Chapter 21- Adaptation & Speciation
21.2 – How Species Form (p.708-713)
Adaptation – any trait that enhances an organism’s fitness or increases its chance of survival
and probability of successful reproduction (Natural Selection).
Speciation – the formation of a species.
A species is a population which can interbreed and produce viable fertile offspring.
Speciation may occur by:
a) Transformation - one species evolves into another. A result of accumulated
changes which occur over long periods of time and one species is
transformed into another. New species is gradually created and old species is
gradually lost.
b) Divergence - one or more species arise from a parent species that continues
to exist. It promotes biological diversity because it increases the number of
species.
Transformation and divergence are a result of natural selection.
In order for a species to remain distinct, they must not interbreed with other species.
Geographical and biological barriers keep species isolated.
Geographical barriers such as rivers and mountains prevent species from interbreeding
because they keep related populations physically separated. Gene pools change over time and
produce species that can no longer interbreed.
Some biological barriers to reproduction may contribute to speciation are:
1. Pre-zygotic barriers (impede mating between species or prevent fertilization)
a) Behavioural isolation - songs of birds, courtship rituals, and chemical signals
(pheromones) are species specific.
b) Habitat isolation - some species live in the same area but have different habitat.
E.g. Northwest Garter snakes: one species prefers water and one prefers open areas.
c) Temporal isolation - many species have their own timing barriers. Some animal
species mate at different times and some plant species only bloom for one day.
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d) Mechanical isolation - some species of insects have genitals that work in a lock and
key formation. Some plant species vary in arrangement of stamen and style. In Sage
plants, one species can only be pollinated by bees that carry pollen on their backs and
other species can be pollinated only be bees that carry pollen on their feet.
e) Gametic isolation - prevents zygote formation even if gametes from different species
meet animals with internal fertilization. Sperm may not survive in female reproductive
tract external fertilization. Sea urchin eggs are released and fertilized in water.
Chemicals on the surface of eggs recognize and reject sperm from other species. In
plants, pollen grains of one species fail to germinate on stigma of another species.
2. Post-zygotic barriers - when sperm of one species successfully fertilizes the ovum of another
species, there are barriers which prevent these hybrid zygotes from developing into normal,
fertile individuals.
a) Hybrid inviability - genetic incompatibility stops development of the hybrid zygote at
some stage during embryonic development. Usually due to genetic incompatibility
which prevents normal mitosis after fusion of nuclei in the gametes.
b) Hybrid sterility - sometimes two species mate and produce hybrid offspring (horse
and donkey produce a mule) but a reproductive barrier exists because the offspring is
sterile. If meiosis fails to produce normal gametes (different structure or number of
chromosomes in parents) this barrier comes into effect.
c) Hybrid breakdown - first generation of cross species are viable and fertile. When
these hybrids mate with each other or with an individual from either parent species
they are sterile or weak. E.g. Cotton - offspring of hybrids die as seeds.
21.3 – The Patterns of Evolution (p.714-726)
Adaptive radiation
This is when a single common ancestral species evolves into a number of different, or variety of,
species (diversification). It is a mechanism for speciation. For example: Darwin’s finches on the
Galapagos Islands.
The conditions necessary for adaptive radiation are:
 New environment for the evolving species. When the new species reach these new
environments, they will enter various ecological niches.
 A way by which the evolving species can reach these new environments. Migration plays a
key role in this process
 The new environment must be free from competition with similar forms
 There cannot be too many new predators
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Convergent Evolution
- Convergent evolution is a process by which unrelated species produce descendants that
display similarities due to the fact that they encountered similar problems in adapting to and
occupying a similar niche within a common environment.
- Similar traits arise in two or more species because each species has independently adapted to
similar environmental conditions and not because they share a common ancestor.
- The environment selects similar adaptations in unrelated species. For example, the wings of
birds, bats and butterflies are all used for flight but have evolved differently. Wings are
examples of analogous structures.
- Birds and bees both have wings but not common ancestors.
Divergent Evolution
- Divergent evolution is a process by which species that were once similar become increasingly
distant. In other words, it is a pattern of evolution in which species that were once similar to an
ancestral species diverge, or become increasingly distinct.
- Divergence occurs when populations change as they adapt to different environmental
conditions. The population becomes less and less alike as they adapt, eventually resulting in
two species.
Co- Evolution
-Co- Evolution is a process by which species that are tightly linked with one another (a flower
and a pollinator) evolve gradually together, each one responding to the changes in the other.
- In other words, it is an evolutionary process in which two organisms that are tightly linked
(predator-prey, pollinators-plants, parasites-hosts) evolve together and respond to changes in
the other population.
1. Plants rely on animals for pollination. Insect pollinated plants usually have strong scents
(birds have poor sense of smell). Bird pollinated plants usually have bright red petals
while insects are colour blind. Bird pollinated plants have nectar in long tubes to suit the
long beaks of birds.
2. Predator – prey: A rough skinned newt produces a poison that can kill 17 adult humans.
It has produced such a toxic chemical because its main predator the red-sided garter
snake has evolved a genetic resistance to the poison.
3. Bacteria and antibiotics
At present, scientists do not agree on the rate at which evolution occurred. Two opposing
viewpoints on the rate of evolution are:
Gradualism is based on Darwin’s theory. It states that the new species arise through the
gradual accumulation of small variations. In other words, evolution occurs slowly and
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continuously over long periods of time. Since Darwin’s time, evolutionary biologists have
supported the model of gradualism, which says that change occurs within a lineage, slowly and
steadily, before and after a divergence.
