depending on the aim of the study, the EC50 (the concentration of
sample which affects 50% of organisms) and the G-value (lowest dilution factor at which nosignificant toxic affect can be measured), are calculated. From all those parameters, the
gravitactic orientation represented with upward swimming and r-value is the most sensitive.[13]
Macroinvertebrate bio-indicators
Macroinvertebrates are useful and convenient indicators of the ecological health of a
waterbody or river. They are almost always present, and are easy to sample and identify. The
sensitivity of the range of macroinvertebrates found will enable an objective judgement of the
ecological condition to be made. Tolerance values are commonly used to assess water pollution.
In Australia, the SIGNAL method has been developed and is used by researchers and
community "Waterwatch" groups to monitor water health.
In Europe,a remote online biomonitoring system was designed in 2006. It is based on
bivalve molluscs and the exchange of real time data between a remote intelligent device in the
67
field (able to work for more than 1 year without in-situ human intervention) and a data centre
designed to capture, process and distribute on the web information derived from the data. The
technique relates bivalve behaviour, specifically shell gaping activity, to water quality changes.
This technology has been successfully used for the assessment of coastal water quality in various
countries (France, Spain, Norway, Russia, Svalbard (Ny Alesund) and New Caledonia.
In the United States, the Environmental Protection Agency (EPA) has published Rapid
Bioassessment Protocols, based on macroinvertebrates, as well as periphyton and fish. These
protocols are used by many federal, state and local government agencies to design biosurveys for
assessment of water quality.[16] Volunteer stream monitoring organizations around the U.S.,
working in cooperation with government agencies, typically use macroinvertebrate
methods.[17] The species identification procedures are conducted in the field without the use of
specialized equipment, and the techniques can be easily taught in volunteer training sessions.
In South Africa, the Southern African Scoring System (SASS) method was developed as
a rapid bioassessment technique, based on benthic macroinvertebrates, and is used for the
assessment of water quality in Southern African rivers. The SASSaquatic biomonitoring tool has
been refined over the past 30 years and is now on the fifth version (SASS5) which has been
specifically modified in accordance with international standards, namely the ISO/IEC
17025 protocol.[19] The SASS5 method is used by the South African Department of Water
Affairs as a standard method for River Health Assessment, which feeds the national River Health
Programme and the national Rivers Database.
The imposex phenomenon in the dog conch species of sea snail leads to the abnormal
development of a penis in females, but does not cause sterility. Because of this, the species has
been suggested as a good indicator of pollution with organic man-made tin compounds
in Malaysian ports.
68
The Problems of Urbanization in Developed and Developing
Countries
The second UN Conference on Human Settlements in 1996 came to the conclusion that
the cities all over the world are facing problems due to urbanization. But how do the type and
scale of problems differ between those in the developed and the developing world?
The initial problems faced by the developing countries are mostly due to demographic
changes. The new migrants from rural areas are usually young males. The existing infrastructure
of the city cannot usually cope with the influx of new migrants and this brings about social
problems. Most of the cities in these countries suffer from chronic housing shortages. This
causes a proliferation of slums and squatter settlements.Central slums are usually in old, substandard buildings, which have been subdivided into tiny, cramped flats. Most newcomers
initially move here in search of work, but access to services is poor. There is a high incidence of
crime, suicide, drugs and alcohol. These central areas are often nicknamed ‘slums of despair.
Later, with more money and joined by the family, the early migrant may move to
peripheral squatter settlements. Conditions here are fractionally better, but the huge informal
settlements are a headache for the city authorities. It may take many years for the settlements to
be provided with adequate water and electricity, education and healthcare. The self-built houses
are often sub-standard with no regard for building regulations.
Cities in the developed world have social problems too, though they are generally more
confined to inner city areas. Immigration is far less rapid, although still noticeable. Housing
shortages are also less severe. Inner city areas are characterized by an increased incidence of
single parents, elderly people and children in care.
69
Death rates and infant mortality are higher and life expectancy is lower while access to
services is poorer than in outer suburbs. However, the scale of the problems is by no means as
bad as in developing countries. The economic problems of developing countries include
widespread under employment and unemployment. The work in the informal sector is
unregulated and poorly paid. The informal sector does not enable people to benefit from social
security, making this support channel useless for the majority.
Many developed cities have suffered a backlash from unsustainable economic growth.
Recent years have seen a decline in the traditional heavy industries as a result of falling demand
and competition. For example, employment in the manufacturing sector fell from 19 per cent in
the early 80s to just 9 per cent by 1991 in London.
The recession at that time was not helped by the modern economic ideas of downsizing
(decreasing staff numbers) and outsourcing (using external contractors as required). The days of
full employment are probably now gone, although the situation has recovered significantly from
the early 1990s. The other major problem facing developed countries is decentralization. In
recent years, companies have chosen sites away from the expensive city centres, in cheaper,
more flexible out-of-town locations. This has been particularly the case with new hi-tech
industries.
Environmental problems are very much a hazard in the developing countries as in the
developed world. Pollution is heavy—thick black smog is common in many countries as a result
of too many cars, taxis and buses. The exhaust fumes from these vehicles blend into a toxic
cocktail of gases. Respiratory problems especially cardiovascular illness and lung cancer are
becoming common in people.
Water supplies are contaminated by bleaching of dangerous chemicals into the soil. In
fact, water supply itself is a severe problem in many of the developing countries. The over70
exploitation of local aquifers has caused them to be almost exhausted. Water must be pumped
from further a field at a much greater cost. The system of water delivery is also flawed. A quarter
or more of water is lost as a result of leaking pipes and illegal ‘tapping.’
Illegal dumping of waste by the developed world has become a health hazard in most of
the developing countries. In some ways, the problems of the developing countries and the
developed countries are very different. While governments in the developing world must try to
increase the supply of services to satisfy the increasing population, those in the developed world
must try to cope with economic slowdown due to decentralization and changing working
patterns.
Sewage
Sewage is a water-carried waste, in solution or suspension, that is intended to be removed
from a community. Also known as domestic or municipal wastewater, it is more than 99% water
and is characterized by volume or rate of flow, physical condition, chemical and toxic
constituents, and its bacteriologic status (which organisms it contains and in what proportions). It
consists
mostly
of greywater (from
sinks,
tubs,
showers,
dishwashers,
and
clothes
washers), blackwater (the water used to flush toilets, combined with the human waste that it
flushes away); soaps and detergents; and toilet paper (less so in regions where bidets are widely
used instead of paper). Whether it also contains surface runoff depends on the design of its route
back to the environment.
All sewage ends up back in the environment (from which its constituents came), by any
of several routes. A basic distinction in its route is whether it undergoes anysewage treatment to
mitigate its effect on the environment before arriving there. Sewage usually travels from a
building's plumbing either into a sewer, which will carry it elsewhere, or into an onsite sewage
facility (of which there are many kinds). Whether it is combined with surface runoff in the sewer
depends on the sewer design (sanitary sewer or combined sewer). Before the 20th century,
71
sewers usually discharged into a body of water such as a stream, river, lake, bay, or ocean. There
was no treatment, so the breakdown of the human waste was left to the ecosystem. Today, the
goal is that sewers route their contents to a wastewater treatment plant rather than directly to a
body of water. In many countries, this is the norm; in some developing countries, it may be a yetunrealized goal. In general, with passing decades and centuries, humanity seeks to be smarter
about the route of sewage on its way back to the environment, in order to reduce environmental
degradation and achieve sustainability. Thus other goals of modern sewage routing include
handling surface runoff separately from sewage, handling greywater separately from toilet waste,
and coping better with abnormal events (such as peaks in use from internal displacement and
peaks in stormwater volumes from extreme weather).
The term sewage is nowadays regarded as an older term and is being more and more
replaced by "wastewater". In general American English usage, the terms "sewage" and
"sewerage" mean the same thing. Both words are descended from Old French assewer, derived
from the Latin exaquare, "to drain out (water)". In American technical and professional English
usage, "sewerage" refers to the infrastructure that conveys sewage.[5]
Classes of sewage include sanitary, commercial, industrial, agricultural and surface
runoff.
The wastewater from
residences
and
institutions,
carrying
body
wastes
(primarily feces and urine), washing water, food preparation wastes, laundry wastes, and other
waste products of normal living, are classed as domestic or sanitary sewage. Liquid-carried
wastes from stores and service establishments serving the immediate community, termed
commercial wastes, are included in the sanitary or domestic sewage category if their
characteristics are similar to household flows. Wastes that result from an industrial processes
such as the production or manufacture of goods are classed as industrial wastewater. Their flows
and
strengths
are
usually
more
varied,
intense,
and
concentrated
than
those
of sanitary sewage. Surface runoff, also known as storm flow or overland flow, is that portion
of precipitation that runs rapidly over the ground surface to a defined channel. Precipitation
absorbs
gases
and
particulates
from
the atmosphere,
72
dissolves
and leaches materials
from vegetation and soil, suspends matter from the land, washes spills and debris from urban
streets and highways, and carries all these pollutants as wastes in its flow to a collection point.
Sewage services
Disease potential
All categories of sewage are likely to carry pathogenic organisms that can transmit
disease to humans and other animals; contain organic matter that can cause odor and nuisance
problems; hold nutrients that may cause eutrophication of receiving water bodies; and can lead
to ecotoxicity. Proper collection and safe, nuisance-free disposal of the liquid wastes of a
community are legally recognized as a necessity in an urbanized, industrialized society. [6] The
reality is, however, that around 90% of wastewater produced globally remains untreated causing
widespread water pollution, especially in low-income countries. Fecal matter can potentially
cause disease.
Increasingly, agriculture is using untreated wastewater for irrigation. Cities provide
lucrative markets for fresh produce, so are attractive to farmers. However, because agriculture
has to compete for increasingly scarce water resources with industry and municipal users, there
is often no alternative for farmers but to use water polluted with urban waste, including sewage,
directly to water their crops. There can be significant health hazards related to using water loaded
with pathogens in this way, especially if people eat raw vegetables that have been irrigated with
the polluted water.
The International Water Management Institute has worked in India, Pakistan, Vietnam,
Ghana, Ethiopia, Mexico and other countries on various projects aimed at assessing and reducing
risks of wastewater irrigation. They advocate a ‘multiple-barrier’ approach to wastewater use,
where farmers are encouraged to adopt various risk-reducing behaviours. These include ceasing
irrigation a few days before harvesting to allow pathogens to die off in the sunlight, applying
water carefully so it does not contaminate leaves likely to be eaten raw, cleaning vegetables with
disinfectant or allowing fecal sludge used in farming to dry before being used as a human
manure. The World Health Organization has developed guidelines for safe water use.
73
Collection and disposal
A system of sewer pipes (sewers) collects sewage and takes it for treatment or disposal.
The system of sewers is called sewerage or sewerage system (see London sewerage system) in
British English and sewage system in American English. Where a main sewerage system has not
been provided, sewage may be collected from homes by pipes into septic tanks or cesspits, where
it may be treated or collected invehicles and taken for treatment or disposal. Properly functioning
septic tanks require emptying every 2–5 years depending on the load of the system.
Sewage and wastewater is also disposed of to rivers, streams, and the sea in many parts of
the world. Doing so can lead to serious pollution of the receiving water. This is common in third
world countries and may still occur in some developed countries, where septic tank systems are
too expensive.
Treatment
Sewage treatment is the process of removing the contaminants from sewage to produce
liquid and solid (sludge) suitable for discharge to the environment or for reuse. It is a form
of waste management. A septic tank or other on-site wastewater treatment system such
as biofilters can be used to treat sewage close to where it is created.
Sewage water is a complex mixture of chemicals, with many distinctive chemical
characteristics. These include high concentrations of ammonium, nitrate, phosphorus, high
conductivity (due to high dissolved solids), high alkalinity, with pH typically ranging between 7
and 8. Trihalomethanes are also likely to be present as a result of past disinfection.
In developed countries sewage collection and treatment is typically subject to local, state and
federal regulations and standards.
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Industrial waste
Industrial waste is the waste produced by industrial activity which includes any material
that is rendered useless during a manufacturing process such as that of factories, mills, and
mining operations. It has existed since the start of the Industrial Revolution.[1] Some examples of
industrial waste are chemical solvents, paints, sandpaper, paper products, industrial by-products,
metals, and radioactive wastes.