Punctuated equilibrium was proposed by Steven Gould and Niles Eldridge. A species remains
in equilibrium (unchanged) for extended periods of time and then in a relatively short period of
time, rapid change occurs. In other words, the long period of equilibrium is interrupted, or
punctuated, by a short period of evolution. See Fig. 20.21, p. 724
Gould and Eldridge:
 Gould and Eldridge developed a model of punctuated equilibrium which proposes that
evolutionary history consists of long periods of stasis or equilibrium ‘punctuated’ or
interrupted by periods of divergence.
 According to the model, most species undergo many of their physical changes when
they first diverge from the parent species. After that they don’t change much, even as
they give rise to other species.
21.4 – The Origins of Life on Earth (p.727-730)
Evidence and arguments pertaining to the origin, development, and diversity of living
organisms on Earth:
1. Chemical evolution
a) Oparin- Haldane Theory (1920-1930)
This is the most widely accepted theory and it suggests that the first organic compounds were
formed by natural chemical processes on the primitive earth and that the first life-like
structures developed from coacervates (aggregates of large protein-like molecules) and were
heterotrophs.
 organic compounds, the building blocks of life could be formed spontaneously
from simple inorganic compounds present on the surface of early Earth
 Earth had a reducing atmosphere containing little or no oxygen and plenty of
hydrogen, ammonia, methane and water vapor. These gases condensed to form
pools (the primordial soup) on the surface of the Earth.
 Energy such as lightning and UV radiation led to the development of organic
compounds from simple compounds in the ‘soup.’
 These compounds combined and evolved, eventually leading to the emergence
of an early life form.
b) Miller-Urey Theory (1953)
Miller and Urey produced an experiment to try and prove the origin of life. They took the
materials present on the earth at that time: methane, ammonia, water and hydrogen and
placed them in a flask. They exposed the flask to sparks to represent the sunlight and
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lightening on the earth at that time. They discovered that from such an experiment it was
possible to create organic compounds (amino acids) that could have been the beginning of life
on earth. These molecules would have provided food for any heterotrophs that evolved. It also
demonstrates a mechanisms for the production of molecules such as amino acids and RNA
needed for controlling any cells that could be produced by the conditions on the primitive
Earth.
 designed an experiment to test the Oparin - Haldane theory
 combined methane, ammonia, water vapor and hydrogen and exposed these
gases to an energy source simulating lightning.
 discovered that urea, amino acids, and other compounds were produced
 amino acids, the building blocks of proteins, make up most living things
2. Heterotroph hypothesis
 first organisms were heterotrophs which fed on plentiful organic compounds in
primordial soup
 As compounds were used up, early bacteria may have consumed each other. The lack
of food led to the evolution of autotrophs.
 as autotrophs carried out photosynthesis oxygen was produced and began to
accumulate in the atmosphere
 Earth’s atmosphere changed from a reducing atmosphere to an oxidizing atmosphere
 large amounts of oxygen led to the development of aerobic bacteria
 bacteria evolved by natural selection forming the first eukaryotic cell
 parts of cell nucleus pinched off to form mitochondria and chloroplasts
3. Symbiogenesis
This was developed as a result of observations of organelles, such as chloroplasts and
mitochondria, and it revealed that they were similar to prokaryotic cells. It is the belief that
through symbiotic relationships, these organelles become incorporated into eukaryotic cells
through partnerships that formed between cells. Through the relationship these organelles
became functional structures within the partner cells in return for nutrition, protection and so
on
 Lynn Margulis knew that mitochondria and chloroplasts in a eukaryotic cell both have
their own genetic material independent of the cell’s nucleus
 symbiosis ( organisms living together) is very common on earth
 Development of Eukaryotic cells and organelles could be a product of symbiogenesis:
The development of a new species through symbiosis.
 The theory is called the Serial Endosymbiosis Theory (SET)
 Millions of years ago an anaerobic bacterium ingested but did not digest an aerobic
bacterium
 Host gained because it could breath oxygen and the guest was protected from the
environment
 Over time the guest bacteria developed into the mitochondrion.
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
Margulis proposed a similar relationship for chloroplasts as a free-living photosynthetic
bacteria was ingested but not digested by other bacteria.
 The host gained a food source as the guest performed photosynthesis.
4. Panspermia theory
This theory suggests that life came from some other source outside of the earth and then
migrated to earth either through intelligent beings or by chance.
 life originated elsewhere in the universe and migrated to our planet
 supporters of this theory disagree on whether life was brought to our planet by
intelligent beings (suggested by Frances Crick) or whether it migrated by chance
(bacteria on meteorites).
5. GAIA theory
This comes from the Greek word, meaning “mother earth”. It was developed by James
Lovelock. It suggests that the earth, including all of its abiotic and biotic components may
constitute a huge, living, self-regulating system. It states that the biota (the sum of all
organisms) controls various properties of the atmosphere, ocean and lands
 proposed by James Lovelock
 views Earth as a ‘superorganism’ named Gaia which is regulated and maintained by life
on its surface
 Earth has systems in place to keep a dynamic balance and regulate atmosphere and
temperature, similar to a living organism’s homeostasis
 It relies on the theory that chemical evolution occurred to explain the origin of life and
then
 Gaia ‘came alive’ and began to regulate Earth processes as bacteria multiplied
6. Intelligent design theory
This is the concept that all biological origins on earth have followed a pattern which set out as a
product of some intelligent cause or agent. It maintains that life and its mechanisms are too
complex to have evolved by chance.
 life and the mechanisms of life are too complex to have evolved by chance
 it must have been directed by an unidentified supernatural form of intelligence
 supported by Behe and Dembski
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