Toxic waste, chemical waste, industrial solid waste and municipal solid waste are
designations of industrial waste. Sewage treatment plants can treat some industrial wastes, i.e.
those consisting of conventional pollutants such as biochemical oxygen demand (BOD).
Industrial
wastes
containing toxic pollutants
require
specialized
treatment
systems.
(See Industrial wastewater treatment).
Management
In Thailand the roles in Municipal solid waste (MSW) management and industrial waste
management are organized by the Royal Thai Government which is then divided into central
government, regional government, and local government. Each government is responsible for
different tasks. The central government is responsible to stimulate regulation, policies, and
standards. The regional governments are responsible for coordinating the central and local
governments. The local governments are responsible for waste management in their governed
area. However, the local governments do not dispose of the waste by themselves but instead hire
private companies that have been granted the right from the Pollution Control Department (PCD)
in Thailand. The main companies are Bangpoo Industrial Waste Management Center, General
Environmental Conservation Public Company Limited (GENCO), SGS Thailand, Waste
Management Siam LTD (WMS), and Better World Green Public Company Limited
(BGW). These companies are responsible for the waste they have received from their customers
before releasing it to the environment, burying it, or using it for energy.
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Social forestry in India
Social forestry means the management and protection of forests and afforestation on
barren lands with the purpose of helping in the environmental, social and rural development. The
term, social forestry, was first used in India in 1976 by The National Commission on
Agriculture, Government of India. It was then that India embarked upon a social forestry project
with the aim of taking the pressure off currently existing forests by planting trees on all unused
and fallow land.
Social forestry programme
Government is trying to increase forest areas that are close to human settlement and have
been degraded over the years due to human activities needed to be afforested. Trees were to be
planted in and around agricultural fields. Plantation of trees along railway lines and roadsides,
and river and canal banks were carried out. They were planted in village common land,
government wasteland, and Panchayat land.
Involvement of common people
Social forestry also aims at raising plantations by the common man so as to meet the
growing demand for timber, fuel wood,fodder, etc., thereby reducing the pressure on the
traditional forest area. This concept of village forests to meet the needs of the rural people is not
new. It has existed through the centuries all over the country but it was now given a new
character.
With the introduction of this scheme the government formally recognised the local
communities’ rights to forest resources, and is now encouraging rural participation in
the management of natural resources. Through the social forestry scheme, the government has
involved community participation, as part of a drive towards afforestation, and rehabilitating the
degraded forest and common lands.
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This is precisely what a movement has been started called Plant a Tree Challenge ( [1]).
Invoking a sense among people to come and participate as this movement and take every
occasion to Plant a Tree as this is our own responsibility.
Need of social forestry
This need for a social forestry scheme was felt as India has a dominant rural population
that still depends largely on fuelwood and other biomass for their cooking and heating. This
demand for fuel wood will not come down but the area under forest will reduce further due to the
growing population and increasing human activities. Yet the government managed the projects
for five years then gave them over to the village panchayats (village council) to manage for
themselves and generate products or revenue as they saw fit.
Types
Social forestry scheme can be categorized into groups; farm forestry, community
forestry, extension forestry andagroforestry.
Farm forestry
At present in almost all the countries where social forestry programmes have been taken
up, both commercial and non commercial farm forestry is being promoted in one form or the
other. Individual farmers are being encouraged to plant trees on their own farmland to meet the
domestic needs of the family. In many areas this tradition of growing trees on the farmland
already exists. Non-commercial farm forestry is the main thrust of most of the social forestry
projects in the country today. It is not always necessary that the farmer grows trees for fuelwood,
but very often they are interested in growing trees without any economic motive. They may want
it to provide shade for the agricultural crops; as wind shelters; soil conservation or to use
wasteland. Farm Forestry is another name for Agroforestry; a part of Social Forestry.
Due to huge requirement of pulpwood for production virgin celluolosic fibre based paper,
Pulp & Paper Industry have become a major demand driver for particular species of tree like
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Eucalyptus Eucalyptus,
Acasia Acacia,
Subabul Leucaena
leucocephala and
Casaurina Casuarina. As a rough estimate, total demand for pulp wood is approximately 10
million ADMT (i.e wood having 10% moisture). Indian Paper Manufacturer's Association [1] is
an umbrela organisation of Indian Pulp and Paper Industry which co-ordinates and drives
plantation efforts by member organisations in India.
Community forestry
Another scheme taken up under the social forestry programme, is the raising of trees on
community land and not on private land as in farm forestry. All these programmes aim to
provide for the entire community and not for any individual. The government has the
responsibility of providing seedlings, fertilizer but the community has to take responsibility of
protecting the trees. Some communities manage the plantations sensibly and in a sustainable
manner so that the village continues to benefit. Some others took advantage and sold the timber
for a short-term individual profit. Common land being everyone’s land is very easy to exploit.
Over the last 19 years, large-scale planting of Eucalyptus, as a fast-growing exotic, has occurred
in India, making it a part of the drive to reforest the subcontinent, and create an adequate supply
of timber for rural communities upon the augur of ‘social forestry’.
Extension forestry
Planting of trees on the sides of roads, canals and railways, along with planting on
wastelands is known as ‘extension’ forestry, increasing the boundaries of forests. Under this
project there has been creation of wood lots in the village common lands, government wastelands
and Panchayat lands. Schemes for afforesting the degraded government forests that are close to
villages are being carried out all over the country.
Agroforestry
In agroforestry, silvicultural practices are combined with agricultural crops like
leguminous crop, along with orchard farming and live stock ranching on the same piece of land.
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In lay man language agroforestry could be understood as growing of forest tree along with
agriculture crop on the same piece of land. In a more scientific way agroforestry may be defined
as a sustainable land use system that maintains or increases the total yield by combing food crop
together with forest tree and live stock ranching on the same unit of land, using management
practices that takes care of the social and culture characteristic of the local people and the
economic and ecological condition of the local area.
Due to huge requirement of pulpwood for production virgin celluolosic fibre based paper,
Pulp & Paper Industry have become a major demand driver for particular species of tree like
Eucalyptus eucalyptus, Acasia acacia, Subabul leucaena,
leuco cephala and Casaurina
casuarina. As a rough estimate, total demand for pulp wood is approximately 10 million ADMT
(i.e wood having 10% moisture). Indian Paper Manufacturer's Association is an umbrela
organisation of Indian Pulp and Paper Industry which co-ordinates and drives plantation efforts
by member organisations in India
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UNIT – IV
Mechanism of Evolution
Biological evolution is not simply a matter of change over time. The central idea of
biological evolution is that all life on earth shares a common ancestor. Through the process of
descent with modification, the common ancestor of life on earth gave rise different diversity of
life. Evolution means that we're all distant cousins: humans and oak trees, hummingbirds and
whales.
Mechanisms:
Evolution is the process by which modern organisms have descended from ancient
ancestors. Evolution is responsible for both the remarkable similarities across all the life and
diversity of that life. Fundamental to the process is genetic variation upon which selective forces
can act in order for evolution to occur.
Descent with modification
Descent and the genetic differences that is heritable and passed on to the next generation.
We've defined evolution as descent with modification from a common ancestor, but exactly what
has been modified? Evolution only occurs when there is a change in gene frequency within a
population over time. These genetic differences are heritable and can be passed on to the next
generation.
Mechanisms of change
Natural selection, genetic drift, Mutation, and gene flow selection are considered as
mechanisms of change. Each of these four processes is a basic mechanism of evolutionary
change.
1) Natural Selection
Natural selection leads to an evolutionary change when some individuals with certain
traits in a population have a higher survival and reproductive rate than others and pass on these
inheritable genetic features to their offspring. Evolution acts through natural selection whereby
reproductive and genetic qualities that prove advantageous to survival prevail into future
generations. The cumulative effects of natural selection process have giving rise to populations
that have evolved to succeed in specific environments. Natural selection operates by differential
reproductive success (fitness) of individuals.
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2) Genetic Drift
Random Drift consists of random fluctuations in the frequency of appearance of a gene,
usually, in a small population. The process may cause gene variants to disappear completely,
thereby reducing genetic variability. In contrast to natural selection, environmental or adaptive
pressures do not drive changes due to genetic drift. The effect of genetic drift is larger in small
populations and smaller in large populations.
CrapsGenetic drift is a stochastic process, a random event that happens by chance in
nature that influences or changes allele frequency within a population as a result of sampling
error from generation to generation. It may happen that some alleles are completely lost within a
generation due to genetic drift, even if they are beneficial traits that conduct to evolutionary and
reproductive success. Allele is defined as any one of two or more genes that may occur
alternatively at a given site (locus) on a chromosome. Alleles are responsible for variations in a
trait.
The population bottleneck and a founder effect are two examples of random drift that can have
significant effects in small populations. Genetic drift works on all mutations and can eventually
contribute to the creation of a new species by means of the accumulation of non-adaptive
mutations that can facilitate population subdivision.
3) Mutation
Mutation can be defined as a change in the DNA sequence within a gene or chromosome
of a living organism. Many mutations are neutral, i.e. they can neither harm nor benefit, but can
also be deleterious or beneficial. Deleterious mutations can affect the phenotype and in turn,
reduce the fitness of an organism and increase the susceptibility to several illnesses and
disorders. On the other hand, beneficial mutations can lead to the reproductive success and
adaptability of an organism to its environment. These beneficial mutations can be spread and
fixed in the population due to natural selection processes if they help individuals in the
population to reach sexual maturity and to successfully reproduce. Mutations are, undoubtedly, a
source of genetic variation and serve as a raw material for evolution to act. Germ line mutations
occur in gametes (eggs or sperm cells) and can be pass on to offspring, whereas somatic
mutations occur in non-reproductive cells and are not pass on to the following generation. Those
mutations that occur in germ line are the most important to large-scale evolution because they
can be transmitted to offspring.
Mutations can be spontaneous (errors during a normal process of DNA replication,
spontaneous lesions and transposable genetic elements), but they can also be induced by
numerous external or exogenous factors like environmental chemical agents or ionizing
radiation, for example. According to their magnitude (mutations can occur at different levels),
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they can be divided into three different groups: Gene mutations, chromosome mutations and
genome mutations. The DNA is constantly subject to mutations, thus its sequence can be altered
in several different ways. A gene mutation can be defined as any change in the sequence of
nucleotides of the genetic material of an organism. A chromosome mutation is a change in the
structure or arrangement of the chromosomes. These mutations can involve duplications or
deletions of chromosome segments, inversions of sections of DNA (reversed positions) and
translocation. Genome mutations are alterations in the number of chromosomes in the genome.
They can be classified into two groups: Aneuploidy and Euploidy. Aneuploidy is the losses
and/or gains of individual chromosomes from the normal chromosome set arising from errors in
chromosome segregation, and Euploidy refers to variations in complete sets of chromosomes.
4) Gene Flow
In population genetics, Gene Flow (also known as gene migration) refers to the transfer
of genes from the gene pool of one population to another. Gene flow may change the frequency
and/or the range of alleles in the populations due to the migration of individuals or gametes that
can reproduce in a different population. The introduction of new alleles increases variability
within a population and allows for new combinations of traits. Horizontal gene transfer (HGT)
also known as lateral gene transfer (LGT), is a process in which an organism (recipient) acquires
genetic material from another one (donor) by asexual means. It is already known that HGT has
played a major role in the evolution of many organisms like bacteria. In plant populations, the
great majority of cases linked to this mechanism have to do with the movement of DNA between
mitochondrial genomes. Horizontal gene transfer is a widespread phenomenon in prokaryotes,
but the prevalence and implications of this mechanism in the evolution of multicellular
eukaryotes is still unclear. Nevertheless, many investigations on HGT in plants have been carried
out during the last years trying to reveal the underlying patterns, magnitude and importance of
this mechanism in plant populations as well as its influence on agriculture and the
ecosystem.Plant populations can experience gene flow by spreading their pollen long distances
away to other populations by means of wind or through birds or insects (bees, for example) and
once there, this pollen is able to fertilize the plants where it ended up. Pollen is a fine to coarse
powder containing the microgametophytes of seed plants, which produce the male gametes
(comparable to sperm cells). Of course, pollination does not always lead to fertilization.
Maintained gene flow also acts against speciation by recombining the gene pools of
different populations and in such a way, repairing the developing differences in genetic variation.
Thus, gene flow has the effect of minimizing the genetic differences between populations.
Human migrations have occurred throughout the history of mankind and are defined as
the movement of people from one place to another. However, in a genetic context, this
movement needs to be associated with the introduction of new alleles into a population through
successful mating of individuals from different populations.
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Lamarckism
(Theory of Inheritance of Acquired Characters)
Lamarckism is the first theory of evolution. This theory was proposed by Jean Baptiste de
Lamarck (1744-1829), a French Biologist. Although the outline of the theory was brought to
notice in 1801, but his famous book “Philo¬sophic Zoologies” was published in 1809, in which
he discussed his theory in detail.
Lamarckism includes four main propositions. They are
1) Internal Vital Force:
All the living things and their component parts are continually increased due to internal vital
force.
2) Environment and New Needs:
Environment influences all types of organisms. A change in environment brings about changes in
organisms. It gives rise to new needs. The new needs or desires lead to change in structure
results in production of new structures and also change in the habits of the organisms.
3) Use and Disuse of Organs:
The new habits involve the greater use of certain organs to meet new needs, and the disuse or
lesser use of certain other organs which are of no use in new conditions. This use and disuse of
organs greatly affect the form, structure and functioning of the organs.
Continuous and extra use of organs makes them more efficient while the continued disuse of
some other organs leads to their degeneration and ultimate disappearance. Hence the organism
acquires certain new characters due to direct or indirect environmental effects during its own life
span. The newly developed characters are called as acquired or adaptive characters.
4) Inheritance of Acquired Characters:
Whatever an individual acquires a character in its life time due to internal urge, effect of
environment, new needs and use and disuse of organs, the acquired characters inherited
(transmitted) to the next generations. This is a continuous process and after several generations,
the variations are accumulated and they give rise to new species.
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Lamarckism was supported by several examples. Lamarck explained his theory by giving the
following examples.
1) Evolution of Giraffe:
The ancestors of giraffe had a small neck and forelimbs and were looked like horses. But as they
were living in places where vegetation on the surface, they had to stretch their neck and forelimbs to feed on the leaves. This resulted in elongation of the neck. Whatever they acquired in
one generation was transmitted to the next generation with the result that a race of long necked
and long fore-limbed animals was developed.
Stages in the evolution of present day giraffe
2) Webbed Toes of Aquatic Birds:
Aquatic birds like ducks have been evolved from the terrestrial ancestors.
3) Disappearance of Limbs in Snakes:
The snakes have been evolved from lizard like ancestors which were having two pairs of limbs.
4) Flat Fishes:
They are flat and bear both the eyes on one side and live at the bottom of the water.
During the embryonic stage their eyes are present laterally, one eye on either side. The body of
these fishes is not flat at this stage but later on both the eyes is shifted to one side and the body
becomes flat to withstand the pressure of water.
5) Flightless Birds:
The ancestors of flightless birds were capable of flying, but due to some environmental factors
such as absence of predators, plenty of food and well protected habitat, they did not use their
wings. This leads to vestigial wing then they lost the flying habit.
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6) Cave Dwellers:
The ancestors of cave dwellers had normal eye sight. On account of living under continuous dark
conditions, the animal lost their power to see.
Criticism of Lamarckism:
Lamarck’s third principle, inheritance of acquired characters was highly criticised. But
the remaining principles such as use and disuse of organs, influence of the environment were
accepted by many scientists.
The first proposition of the theory does not have any ground because there is no vital force in
organisms which increases their body parts. As regards the second proposition, the environment
can affect the animal but it is doubtful that a new need forms new structures. The third
proposition, the use and disuse of the organs is correct up to some extent. The fourth proposition
regarding the inheritance of acquired characters is disputed.
Mendel’s Laws of Inheritance and Weismann’s Theory of Continuity of Germplasm (1892)
discarded Lamarck’s concept of inheritance of acquired characters.
1) Theory of Continuity of Germplasm. August Weismann (1834-1914), a German biologist, was
the main opposer of the inheritance-of acquired characters. He put forward the theory of
continuity of germplasm. According to Weismann, the characters influencing the germ cells are
only inherited. There is a continuity of germplasm (protoplasm of germ cells) but the somatoplasm (protoplasm of somatic cells) is not transmitted to the next generation hence it does not
carry characters to next generation. Weismann cut off the tails of rats for as many as 22
generations and allowed them to breed, but tailless rats were never born.
2) Boring of external ear and nose of Indian women is never inherited to the next generations.
3) The wrestler’s powerful muscles are not transmitted to the offspring.
4) European ladies wear tight waist garments in order to keep their waist slender but their
offspring at the time of birth have normal waists.
5) Chinese women used to wear iron shoes in order to have small feet, but their children at the
time of birth have always normal feet.
6) Circumcision of penis is in Jews and Muslims but it is not inherited to the next generation.
Evidences in Favour of the Inheritance of Acquired Characters:
1) Formation of Germ Cells from Somatic Cells:
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In certain cases somatic cells can produce the germ cells, which is against Weismann’s theory of
continuity of germ-plasm. This occurs in vegetative propagation in plants and regeneration in
animals.
2) Effect of Environment directly on Germ Cells:
Tower exposed the young developing Potato Beetles to extremes of temperature and humidity at
the time of the development of their reproductive organs. This did not produce any change in the
beetles themselves. Their offspring, however, had colour variations, which were passed on to the
succeeding generations. Tower’s observations indicate direct effect of environment on germ
cells.
3) Effect of Radiation:
Exposure of organisms to high energy radiations (ultra-violet rays, X-rays, gamma rays, etc.) or
feeding them with mutagenic chemicals, produces sudden inheritable variations or mutations.
For example, Auerbach et al obtained a number of mutations and chromosome aberrations in
Drosophila with the help of mustard gas.
4) Agar:
Agar reared water fleas in a culture of green flagellates and found that some abnormalities were
developed in their structures. The parthenogenetic eggs of such individuals when kept in
ordinary water and allowed to hatch produced individuals with the same abnormalities.
5) Effect of Chemicals:
There is no isolation of somatic and germ cells. Rather one part of the body affects other parts of
the body through chemicals called hormones. Change in the secretion of hormones results in the
change of different parts of the body.
6) Guyar and Smith:
Guyar and Smith took the solution of the eye lens of rabbit and inoculated the same into fowl.
The fowl’s serum containing antibodies was injected into pregnant rabbits. Some of the offspring
were found to have malformed or degenerate eyes.
Neo-Lamarckism:
Modified form of Lamarckism is called Neo-Lamarckism (neo = new). Neo-Lamarckism.
It states that 1) environment does influence an organism and change its heredity, 2) At least some
of the variations acquired by an individual can be passed on to the offspring 3) Internal vital
force and appetency do not play any role in evolution and 4) only those variations are passed on
to the offspring which also affect germ cells or where somatic cells give rise to germ cells.
Evidences in favour of the inheritance of acquired characters support the Neo-Lamarckism.
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Darwinism
(The theory of natural selection)
Darwinism is a theory of biological evolution developed by Charles Robert Darwin and others,
stating that all species of organisms arise and develop through the natural selection of small,
inherited variations that increase the individual's ability to compete, survive, and reproduce.
Charles Darwin (1809-1882) was a great English naturalist. Darwin was born in
Shrewsbury, England, on February 12, 1809. In 1831, He made wide travels for five years in a
ship called H.M.S Beagle and observed a large number of animals while he was visiting
Galapagos Islands. There he observed a lot of variation among the plants and animals, especially
in a group of birds called finches. They were later called as Darwin’s finches. The idea of
evolution was struck in the mind of Darwin after his visit to this island. Moreover his idea of
evolution was influenced by his study on Malthus’s ‘Essay on Population’. He deeply studied
various aspects of biology and fossils for about twenty years. All his observation and studies
helped him to reveals a number of facts and make deduction there from for the idea of evolution.
Darwin published his views in 1859 under the title “On the Origin of Species by means of
Natural Selection or the Preservation of Favoured Races in the Struggle for life”. In short it is
called “Origin of Species” or “Theory of Natural Selection”. Since Darwin predicates cretin facts
and inference about the mechanism of evolution, they are collectively called as “Darwinism”.
Darwin’s theory of natural selection was based on three sets of facts and two deductions there
from.
Charles Robert Darwin
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Rapid multiplication: The first fact which Darwin had observed that all animal and plants tent to
reproduce at a very rapid rate. So the offspring were more in number than the parent. Thus
prodigality of production (over production) was seen in nature. It can be explained with a few
examples.
1) Paramecium is a minute and microscopic creature. It reproduces by binary fission and
conjugation. If paramecium is allowed to reproduce, in five years there were thousands of
generation. If all of them were alive they would possess a volume of the earth.
2) An oyster may lay as many as 114 lakhs of eggs at a single spawning. If all these eggs are
hatched and if all the young ones reproduce for 5 generation, they will form a volume of about
eight times the size of the earth.
4) A single salmon produces 280 lakhs of eggs in a season. If all of them are alive and their
young ones also produce at the same rate without any mortality or loss, the aquatic environment
will be occupied by these fishes.
5) All the animals mentioned above reproduce at a prolific rate. The same trend can be seen in
the moderately breeding animals also. For example, a frog lays many hundred of eggs in each
season. If the entire frog in the world were taken in to account and if all the eggs laid by them
metamorphosed and if this process is repeated without death, what will happen to the earth?
6) The slowest breeder in the world today is the elephant. It begins to reproduce at its 40 th year
and almost stop at its 90th year. Thus it will produce about 6 young ones in its life time. It is
calculate that after 750 years there would be 19 million of elephants alive, descended from a pair,
thus the animals tend to increase in a geometric ratio.
Struggle for existence: the second observation made by Darwin was about the check in the rise of
population. This was made possible by the struggle for existence. Since space, food, mate, and
others factors are much limited, they not be enough for such huge members of animals and so
there is competition to get these requirement. While doing so, the population level is checked
against enormous rise. There are three types of struggle.
i) Intra specific struggle: It is a completion between the members of the same species. Since all
the members are living in the same environment, their need and requirements are same and so
there will be heavy completion. Eg. Some of aquarium fishes eat their own young ones.
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ii) Inter specific struggle: It is a completion between the members of different species. Lion
feeding on the sheep, birds eating on the insects are some of the example for inter specific
struggle.
iii) Environmental struggle: It is a competition between the animal and environment. Animals
have to fight against certain environmental factors such as excess of heat and cold, lightning,
earthquakes, floods, volcanoes, diseases, accidents etc. Such natural calamities result in the death
of a large number of animals.
Due to these three types of struggle for existence an effective check is there on the undue growth
and multiplication of animals.
Variation: the third observation which Darwin made was the presence of variation in animals.
Variation are noting but difference in characters between individuals. Darwin could see that one
animal was different from the other. According to Darwin, these variations were as s result of
struggle for existence.
Darwin said that those animals with favorable variation have a better chance of survival than the
organisms with unfavorable variation. Animal with unfavorable variation would be eliminated.
For example, if there is a fight between a lion and a lamb, there is no doubt that the lion wins the
flight since it has favorable variation like vigorous strength, powerful claws and teeth etc. The
lamp has unfavorable variations and so it is eliminated from the struggle. Thus Darwin
recognized the two type of variations unfavorable or favourable variation.
Darwin insisted upon the minute fluctuation in variation. Since He believed that even these small
variations would be preserved. Though they are fluctuating in nature, species would depart
further and further from its original and slowly form them stronger generation after generation.
Thus Darwin believed that all variations were heritable.
Survival of the fittest and natural selection:
Darwin observed that only those animals which have favourable variation or fittest adaptations
will survive in the struggle for life. In other words, Nature will select only those animals to
survive which have fittest adaptations and other which will have unfavorable variation will be
eliminated. Thus this automatic selection of animals with fittest adaptations by nature was called
Natural Selection by Darwin. In natural selection, nature is the selecting agent. It works silently
and insensibly whenever and wherever opportunity is there. Nature never commits a mistake.
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Natural selection is a continuous process of trails and hourly. The process of this selection cannot
be seen since it takes longer ages.
Origin of species:
According to Darwin, new species will arise when all the above principles work together. Rapid
multiplication will lead to the struggle for existence. In their struggle, animals will develop
variations. Animals with fittest variation will be selected by nature and they will be surviving.
The fittest character will be transmitted to the next generation. Thus in each succeeding
generations better and better characters are transmitted and thus in one of the generations the
animal will have entirely new characters, differing from the original ancestor. Animals with new
characters will from a new group and they are included under species. Thus new species arise by
gradual accumulation of fittest variation for number of generation. Thus different animal forms
came into existence, said Darwin. Thus Darwin explained the origin of new species and it was a
plausible explanation for the mechanism of evolution.
Objection to Darwinism:
Several objections had been raised against Darwinism. Some of them are:
Darwinism speakers about useful variations only and it does not explain the occurrence of
vestigial and useless organs.
There is no explanation about the power and phenomenon of mimicry.
There is no information or explanation about the power of regeneration of the lost parts.
Presence of over-specialized structures such as the huge horns of extinct Irish deer and the spiral
tusks of mammoths had no explanations.
This theory cannot explain the development of hybrid sterility.
This theory did not distinguish between heritable and non-heritable variation. Darwin considered
the all variation is heritable.
There is no information about the origin of variations. But it explained only the survival of the
fittest and not the arrival of the fittest.
It did not also explain how variations are passed on from one generation to the other.
In addition to Natural Selection Theory Darwin proposed three more theories to explain certain
facts which he could not explain through his Natural Selection theory.
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1. Artificial selection theory: Darwin proposed this theory to understand the principles of his
Natural Selection Theory. The process of artificial selection theory is more or less similar to that
of Natural Selection Theory. In Natural Selection Theory, nature is the selecting agent while in
this artificial selection theory man is the selecting agent. As nature selects only those animals
with favorable character, man selects only those plants and animals with desirable characters
according to his wishes and fancies. For example, man will select only those plants which yield
more fruits, those cows which give more milk, those dogs which are healthier. In this process
man may go wrong but nature will never fail in its action. Man’s action is unstable and there is a
possibility of reverting to the original type but these never occur when nature operates. In Man’s
action there is conscious selection of certain individual desired characters but in nature it never
happens so!
Sexual selection theory:
Darwin proposed this theory to explain the presence of secondary sexual characters in animals.
Secondary sex characteristics are features that appear at sexual maturity in animals. They are
sexually dimorphic phenotypic traits that distinguish the two sexes of a species (male and
female). Secondary sexual characters help in attracting the opposite sex and thus help
reproduction. They are the product of sexual selection for traits which display fitness, giving an
individual an advantage over its rivals in courtship and aggressive interactions.
Well-known secondary sex characteristics include, brightly colour red feathers in birds, mane of
the male lion, antlers of the deer, vocal sacs and nuptial pad of male frog, sound and light
producing organs of insects, long feathers of male peacock, the bright facial and rump coloration
of male mandrills, and horns in many goats and antelopes. Male birds and fish of many species
have brighter coloration or other external ornaments. Differences in size between sexes are also
considered secondary sexual characteristics. Due to the presence of secondary sexual characters
in the males and the females, they may look differently.
Now doubt arises whether the males will be selected by nature or females. By putting forth the
principle of Natural Selection, it cannot be explained because natural selection involves survival
value which must be the same for both sexes. Hence Darwin’s proposed this theory to give a
possible explanation to these secondary sexual characters.
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In natural selection, nature is the selecting agent whereas in sexual selection, the female is the
selecting agent. Nature will select only those animals have favorable characters. When female
selects, it will select those male which have more attractive features. As the females prepare
males which have more attractive characters, then in the course of time the males will become
more and more attractive with better developed secondary sexual characters. In this case the
fighting will be among the males which can be considered as the intra specific struggle. This
fight is not for food or shelter but for a chance to mate! Males with unfavorable secondary sexual
characters are automatically rejected by the female. These disadvantaged males are discarded by
nature.
The factors employed in the sexual selection theory are:
1. Many secondary sexual characters are not explicable by natural selection. They are not useful
in the struggle for life.
2. The male seeks the female for the sake of paring.
3. The males are more in the number than the females.
4. In many cases there is a struggle among the males for the possession of the females.
5. Many males sing or dance or produce light or otherwise draw to themselves the attention of
the females.
Objection to sexual selection theory
As in the case of natural selection theory, sexual selection theory was also objected by many.
1. Sexual selection theory could not be applied to all animals, since secondary sexual characters
are not found in all animals.
2. According to Darwin, males are outnumbered than the female. But according to the modern
observation male and female are in equal number. So normally all males can succeed in finding a
female.
3. In nature, it is generally observed that there is no such selection exercised by the female.
Though there is such a courtship display among the animals and this excites the females and puts
her in a mood to mate. There is hardly any selection. Fur seal offers one of the best examples to
explain this. The males arrive at breeding ground before the females.
Then they engage
themselves in a fierce fight. During this fight males ate either killed or driven away from the
breeding grounds. When the females arrive there later on, the males easily get the females. The
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males have powerful tusk which are considered as secondary sexual characters. The females are
not attracted by this character but they simply accept the males which approach them.
Thus according to a few biologists, sexual selection plays no role in evolution but a few attach a
really important role.
Theory of Pangenesis:
Darwin thought that his explanation to the theory of evolution would be incomplete if he did not
explain the inheritance of characters. So he tried to explain the process of heredity in his own
way through a theory, called pangenesis. Pangenesis was Charles Darwin's hypothetical
mechanism for heredity. According to this theory, all organs, tissues and each and every call
produce minute particles called “gemmules”. These gemmules circulate throughout the body and
family assembles in germ cells and each gemmule is capable of giving rise to new individual.
These individual would be miniature replica of the parent body and they would development just
like their parent. Darwin further believed that certain gemmules remained dormant for several
generations and then develops. Thus in an individual its ancestral characters are sometime seen.
Though Darwin had some idea about heredity, it was unscientific and so this theory was not at all
accepted today.
Mutation Theory of Evolution by Hugo De Vries
Hugo de Vries (1848—1935), a Dutch botanist. He put forward his views regarding the
formation of new species in 1901. He also met some of the objections found in Darwin’s theory.
According to De vries, new species are not formed by continuous variations but it is through
sudden appearance of variations. He named this phenomenon as as mutations. Hugo de Vries
stated that mutations are heritable and transmitted from one generation to next generation.
Experiments Conducted by Hugo de Vries:
Drvries conducted his experiments on Oenothera lamarckiana, (Evening Primrose) and found
several aberrant types. When O. Lamarckian was self-pollinated and its seeds were allowed to
grow, majority of F1 plants were similar to the parents, but a few were different plants.
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The different plants were also self-pollinated and when their seeds were sown, the majority of
the plants were similar to the parents while a few were still more different plants and this
continued generation after generation. These plants appeared to be new species, Hugo de Vries
suggested from his experiments that new types of inherited characteristics may appear suddenly
without any previous indication of their presence in the race.
Hugo de Vries believed that mutation causes evolution and not the minor heritable varia-tions
which was mentioned by Darwin. Mutations are random and directionless while Darwin’s
variations are small and directional. According to Darwin evolution is gradual while Hugo de
Vries believed that mutation caused species formation and hence known as saltation (single step
large mutation).
Salient Features of the Mutation Theory:
On the basis of above observations, Hugo de Vries (1901) put forward a theory of evolution,
called mutation theory. The theory states that evolution is a jerky process where new varieties
and species are formed by mutations (discontinuous variations) that function as raw material of
evolution. The salient features of mutation theory are:
1. Mutations or discontinuous variations are the raw material of evolution.
2. Mutations appear all of a sudden and become operational immediately.
3. Unlike Darwin’s continuous variations or fluctuations, mutations do not revolve around the
mean or normal character of the species.
4. The same type of mutations can appear in a number of individuals of a species.
5. All mutations are inheritable and appear in all conceivable directions.
6. Useful mutations are selected by nature. Lethal mutations are eliminated.
7. Accumulation of variations produce new species. Sometimes a new species is produced from a
single mutation.
Points in Favour of the Mutation Theory:
(1) Mutations are actually the source of all variations and hence fountain head of evolution.
(2) Mutation theory can explain both progressive and retrogressive evolution.
(3) As the ratio of mutations is not the same in all indi-viduals and their parts, mutation theory
can explain the occurrence of both changed and unchanged forms.
(4) A number of mutations have appeared in the past.
Mutations are also induced.
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They have given rise to new varieties.
(a) Ancon Sheep (Fig. 7.49) is a short legged variety which appeared suddenly in Massachusetts
in 1791.
(b) Hornless Cattle developed as mutation from the homed cattle in 1889.
(c) A single mu-tation can give rise to a new variety and even species of plants, e.g., Delicious
Apple, Cicer gigas, Noval Orange, Red Sunflower,
(d) Hairless cats and double-toed cats have developed through mutations.
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Points against the Mutation Theory (Criticism of the Mutation Theory):
(1) Oenothera lamarckiana of Hugo de Vries was not a normal plant but a complex heterozygous
form with chromosome aberrations.
(2) Natural mutations are not common
as Hugo de Vries thought them to occur.
(3) Most of the mutations are negative or retro-gressive.
(4) Mutations are generally recessive while traits taking part in evolution are usually dominant.
(5) Mutation theory cannot satisfactorily explain the development of mimicry, mutual
dependence of flowers and pollinating insects.
(6) This theory does not explain the role of nature.
Significance of Hugo de Vries’ Mutation Theory:
This Theory gives direct attention to the mutations. But later on it was thought that evolution
cannot occur by mutations alone. Natural selection and isolation of mutants were also essential
for evolution.
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Modern Synthetic Theory of Evolution
Modern synthetic theory of evolution is as a result of the concept of various scientists namely.
The modern syntheric theory is proposed by T. Dobzhansky, R.A. Fisher, J.B.S. Haldane, Swall
Wright, Ernst Mayr, and G.L. Stebbins. This theory was discussed in Stebbins book of evolution
“Process of Organic Evolution”.
Modern syntheric theory of evolution includes the following key factors:
1) Gene mutations
2) Variations
3) Heredity
4) Natural selection and
5) Isolation.
In addition to the above key factors number of accessory factors influences the basic factors.
Migration of individuals from one population to another as well as hybridization between races
or closely related species both increase the amount of genetic variability available to a population.
1. Mutation
An alteration in the chemistry of gene (nucleotide level) is able to change its phenotype. This
process is known as point mutation or gene mutation. Mutation can produce drastic changes
which may be deleterious or harmful and lethal or can remain insignificant. There are equal
chances of a gene to mutate back to normal (reverse mutation). Since most of the gene mutations
are recessive in nature and these are able to express phenotypically only when they are in
homozygous condition. Thus, gene mutation tends to produce variations in the offspring.
2. Variation or Recombination
Recombination that is, new genotypes from already existing genesis of several types: 1) the
production of gene combinations containing the same individual two different alleles of the same
gene, or the production of heterozygous individuals (meisois); 2) the random mixing of
chromosomes from two parents during sexual reproduction to produce a new individual; 3) the
exchange between chromosomal pairs of particular alleles during meiosis, called crossing over,
to produce new gene combinations. Chromosomal mutations such as deletion, duplication,
inversion, translocation and polyploidy also result in variation.
3. Heredity:
The transmission of variations from parent to offspring is an important mechanism of evolution.
Organisms possessing helpful hereditary characteristics are favoured in the struggle for
existence. As a result, the offspring are able to benefit from the advantageous characteristics of
their parents.
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4. Natural selection:
It brings about evolutionary change by favouring differential reproduction of genes which
produces change in gene frequency from one generation to the next. Natural selection does not
produce genetic change, but once it has occurred it acts to encourage some genes over others.
Further, natural selection creates new adaptive relations between population and environment by
favouring some gene combinations, rejecting others and constantly modifying and moulding the
gene pool.
5. Isolation:
Isolation of organisms of a species into several populations or groups under psychic,
physiological or geographical factors is supposed to be one of the most significant factors
responsible for evolution. Geographical barriers include physical barriers such as rivers, oceans,
high mountains which prevent interbreeding between related organisms. Physiological barriers
help in maintaining the individuality of the species, since the isolations known as reproductive
isolation do not allow the interbreeding amongst the organisms of different species.
Speciation (origin of new species):
An isolated population of a species independently develops different types of mutations. The
latter accumulate in its gene pool. After several generations, the isolated population becomes
genetically and reproductively different from other so as to constitute a new species.
Mimicry and Evolution
Mimicry is an interesting phenomenon in animal kingdom. Mimicry is defined as “the
resemblances of one organism to another or to any natural object”. Through mimicry, the mimic
gets concealment and protection or for some other advantages. Adaptive colouration and
protective resemblance are the twin aspects of mimicry. Adaptive colouration gives concealment
to animal when their colour resembles other object. In protective resemblance colouration is not
involve, but the animal resemble other object as such.
The organism which exhibits mimicry is called a mimic. The organism or the object which is
mimicked or imitated is called a model.
Mimicry can be classified into three types. They are 1) Protective mimicry, 2) Aggressive
mimicry and 3) Conscious mimicry.
1. Protective mimicry:
It is of highly specialized character and the organisms mimic themselves in the form, as well as
in colour to protect the animals from enemies or predators. By this kind of mimicry, the mimic
gets protection due to mimicry. Hence this type of mimicry is known as productive mimicry.
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Stick insect
The best example of this type of mimicry is the stick-insect. Stick insects have slender body,
jointed limbs, slow movement and it almost resembles the twig of a tree and it is very difficult to
differentiate or identify stick insects from its environment. Thus this insect protects themselves
by mimicking the twig of the tree. The twig is the model and the stick insect is the mimic.
Phyllium
Another example for protective mimicry is the leaf insect, Phyllium which looks exactly like a
green leaf. It lives among the green leaves and so it is very difficult to distinguish leaf insect.
Flattened wings and expanded body and limbs are green in colour and thus they imitate the
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colour of the leaves. The venation of the wings is also similar to leaves. By acquiring this kind
of mimicry, the leaf insect is capable of escaping from its enemies.
Kallima
Kallima is a butterfly which mimics the dead dry leaves. The wings of this insect are brightly
coloured on the upper side but their undersides are dull brown in colour. During flight the bright
colour will be exposed but when it rests, the wings are folded and kept vertically upwards. In
such position the dull colour is exposed and which matches well with the colour of the dead
leaves. Thus the Kallima butter fly gets protection from its enemies.
Another example is that of the caterpillar of geo-metric moth. It rests on a tree in such a position
and angle that it resembles exactly a twig.
There are also instances where a mimic resembles its model even in behavior. For example a
particular species of spider mimics the ant. When undisturbed it moves about in a slow and
steady manner very much like the worker ants in search of food. When disturbed, it runs about
excitedly here and there just like the ants.
2. Warning mimicry: If harmless animal or non-poisonous animals or tasteful species of animal
mimic the harmful animal or poisonous animals or distasteful species of animals, that type of
mimicry is called warning mimicry. By acquiring these characters, harmless animals warn other
enemies for their protection. Some of the other examples warning mimicry are:
The non-poisonous snake Lycodon mimics the deadly poisonous snake, Krait (Bungarus) in its
coloration and banding pattern.
Another well knows example is North American Viceroy and Monarch butterflies. The Palatable
Viceroy butterfly mimics the unpalatable Monarch butterfly. By this mimicry the palatable
viceroy butterfly escapes from predation. Both butterflies have a reddish brown colour with
black edges and white dots.
3. Aggressive mimicry: Carnivorous animals such as spider and fishes exhibit aggressive
mimicry. In this mimicry, either they conceal themselves so that they are not easily recognized
in their surroundings or attract their prey. For example, certain yellow bodied spiders resemble
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very much the colour of the flower upon which they rest. They are invisible to the visiting insects
and thus they are unnoticed and they escape from insects. Here the model is the flower and the
mimic is the spider. Another case is about certain spiders which also resemble the flowers very
closely. The insects which are attracted to collect honey are devoured by spiders
.
The importance of mimicry in evolution was discussed by Bates and Muller. According
to them the mimicry is named as Batesian mimicry and Mullerian mimicry.
Batesian mimicry
Batesian mimicry was proposed by Henry Walter Bates in 1862 his most famous
expedition in the Amazon rainforest from 1848 to 1859. His views were based on the fact that
edible or harmless species resemble inedible or harmful species. Batesian mimicry describes a
relationship between two organisms - where one that is harmless looks almost exactly like one
that is harmful. The conditions of Batesian mimicry are:
1. The model must be relatively inedible or otherwise protected
2. It must have a conspicuous colour pattern.
3. It must be common, usually very much more common than the mimic.
4. Both model and mimic must be found together in the same area at the same time.
5. The mimic should bear a very close resemblance to the model.
6. The resemblance only extends to visible structures, colour pattern or behavior. That is
to say that it should deceive the predator but not the anatomist.
Bastesian mimicry can be explained with an example. In North America there are two
types of butterflies. They are Monarch butterfly and Viceroy butterfly. Both of them live the
same area. Though these butterflies belong to different species, they have a similar colour
pattern. But the Monarch butterfly is inedible and the Viceroy butterfly is edible. Since the
monarch butterfly is inedible, it is the model and the viceroy butterfly which is edible is the
mimic.
Experimentally it has been proved how mimicry works in these two types of butterflies
which live in the same areas. Predator birds were allowed to eat these butterflies. It was found
that birds which came in contact first with the inedible monarch butterflies, tries to avoid the
edible viceroy butterflies also when they next came in contact with them. Thus the edible mimics
were able to survive due to the inedible models. This was possible only when the number of the
model was greater than the mimic. Otherwise the chance of survival was much less. Since the
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mimic was found fit to live in that environment, it was selected by nature, selection operates in
the process of mimicry.
Mullerian mimicry
Mullerian mimicry was explained by Muller. It is named after the German naturalist Fritz
Müller, who first proposed the concept in 1878. His views were based on the fact that two or
more inedible species resemble each other. Müllerian mimicry is a natural phenomenon in which
two or more poisonous species, that may or may not be closely related and share one or more
common predators, have come to mimic each other's warning signals. The conditions of
Mullerian mimicry are;
1. Both the model and the mimic are inedible or warningly coloured.
2. Both the model and the mimic can be equally common. It is not necessary that the
model must be much more common than the mimic.
3. Both the model and the mimic should be found in the same area at the same time.
4. The resemblance between the forms need not be very exact.
Mullerian mimicry can be explained with an example of wasp and moth. There is a moth
which resembles a wasp. Both are inedible and they live in the same area. According to Muller, a
bird which is a predator, will take about 10 attacks to learn whether an animal is tasteful or not.
When a bird comes in contract first with the moth it will learn that they are not tasty after killing
about 10 moths. If the same bird comes in contact with the wasp later, the wasp will not be
touched since the wasp resembles the moth. In the same way, if the birds learn first that the
wasps are distasteful, they will avoid the moth also. Thus both the distaste moth and wasp have
reduced number of change of destruction.
A common example of Mullerian mimicry can be seen butterflies such as H. erato and H.
melpomene. They are two different species of butterflies that exhibit Mullerian mimicry. Both of
them have evolved to have mostly black bodies and wings, but they have a similar pattern of redorange dots and markings on their wings. Both of these species of butterflies have a taste that is
very undesirable to predators. Their bad taste is derived from the food that their caterpillar form
eats before they undergo metamorphosis and become butterflies. Since both of these species have
the same bad taste, most predators will need to only try one to learn to avoid the other. If you
have ever eaten at a restaurant and had a bad meal and decided to avoid the restaurant altogether,
it would be similar to predators avoiding all butterflies that have this coloration.
Industrial Melanism
In European countries it was observed that after the development of industries there, the
environment was changed and only melanic varieties could survive. If a non-melanic variety
comes over there, it will be in a disadvantageous position. Unless some mutation takes place in it
and converts in to a melanic variety, its survival change is less. So, having come over to this
environment, it acquires black colour by mutation and tries to resemble the melanic variety. Thus
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mimicry slowly develops here for protection. As a result of this mimicry polymorphic species are
produced.
Mimicry and Evolution
Whatever may be the type of mimicry, whether Batesian or Mullerian or Mimicry due to
industrialization, there must come Natural Selection to select the most advantageous ones. Thus
Natural Selection will automatically select those animals and plants which have tried to protect
themselves in their environment.
Co-evolution
Co-evolution is studied in connection with mimicry of animals. This term co-evolution
was coined by Ehrlich and Raven. The most important part of an organism's environment is other
organisms. Co-evolution occurs when, in adapting to their environments, two or more organisms
evolve together. To "make the best of" where they live, organisms make use of other organisms
by eating them, living on or in them, and/or building a "partnership" with them. Organisms coevolve with many species at the same time, because an environment includes many different
types of organisms. It is an established fact that all the closely related species are not appeared at
the same time. For example, although the first mammals appeared approximately 225 million to
180 million years ago, not all mammal species appeared at that time. The first segmented
flatworms originated millions of years ago before the appearance of mammals. But the tapeworm
is a parasite of humans, cows, and other mammals could not have evolved before the first large
mammals, because it adapted so much to the parasitic relationship that its ancestors before the
relationship were not of the same species of worm. Relationships formed through co-evolution
may be called symbiotic relationships. Three types of symbiotic relationships are predator-prey
relationships, mutualistic relationships, and parasitic relationships.
Polymorphism and Evolution
Polymorphism is a phenomenon in which two or more variations of a species co-exist in a region
in a given time. Polymorphism in biology is said to occur when two or more clearly different
phenotypes exist in the same population of a species—in other words, the occurrence of more
than one form or morph.
Polymorphism is commonly found in nature. It is related to biodiversity, genetic variation and
adaptation. Polymorphism usually functions to retain a variety of forms in a population living in
a varied environment. Sexual dimorphism is a well known example of polymorphism which
occurs in many organisms. Mimetic forms of butterflies, human hemoglobin and blood groups,
banding pattern of Cepaea are some of the examples of polymorphism.
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Polymorphic forms of Cepaea
Evolutionary theory state that polymorphism is result of evolutionary processes. It is heritable
and is modified by natural selection. In polyphenism, an individual's genetic make-up allows for
different morphs, and the switch mechanism that determines which morph is shown is
environmental. In genetic polymorphism, the genetic make-up determines the morph.
Polymorphism also refers to the occurrence of structurally and functionally more than two
different types of individuals, called zooids within the same organism. It is a characteristic
feature of Cnidarians. For example, in Obelia there are feeding individuals, the gastrozooids; the
individuals capable of asexual reproduction only, the gonozooids, blastostyles and free-living or
sexually reproducing individuals, the medusae.
Polymorphism was crucial to research in ecological genetics. The results of research on this
aspect had a considerable effect on the mid-century evolutionary synthesis, and on present
evolutionary theory. The work started at a time when natural selection was largely discounted as
the leading mechanism for evolution, continued through the middle period when Sewall Wright's
ideas on drift were prominent, to the last quarter of the 20th century when ideas such as Kimura's
neutral theory of molecular evolution was given much attention. The significance of the work on
ecological genetics is that it has shown how important selection is in the evolution of natural
populations, and that selection is a much stronger force than was envisaged even by those
population geneticists who believed in its importance, such as Haldane and Fisher.
There are two types of polymorphism: transient polymorphism and stable or balanced
polymorphism.
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Transient Polymorphism
In transient polymorphism, one form is gradually being replaced by another. As the name
implies, it represents a temporary situation as a by-product of directional natural selection. For
example, during the course of industrial melanism, it was held that the melanic form of peppered
moth gradually predominated the non-melanic form in the trees of Manchester, England due to
selective pecking of the latter (lighter form) by the birds. This polymorphic form will not be so
commonly found because they will be present in a population for a short span.
Another example of transient polymorphism is the development of insecticidal resistance by
insects. Insecticides were widely used to kill many insect pests. But certain insects developed
resistance y acquiring mutation and they could able to survive successfully. A study was made in
1963 showed that 46 species developed resistance to DDT and other relevant chemicals. Another
65 insect species developed resistance against dieldrin.
Stable Polymorphism
Balanced polymorphism occurs when different phenotypes are maintained at relatively stable
frequencies in the population and may resemble a population in which disruptive selection
operates. Sickle-cell anemia results from a change in the structure of the hemoglobin molecule.
Some of the red blood cells of persons with the disease are misshapen, reducing their ability to
carry oxygen. In the heterozygous state, the quantities of normal and sickled cells are roughly
equal. Sickle-cell heterozygotes occur in some African populations with a frequency as high as
0.4.The maintenance of the sickle-cell heterozygotes and both homozygous genotypes at
relatively unchanging frequencies makes this trait an example of a balanced polymorphism.
Blood group of man is another example of stable or balanced polymorphism. All the human
beings belong to the species, Homo sapiens. But they are with different types of blood groups.
Once such a blood group, is ABO type. In this type four different forms are exists. They are A,
B, AB and O. Since the character is not seen externally, it is referred as hidden polymorphism.
African butterflies offer another example for balanced polymorphism. The males of African
papilio species are all of one type (monomorphic). But the females occur in several forms. They
are different from the males and they also differ from each other.
Origin of Polymorphism
In a population of polymorphism appears in many ways. Some of them are:
1. Environmental changes are major sources of polymorphism.
2. Population pressure like predation is also known to bring out polymorphism
3. Heterozygous superiority maintains polymorphism in a population.
4. Mutation pressure is another factor that establishes polymorphism. It produces a variety of
alleles which result in polymorphic species.
5. Natural selection is another major factor which brings polymorphism.
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UNIT – V
Polyploidy and Evolution
One of the most striking features of genome structure is its liability. From small-scale
rearrangements to large scale changes in size, genome comparisons among species reveal that
variation is commonplace. Even over the short time course of laboratory experiments,
chromosomal rearrangements, duplications/deletions of chromosome segments, and shifts in
ploidy have been observed and have contributed to adaptation. Changes in genome
structure typically have immediate effects on the phenotype and fitness of an individual.
Beyond these immediate effects, changes in genome structure might allow evolutionary
transitions that were previously impossible. For example, by introducing an additional
complement of chromosomes, polyploidization might release gene duplicates from the
constraints of having to perform all of the functions of a gene (pleiotropy), providing extra
“degrees of freedom” upon which selection can act to favor new functions. Polyploidization
can also stimulate further structural changes in the genome, providing polyploid lineages with
genomic variation not available to diploid organisms. Indeed, it has been proposed that
tetraploidy may be an intermediate stage in some cancers, facilitating a cascade of structural
changes that disrupt normal controls to cell growth.
Polyploidy, the condition of possessing more than two complete genomes in a cell, has intrigued
biologists for almost a century. Polyploidy is found in many plants and some animal species and
today we know that polyploidy has had a role in the evolution of all angiosperms. Despite its
widespread occurrence, the direct effect of polyploidy on evolutionary success of a species is
still largely unknown. Over the years many attractive hypotheses have been proposed in an
attempt to assign functionality to the increased content of a duplicated genome. Among these
hypotheses are the proposal that genome doubling confers distinct advantages to a polyploid and
that these advantages allow polyploids to thrive in environments that pose challenges to the
polyploid’s diploid progenitors.
Polyploidy-Types
Polyploidy is classified in to two types: 1) Based on the number of sets of chromosomes and 2)
Based on the nature of chromosomes.
1) Based on the number of chromosomes
On the basis of number of chromosomes present polyploidy is classified in to
a) Triploidy (3n) where 3 sets of chromosomes are present in a cell
b) Tetraploidy (4n) where 4 sets of chromosomes are present in a cell
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c) Pentalploidy (5n) where 5 sets of chromosomes are present in a cell
d) Hexaploidy (6n) where 6 sets of chromosomes are present in a cell
e) Hectaploidy (7n) where 7 sets of chromosomes are present in a cell
f) Octoploidy (8n) where 8 sets of chromosomes are present in a cell
g) Nanoploidy (9n) where 9 sets of chromosomes are present in a cell
h) Decaploidy (10n) where 10 sets of chromosomes are present in a cell
2) Based on the nature of chromosomes
In this type polyploidy is divided into autopolyploidy and allopolyploidy.
a) Autopolyploidy. In this type all the chromosomes sets are derived from the same species.
b) Allopolyploidy. When chromosomes sets are derived from two different species, then this
kind is known as allopolyploidy.
Polyploidy-Reasons of origin
Polyploidy is caused by various factors and some of the factors which are responsible for the
production of polyploidy is discussed here.
a) Abnormal Mitosis
During mitotic cell division, the chromosomes duplicate in the normal manner. Following this
the cytoplasm divides into two. When cytoplasm fails to divide both sets of chromosomes are
present in the same cell. This leads to the cell to have additional set of chromosome.
b) Abnormal Meiosis
During meiosis, under abnormal circumstances, all chromosomes enter into a single cell. This
leads to production of a diploid gamete. If the diploid gamete is fertilized by a haploid gamete,
triploid cell is produced.
c) Chemically Induced Polyploidy
Chemicals like colchicines, indole acetic acid (IAA), sulphanilamide induce cells to produce
polyploidy.
d) Temperature
Temperature also induces production of polyploidy
Origin of Polyploidy
Polyploidy originates in two ways. They are a) natural polyploidy and induced polyploidy.
a) Natural Polyploidy
This type of polyploidy is originated due to abnormal meiosis where one gamete is produced
with diploid number of chromosomes. If this diploid gamete (2n) is fertilized by a haploid
gamete (n) a triploid cells (3n) is produced. In this manner when a diploid gamete fused with
another diploid gamete a tetraploid cell will be produced. If both gametes are originated from a
same species, this type of ploidy is known as auto-polyploidy. If the gametes belong to two
different species, then this kind of polyploidy is known as allopolyploidy.
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b) Induced Polyploidy
When polyploidy is originated as a result of physical and chemical agents, this type of
polyploidy is known as induced polyploidy. Chemical agents, colchicines, sulphanilamide,
indole acetic acid are the chemical agents induce the polyploidy. It is known that high
temperature also induces polyploidy. Abnormal mitosis is the cause of polyploidy which is
induced by the physic-chemical factors
Characteristics of Polyploidy
Cells of polyploidy organisms are larger than normal organisms. Polyploidy plants are larger in
size and they yield more. Leaves of the polyploidy plants are larger, darker and thick in size.
Flowers, fruits and seeds also larger bigger. It is observed more amount of vitamins in
polyploidy plants of tomato, nicotine content of tobacco and sugar content in beets. But
polyploidy plants show low fertility, stunted growth and delayed flowering.
Significance of Polyploidy
Polyploidy is a way of speciation where speciation occurs in a single step. Studies on polyploidy
provide much information on past history of plant species. Polyploidy results in the fixation of
heterozygous combinations, particularly those derived by hybridization. Whenever heterozygotes
have an adaptive advantage, polyploidy will be favoured because it makes heterozygous
populations easier to maintain by selective elimination of the few homozygotes. Polyploidy is
therefore a conservative process of evolution. Polyploidy has much stabilizing effect. Evolution
through polyploidy is irreversible. Evolutionary steps that give rise to new polyploidy species
can be repeated experimentally. The formation of new polyploidy plants in gardens provides
opportunities for conducting synthetic experiments on evolution.
Polyploidy and Speciation
Polyploidy brings about instantaneous speciation which means formation of a new species in a
single step. This method of speciation is more common in plants than animals.
Polyploidy is generally produced by the diploid ancestors. Polyploidy arises in many ways. A
simple method of polyploidy origin is production of diploid gamete by a diploid parent. The
diploid gamete is produced due to failure in meiosis. Thus when a diploid parent contains 2n
chromosomes, the diploid gamete is also contains 2n chromosomes. When these diploid gametes
fuse with one another, resulted individual will have 4n chromosomes. This condition is known as
tetraploidy. Many tetraploidy are fertile among themselves, but they cannot produce fertile
hybrids with their diploid ancestors. Thus reproductive isolation appears at one stroke and
without any geographic isolation. Hence polyploidy leads to production of sympatric speciation.
Polyploidy is the most common method of speciation in plants. Most of the wild plants are
polyploids of diploid ancestors. Cotton, wheat, tobacco, potato, coffee, sugarcane are some of the
examples. Plant breeders produce new varieties through artificially induced polyploidy.
The earlier researches in this field have shown that about 2-4% of the speciation events in
angiosperms were due to polyploidy. The phenomenon was also found in 7% of the speciation
events in case of ferns. Sympatric speciation in plants was thought to be mainly due to
polyploidy.
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With the new sequencing methods and comparison algorithms, polyploidy was found to be
involved in the speciation of flowering plants and eukaryotes too. Presence of gene redundancy
in these organisms was attributed to either polyploidy or whole genome duplication. Polyploidy
has been found to be important in the evolution of amphibians also.
The evidences point out to the lineages undergoing repeated polyploidization followed by
diploidization. The process of polyploidization is thought to be followed by other processes such
as genomic rearrangement or gene silencing which will facilitate diploidization.
Conservation of species
Although the recurrence of polyploidization was evident, the frequency and importance of such
events need to be studies in detail. The recurrence was found to counterbalance the local
extinction of initial autopolyploid species.
Difference in gene composition
Soybean species and its relatives are complex paleopolyploids which had undergone two
polyploidization events in their evolutionary history. The homeologous species of soybeans were
found to differ in the transposon insertions and R-gene composition. Further evidences were
found for polyploidy to start divergent evolution within the species.
Variations in gene expression
Gene expression pattern in gene duplication have shown that polyploidy genomes respond in
similar manner to environmental stimuli. Hence, polyploidy and whole genome duplication has
also been proposed to be the result of external factors.
Epigenetic changes
Chromosomal rearrangements and DNA methylation pattern changes are frequent in polyploids
and this has resulted in variations in the process of gene expression and regulation. Such changes
in Brassica have resulted in considerable morphological variations.
Applications of Polyploidy
Study of adaptive evolution among polyploids reveal significant information on the evolutionary
history of plants and hence can lead to better conservation methodologies.
Crop domestication is perhaps the most significant application of polyploidy since polyploids are
found to be high in vegetative content. The only drawback is that further propagation needs
clonal means.
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Studies on polyploidy also can reveal information on how the plant genomes manage to succeed
the effects of genome obesity (increase in genome dosage).
The effect and process of polyploidy need to be evaluated more. Future prospects and further
applications of polyploidy are increasingly being discovered. The phenomenon once reserved for
plants, has now been found to occur in animals alike.
ISOLATION AND ISOLATING MECHANISM
Evolutionists in the beginning were the belief that mutation and selection alone are
sufficient for the process of evolution. But later on, after the work of many scientists like
Wagner, Dobzhansky, Stebbins and Mayr, it was discovered that isolation has a key role origin
of species. Even Darwin was aware of the importance of isolation when he visited Galapagos
Islands but he did not emphasize on it.
Isolation is a phenomenon in which there are barriers. These barriers prevent
interbreeding between populations. When interbreeding is prevented, the gene flow between the
populations is also prevented. Thus when exchange or mixing up of genes between populations
is prevented, it becomes the starting point for the origin of new species.
For the initiation of the species, variations are raw materials and to preserve these
variations isolation is necessary. But it should not be too early. The early isolation of a particular
species will ultimately lead to extreme specialization, whose final result will be extinction.
Without isolation the ravages of Natural Selection will be very great.
New speciation can arise in two distinct ways, shown diagrammatically below:
A
B
A
B
C
C
D
In the first case, only one species (A) exists at any point in time. Species A evolves in to
B, B into C and so on and this case is known as the transform of species in time. It is also
referred to as anagenesis. In the second case, a single species (A) evolved in to two
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contemporary species B and C where a splitting had occurred. This is known as the
multiplication of species in space. It is otherwise known as cladogenesis. In the first case
mutation, natural selection and genetic drift play key role but in the second case in addition to
these factors, isolation plays a vital role.
While we recognize isolation as a factor in evolution, there are three important types of isolation.
They are geographical isolation, ecological isolation and reproductive isolation.
Geographical isolation
Wagner observed the occurrence of geographical variation and emphasized the
importance of geographical isolation in evolution. He said that if a population became separated
from the rest of the species it cloud diverge in isolation and became a different species.
Following Wagner, Dobzhansky, Huxley, Stebbins and Frod gave importance to this
factor. Geographical isolation is brought about by geographical barriers. These barriers play a
vital role in bringing about geographical isolation of races and species from one another.
Ernst Mayr gave a clear idea of geographical isolation. The following sequence in
speciation is now universally accepted for sexually reproducing animals.
Species
Isolated Population
Isolate
Race or Incipient Species
Species
The waved lines between species and isolated population represent an extrinsic barrier
restricting or preventing gene flow between the isolated population and its parental species. This
allopatric isolation is the primary step in the origin of new species.
Distribution of animal from one place to another is an unavoidable one. It can take place
at any stage of life cycle of animals. During their movement from one place to another place,
animals are likely to meet with some kind of physical barriers (geographical barriers). The sea
and freshwater bodies are effective barriers to the land animals and land will from an effective
barrier to aquatic animals. Mountain ranges, valleys, ice masses, forests etc, are also effective
geographical barriers. Due to these barriers isolation of population is caused and so gene flow
between populations is prevented. This will give rise to variation in the isolated population and
thus a new species would evolve in due course.
At time a single population with a particular genetic composition may be isolated into
two populations by the sudden appearance of a physical barrier. Each isolated population will
have the original genetic composition. Then, How each of the isolated populations with the same
genetic composition could diverge and give rise to subspecies and then to species? The
explanation to these questions is that even though the genetic composition of these populations is
similar, environment may be different. For example, if one of the isolated populations occupies
one type of environment, another population may occupy a different type of environment. Thus
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selection pressures in these two types of environments will be acquiring mutation. Mutation will
give rise to new variation and useful variation will be selected by nature. Thus subspecies may
arise.
Thus geographical isolation is a prerequisite and by this alone a new species will not be
formed. This is only an extrinsic factor and it should be followed by intrinsic factors like
reproductive isolation for the formation of new species. Though mutation and selection are
considered more important for the origin of species, isolation will give more chance for these to
act.
Reproductive isolation
(Genetic isolation; Physiological isolation)
Whatever may be the type of isolation, whether it is geographical or ecological isolation,
there must be a reproductive isolation to form a new species. Thus the geographical isolation is
of final and utmost importance in the formation of species. Since reproductive isolation is
genetically and physiologically controlled, this isolation may well be called as genetic isolation
or physiological isolation. To bring about effectiveness of reproductive isolation, there are
certain special devices which are called the isolating mechanisms. These isolating mechanisms
prevent or reduce the interbreeding within the species. The isolating mechanisms can be defined
precisely as biological properties of individuals which prevent the interbreeding populations.
From this definition it is clear that isolating mechanism does not include geographical or any
other purely extrinsic isolation. A mountain or a steam that separates two populations is not an
isolating mechanism. Thus geographical isolation is ultimately caused by special factors and
reproductive isolation by biological factor.
This term isolating mechanism was coined by Dobzhansky. There are mainly two categories
of isolating mechanisms. They are a) pre-mating mechanisms and b) post-mating mechanisms.
As the word indicates, all the isolating mechanisms before mating will be included here. They
are of various types.
Premating Isolation
i) Habitat isolation:
It is nothing but ecological isolation. Mating inability between the organisms of closely related
species or sub-species occupying the same region but different habitats. Certain species are
isolated due to their preference to the habitat. In Florida, there is a water snake, Natrix sipedon.
One race other this snake is found in freshwater and another race is found in saltwater. Even
though they come closer, they do not mate. Thus they have habitat isolation which prevents
interbreeding.
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ii) Seasonal isolation (Temporal isolation):
Interbreeding is prevented because of difference in breeding seasons. Due to the effect of factors
like photoperiod, temperature and humidity, the breeding season of the animals of one species is
different from that of other species, so are reproductively isolated. Seasonal isolation is common
in plants, insects and other invertebrates. It has been observed in two species of American toads.
The Red-legged Frog Rana aurora breeds from January to March and the closely related Yellowlegged Frog Rana boylii breeds from late March through May. One breeds early in the season
and another breeds late in the season. Hence they do not interbreed. Thus season has become a
barrier for this case.
In America, 3 species of frog Rana sp. live in the same pond and each species breeds separately
depending upon the temperature of the pond but do not interbreed. One species breeds when the
temperature of the pond is 44⁰F and another species breeds at a temperature of 55⁰F. The third
species breeds at 60⁰F.
iii) Ethological isolation (Sexual isolation; psychological isolation; Behavioural isolation);
The word “ethology” refer to behaviour of animals. Here the behavioural patterns in connection
with the courtship of animal are discussed. Differences in sexual behavior in certain species
prevent interbreeding. Ethological barriers to mating constitute the largest and most important
category of isolating mechanisms in animals. There are definite sexual behaviours between the
males and females of a species to attract them for mating. This behaviour may be in the from of
nuptial dance, sound production, light emission etc. There are two species of tree frog Hyla sp.
living in the same pond but they do not interbreed due to their difference in sexual behavior. The
females respond to the sounds produced by the males. The sounds produced by the males of
these 2 species differ in their intensity and duration and so the females of the particular species
approach the males of the species alone.
iv) Mechanical isolation:
If there are differences in the external genital organs or floral parts, they will not correspond to
each other. Thus there is a barrier for copulation. This prevents interbreeding. It has been
suggested that the female and male genital organs should act like ‘lock and key’. There will be
one type of key for a particular lock. Thus genital organs are species specific. Even slight
variation in the genital organs will make copulation impossible. It has been observed in several
species of Drosophila that if mating occurs between unrelated species, it will cause injury to the
genital parts and some times it may be fatal also. The non-correspondents of external genitalia
have been observed in butterflies, moths, bees, wasps etc.
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Post-mating mechanism
As the word itself indicates all, isolating mechanisms after mating will be included here. If the
efficient premating mechanisms fail, then post mating mechanisms prevent hybrid formation.
They are of various types.
i) Gametic mortality:
Here the sperms may enter the genital tract of the female, but by some chemical reaction they
could not move and they are killed before they reach the egg. Some time the sperm may die
because it cannot penetrate the egg membrane of the other species. It has been observed that
when mating occurs between two species of Bufo where the gametes of one species die in the
genital passage.
ii) Zygotic mortality:
Even when fertilization occurs successful, zygote will not survive. Development may stop at any
stage between fertilization and adulthood. It has been observed in the case of sea urchin species
that most of the embryos die before the gastrula stage.
iii) Hybrid inviability:
In this type isolating mechanism, the hybrids are viable, fertile but their young ones are inviable.
The hybrids produce normal eggs or sperms. The embryo develops into the adult but it dies
before reaching maturity. The reason for the reproductive failure of fully fertile hybrid species is
that they are less adjusted to the environment than their parental species. This is due to the
genetic incompatibility of the two parents. This has been observed in fishes, frogs, beetles and
moths. Hybrids between the Rana pipiens and Rana sylvatica do not survive beyond the early
gastrula stage.
iv) Hybrid sterility:
In this type of isolating mechanism hybrids are viable, but sterile due to incompatibility of
chromosomes. Incompatibility of chromosomes results in faulty meiosis. The best example for
this is the mule which is the product of male ass and female horse.
There are number of examples where interspecific crosses may produce fertile interspecific
hybrids in captivity as no reproductive barrier has been developed between these species even
after their long isolation from each other e.g. a) Fertile ‘tigons’- a hybrid between African lioness
(Panthera leo) and Asian tigers (P. tigris) but do not breed in nature due to different
geographical distributions. b) Fertile hybrids between mallard duck (Anas bascas) and pintail
duck (A. acuta) but do not breed in nature due to different mating and nesting behaviour. This
shows that reproductive isolation is not a universal phenomenon.
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The role of isolating mechanisms in evolution
It has been noted that geographical isolation and ecological isolation are only
prerequisites but they are alone not enough to form a new species. They should be followed by
reproductive isolation. The isolating mechanisms are genetically controlled. Only in Drosophila
the genetic of isolating mechanisms has been studied in detail. But even this information is not
enough to clearly state the role of isolating mechanism in evolution because it is much varying.
Geographical isolation will help to isolate the population. If these populations occupy different
environments, mutation will take place to give rise variations. Thus genetic divergence will be
there. Due to this there is a reproductive isolation ie. any one of the pre-mating or post-mating
mechanisms will work. If the animals acquire useful variations, then they will be selected by
nature. Thus geographical isolation, mutation, isolating mechanisms and natural selection must
work together to give rise to new species.
The origin of isolating mechanism
Generally, no one isolating mechanism is completely defective, but their combined
effects cause total reproductive isolation. One of the major problems is to explain the origin of
isolation mechanism. There re mainly two views.
1. Muller’s view: According to Muller due to geographical barrier, there will be geographical
isolation of population and thus allopartic population will be formed. These separated
populations may meet with different environment condition. So each population may try to
adjust to that particular environment condition. In this process mutation, recombination of genes,
genetic drift ect, will play their role and thus genetic divergence will take place. These
population with genetic divergence con not interbreed even if the geographical barriers disappear
now because they are isolating mechanisms is operating and thus it explain the origin of isolation
mechanism. Due to this isolating mechanism these 2 populations will became 2 new species.
2. Dobzhansky’s view: His view is mainly based on the role played by Natural selection. He laid
emphasis on hybrid. When genes from different population are brought together, the hybrid
formed will not be successful. They will be either poorly adapted or partially sterile and so
natural selection will not select them. This natural selection eliminates not only the hybrids but
the genes of the parents that formed the hybrids. Thus natural selection is against hybridization
according to Dobzhansky, natural selection thus acts to reduce the wastage of gamete on the less
fit hybrids.
Conclusion
From the account given above about isolation, it can be understood that isolation is an important
factor in evolution. Only recently its important has been recognized. Geographical or spatial
isolation effectively prevent gene exchange between populations and should be followed by
reproductive isolation if a subspecies or species is to arise. The reproductive isolation is brought
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about by isolating mechanisms. Since isolating mechanisms are genetically controlled, they will
maintain reproductive isolation. Virtually all of the evidence suggests that the initial stages in the
development of geographical isolation. So it is said that geographical isolation is a prerequisite
and reproductive isolation with genetical divergence is of crucial importance in speciation. Thus
isolation is an absolutely essential mechanism for species formation, nearly as important as
natural selection. Hence Wagner has right pointed out that ‘without isolation or the prevention of
interbreeding organic evaluation is in case possible.
Speciation and Concept of Species
Speciation is the origin of new species. Generally, it refers change of one species, over the time
and eventually becoming two species. It consists of the evolution of biological barriers to gene
flow (reproductive isolation) between two populations of a same species. As a field of scientific
investigation, it links the fields of macroevolution and microevolution, including the fields of
genetics, ecology, behavior and biogeography.
A species concept is a way of defining a species, and a recent reviewer found 24 different species
concepts that have been proposed over the past century. The most famous, and the one that most
biologists use today, is the biological species concept, which states that “species are groups of
actually or potentially interbreeding populations, which are reproductively isolated from other
such groups”.
In other words, speciation is the evolution of reproductive isolation between two groups.
Operationally, this definition works well for most animals. However, it has limitations: it does
not always work with plants, and it cannot be applied to extinct species (e.g. fossils) or asexually
reproducing species (e.g. bacteria).
Causes Speciation
Speciation, or the evolution of reproductive isolation, occurs as a by-product of genetic changes
that accumulate between two previously interbreeding populations of the same species. For
example, let us start with two populations of the same species that do not differ genetically.
Initially, an individual from population A is able to successfully breed with an individual from
population B. As these populations evolve, they each gradually accumulate genetic changes that
are different from the other populations' genetic changes. In other words, the two populations
genetically diverge from each other. These changes can be due to different selection pressures
because of different environments, or because of genetic drift/founder events.
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At some point in this process, some of these genetic changes cause the two populations to
become reproductively isolated from each other. In other words, these genetic changes no longer
allow an individual from population A to successfully breed with an individual from population
B. They prevent gene flow between populations. These specific genetic differences that confer
reproductive isolation are called reproductive isolating mechanisms.
There are several different types of reproductive isolating mechanisms, which are classified
according to when in the life cycle of the organism isolation occurs. Isolation can occur before
fertilization (prezygotic barriers) or after fertilization (postzygotic barriers).
Prezygotic isolation can occur either before mating occurs (premating barriers) or after mating
occurs (postmating barriers). One type of premating prezygotic isolation occurs when potential
mates from the two populations do not meet, either because they are separated in time (temporal
isolation) or in space (habitat isolation). Temporal isolation can occur if individuals in two
different populations mate at different times of the day or in different seasons, or even years (e.g.
species of periodical cicadas mate either every 7 years or every 13 years). Habitat isolation
occurs, for example, when herbivorous insects from two populations feed and mate on different
host plants. Another type of premating prezygotic isolation occurs when individuals from two
populations meet, but they do not mate (behavioral or sexual isolation). This occurs, for
example, when courtship behaviors differ between individuals of two populations (e.g. songs in
birds, pheromones in moths, light displays in fireflies, etc.).
One type of postmating prezygotic isolation occurs when mating actually takes place, but male
gametes are not actually transferred to the female (mechanical isolation). This happens when
there is an anatomical incompatibility between individuals from two populations. For example,
the floral anatomy of some plant species prevents some pollinators that visit the plant from
actually transferring pollen. In this case, mating (pollination) occurs, but the male gametes
(pollen) are not able to reach the eggs. A second type of postmating prezygotic isolation occurs
after mating taking places, male gametes are actually transferred, but the egg is not fertilized
(gametic isolation). This can be an important isolating mechanism in externally reproducing
species that send out their gametes en masse. Sea urchins, for example, release their gametes
into the water column. In reproductively isolated species, male and female gametes actually
meet, but the sperm does not fertilize the egg. Another example of this would be when pollen
from a plant of one species lands on the stigma of a plant from another species, and a pollen tube
is not completely formed. In both of these cases, a genetic mismatch between the gametes
prevents successful fertilization.
There are three types of postzygotic isolating mechanisms. In the first type, mating occurs, a
zygote is formed, but the hybrid has reduced viability (hybrid inviability). In other words,
hybrids do not survive long enough to reproduce. The other type of postzygotic isolation occurs
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when hybrids are viable, but they have reduced fertility (hybrid sterility). A classic example is
the mule, which is the result of a cross between a donkey and a horse. Mules are viable, healthy
animals, but they are always sterile (i.e. they are unable to successfully reproduce). The third
type of postzygotic isolation occurs when hybrids are viable and fertile, but the offspring of the
hybrids are inviable or sterile (hybrid breakdown). In all of these postzygotic examples,
individuals from the two populations will mate with each other, and the gametes fuse, but the
genetic material in each of the gametes differs enough that the combinations of alleles are not
compatible.
How does Speciation Occur?
There are several different ways in which the evolution of reproductive isolation is thought to
occur. These can, however, be generalized into a series of events, or steps.
The "Steps" in a speciation event:
Step 1: gene flow between two populations is interrupted
(populations become genetically isolated from each other)
Step 2: genetic differences gradually accumulate between the two populations
(populations diverge genetically)
Step 3: reproductive isolation evolves as a consequence of this divergence
(a reproductive isolating mechanism evolves)
The main difference between the different models of speciation is in the first step, or how the
populations become genetically isolated from each other.
Types of speciation
Formation of a new species is called speciation. New species are formed in a variety of ways.
There are four main types of speciation. They are a) allopatric speciation, b) sympatric
speciation, c) quantum speciation and d) parapatric speciation.
Allopatric Speciation
The prefix allo- means "other". When paired with the suffix -patric, meaning "place", it becomes
clear that allopatric is a type of speciation caused by geographic isolation. The individuals that
are isolated are literally in an "other place". The most common mechanism for geographic
isolation is an actual physical barrier that gets between members of a population. This can be
something like as small as a fallen tree for small organisms or as large as being split by oceans.
Allopatric speciation does not necessarily mean the two distinct populations cannot interact or
even breed at first. If the barrier causing the geographic isolation can be overcome, some
members of the different populations may travel back and forth. However, a majority of the
populations will stay isolated from each other and as a result, they will diverge into different
species.
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Sympatric Speciation
The final type of speciation is called sympatric speciation. Putting the prefix sym-, meaning
"same" with the suffix -patric which means "place" gives the idea behind this type of speciation.
Amazingly enough, the individuals in the population are not separated at all and all live in the
"same place". So how do the populations diverge if they live in the same space?
The most common cause for sympatic speciation is reproductive isolation. Reproductive
isolation may be due to individuals coming into their mating seasons at different times or
preference of where to find a mate. In many species, choice of mates may be based on their
upbringing. Many species return to where they were born to mate. Therefore, they would only be
able to mate with others who were born in the same place, no matter where they move and live as
adults.
Quantum Speciation:
It is the rapid and abrupt mode of species formation. Grant (1971) defined quantum speciation
“the budding off a new and very different daughter species from a semi-isolated peripheral
population of the ancestral species”. This type of speciation is based on the observation of H.L.
Carson on Drosophila inhabiting Hawaii Island.The quantum speciation is a sudden and rapid
speciation. It does not produce subspecies or intermediate stage. Genetic drift or chance plays a
major role in quantum speciation.
Parapatric Speciation
The suffix -patric still means "place" and when the prefix para-, or "beside", is attached, it
implies that this time the populations are not isolated by a physical barrier and are instead
"beside" each other. Even though there is nothing stopping the individuals in the entire
population from mixing and mating, that does not happen in parapatric speciation. For some
reason, individuals within the population only mate with individuals in their immediate area.
Some factors that could influence parapatric speciation include pollution or an inability to spread
seeds for plants. However, in order for it to be classified as parapatric speciation, the population
must be continuous with no physical barriers. If there are any physical barriers present, it needs
to be classified as either peripatric or allopatric isolation.
Factors Influencing Speciation:
Some of the factors that influence the speciation are mutation, recombination, natural selection,
hybridization, genetic drift, poly-ploidy and isolation.
.
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Founder Principle
The origin of species (speciation) the process by which two or more species evolve from a single
ancestral species is a central problem in evolutionary biology. During the evolutionary synthesis
of the 20th century, the dominant theory of speciation for those working on sexually reproducing
animals was allopatric speciation. Allopatric speciation posits that an ancestral species becomes
subdivided into two or more geographical subpopulations by changing climates, colonization of
new areas, the erection of geological barriers, etc. If these geographical subpopulations have
little to no genetic interchange, they will begin to evolve separately. Speciation then arises as an
incidental by-product of the independent evolution occurring within the geographical isolates.
Evolution within species (microevolution) was often envisioned as being dominated by natural
selection leading to adaptive divergence between the geographical isolates. However, the modern
synthesis made it clear that microevolution involved many processes in addition to natural
selection. One of these processes was genetic drift, the random changes in a population’s gene
pool (the set of alleles or gametes collectively shared by a reproducing population) that
inevitably arise from random sampling of a finite number of gametes to form the next generation.
Just by chance, a particular form of a gene can decrease or increase in frequency in the
population, including being completely lost or fixed. The impact of random sampling increases
as the population size decreases. One special case of strong genetic drift is the founder effect, in
which a population is established by a small number of founding individuals from a much larger
ancestral population. Strong genetic drift in the founder population could lead to an immediate
evolutionary divergence from the ancestral population. This accelerated divergence is the
essence of founder effect speciation models. Founder effect speciation is a special case of
allopatric speciation in which one of the geographical isolates was established from a small
number of individuals. This does not mean that other microevolutionary forces, such as natural
selection, are not operating, but rather that the founder effect enhances and accelerates
microevolutionary divergence in concert with natural selection and other microevolutionary
forces, thereby making speciation more likely.
Founders Principle is proposed by Mayer and Sheppard. It states that when a new population is
established by isolation its gene poll is not identical with that of parent population because of
sampling error. This difference is further improved by the different types of evolutionary forces
operating on the two populations independently. This leads to genetic divergence or variations.
Founder principle leads to small isolated populations that have unsusual characteristics as
compared to the characteristics of large population of their relatives and the large populations
descended from a few immigrants may differ from the population which the immigrants came.
Example:
Consider a large population has in its gene pool equal number of genes P and p. Such population
is expected to consist of 25% PP individuals, 50% of Pp individuals and 25% pp individuals
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showing a 1:2:1 ratio. Assume that 10 members of the population migrate to an isolated place, an
island. These 10 members are called founders because they are going to establish a new
population there. The founders are drawn by chance and hence they may not exhibit the 1:2:1
ratio of the parent population. The founder population may have 6PP, 2Pp and 1pp individuals or
in any other combinations. In extreme cases all the 10 individuals may be PP, pp or Pp. So it is
an error of sampling. When all the individual are PP, the isolated population descended from the
10 individual would lack the p gene completely. In this manner the gene pool of the island
population might be very different from which the founders came.
Fossil Evidences
Paleontology is the science that deals with the study of fossils of animals and plants in order to
draw inferences in support of evolution. Fossil can be anything that can give an indication of the
existence of prehistoric organisms. Majority of them are bones buried deep in the soil, which in
the course of time turns into rock. Very old bones get petrified and no organic matter is left in
them. Often, impressions, footprints or molds and casts give a fairly clear idea of the animals to
which they belonged. Most of the bird fossils, including that of Archaeopteryx, are impressions
on the rocks as their bones are too fragile to be fossilized. Fossil footprints of dinosaurs found in
America, Australia and also in India, give an idea of not only their size but also the way they
walked. Rarely though, we are sometimes lucky to find complete animal preserved including its
skin and hairs intact. Discovery of a frozen woolly mammoth in Siberia was such a lucky event
but complete insect fossils preserved in amber are not a rarity for entomologists.
Fossil
Study of the past lived organisms which are not now on the globe or fossils is called
Paleontology. Normally hard parts like spicules, setae, chitinous exoskeleton, shells, spiny
exoskeleton, scales, bones, feathers, teeth and hair of past lived organisms were preserved as
fossils.
Fossilisation
It is the process of formation of fossils of past lived organisms. Normally an animal becomes
fossil due to sudden environmental change. Majority of the fossils are formed by Petrifaction. It
is the process of replacement of organic matter by minerals like sand, lime, iron oxides etc. In
petrifaction decomposers decompose the organic matter of dead organism. In such places
minerals precipitate. The minerals become hard to form into a rock. This rock resembles to the
dead organism.
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Fossil Record:
The totality of fossilized artifacts and their placement within the Earth’s rock strata, which
provides information about the history of life on Earth, e.g., what organisms looked like, where
and when they lived, how they evolved, etc. is the sequence of fossils as they appear in the
geologic strata (layers). By dating the strata, the approximate age of the fossils can be
determined. The fossil record preserves the fossils in the order in which they appeared on Earth,
providing direct visual evidence of evolution. The strata appear in descending order, thus the
deepest strata are the oldest (relative to the present, or the earliest/youngest relative to Earth's
history).
Due to the complexity of the fossilization process, the more prevalent the species and the longer
that it survived, the more likely that it will be represented in the fossil record. Some short-lived
species or transitory forms might have never been fossilized and are therefore missing from the
fossil record. Their existence however, can be deduced by the fossils of their predecessors and
descendants.
Fossils Collection
Fossils are exposed due to natural erosion of the soil or by excavation (digging of the soil). In
India fossil collection and observation are being conducted by Birbal Sahni Institute of
Palaeobotany, Lucknow. It observed three fossil parks in India.
A. Deccan plateau of Mandla district, M.P
The paleontologists collected 50 million years old fossil forest trees.
B. Rajmahal Hills, Bihar:
Scientists collected 100 million years old fossils.
C. Coal mines of Orissa:
Scientists collected 260 million years old fossils.
Fossils-Types
The fossils are four types. They are unaltered, altered, Moulds and coprolite fossils.
A. Unaltered fossils:
These are unchanged fossils. These are dead bodies of past lived organisms, preserved as fossils
without any change.
Examples:
Wooly mammoths found in Ice Mountains of Siberia, Hexapod fossils found in the amber resin
of California in U.S.A.
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B. Altered Fossils:
These are the changed fossils. These are formed by Petrification. Majority of the fossils are of
altered type.
Examples:
Archaeopteryx fossil was collected from calcium mines of Bavaria State in Germany.
Eusthinopteron and Ayshaea are also altered type.
C. Moulds: Moulds-fossils
These are the impressions of body, foot and leaves of past lived organisms. Normally plant
fossils are of this type.
D. Coprolites: Coprolites-fossils
These are the faeces fossils of past lived organisms. The coprolites help to decide that the faeces
are of an herbivore or carnivore or omnivore but not of a particular animal.
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