Year 10, General Science
Area of Study 2, Section 2 – Evolution
2.1 Evolution of a Theory
Nearly two million different kinds of organisms, plants, animals and micro-organisms, are known
to be currently living on Earth. More are being found each year. Many more organisms have come
and gone. Some, like the dinosaurs, are long extinct. The extinction of many others is far more
recent and a direct result of human activity. How did this tremendous diversity of life come to
exist on our planet? The theory of evolution suggests that all forms of life stem from the same
remote beginnings and that the different species we now know have developed gradually over
millions of years.
Surviving in different environments
Adaptations
Organisms are able to survive and breed in their environments because they are suited to them.
Specific structures, functions and behaviours increase their chances of surviving, at least until the
organism is able to reproduce. These characteristics are called adaptations.
They are inherited; that is, passed from parents to offspring. Adaptations take a variety of forms
but many can be classified as either:
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
structural (where the adaptation is physical)
functional (where the adaptation involves the internal function of an organism)
behavioural (where the adaptation involves the way an organism acts).
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Structural adaptations are physical adaptations. They can take many forms.
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Many animals are camouflaged to blend with
their background so that predators cannot
see them.
Some animals resemble objects such as
leaves, twigs or even bird droppings.
Some extremely colourful animals look like
they would be easy prey. These animals
usually sting, taste bad or are poisonous and
their bright appearance warns predators to
stay away.
 With some animals it is difficult for a
predator to tell which end is which. The
predator attacks the wrong end, giving the
prey a chance to escape.
 A tricky variation is the ‘mimic’. The
mimic is not dangerous to predators, but
has copied the colourings of another
dangerous animal, so that predators avoid
it.
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
Some animals have features that make them look larger and more frightening to predators;
for example, the neck frills of some lizards can be opened to make the head seem like that
of a much larger lizard.
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Functional adaptations affect how the internal functions of an animal work. Their internal
function changes depending on their immediate environment. For example, the chameleon
changes colour to blend with changing backgrounds.
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Behavioural adaptations involve the way an
animal acts. They too can take many forms.
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Some animals sit very still or move slowly to
avoid predators.
Others are active only at certain times of the
day or year to avoid unfavourable
conditions such as extremes of heat or
cold.
Some collect and store food for future use.
Many larger animals form herds to provide
some protection from predators.
Some have learnt to use tools to access food
that is hard to get at; for example,
chimpanzees commonly use broken twigs to
extract termites.
Adaptations serve many purposes. Arctic fish contain a kind of anti-freeze in their blood, allowing
them to survive in waters that would freeze the blood of other fish. The long mane of a male lion
makes it appear larger to opponents. This kind of adaptation for intimidation is common.
Intimidation also involves behaviours such as puffing out the chest and standing up as tall as
possible.
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Plants have adaptations too. The silvery
coloured, narrow-shaped leaves of the wattle
tree help reduce water loss by evaporation.
Some flowers achieve pollination by imitating
the shape, colour and smell of a female insect.
When male insects attempt to mate with the
flowers, they transfer pollen.
All organisms have adaptations that assist their
survival in their particular environment.
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Variation
The individuals within a species are very
similar; all Sumatran tigers, for example, are similar, as are all chimpanzees. They are not
identical, however; variation occurs within all species. Much of this variation comes from the
differences in genes and chromosomes that each individual inherits from their parents. Variation
can also result from genetic mutations and from environmental factors such as differences in diet,
availability of water and habitat.
Favourable characteristics
The survival of a species relies on at least some individuals producing offspring. The organisms
best suited to their environment are those most likely
to produce offspring. These are the
organisms that have favourable characteristics, enhancing their ability to survive and
reproduce. Their offspring
will inherit these favourable characteristics. Over several
generations, individuals with favourable characteristics will become the most common. In
contrast, those with less favourable characteristics will find the environment inhospitable. They
will be more likely to die before they get a chance to reproduce and so will become less common.
It can be said that favourable characteristics are selected.
Variation in a species is particularly important if environmental conditions change. While others
die, some individuals will have characteristics that are favourable, allowing them and the species
to survive the change.
Questions: Surviving in different environments
Question 1
survive.
Give an example of an adaptation and how it helps an animal and its species to
Question 2
Give three examples each of an adaptation that is:
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a structural b behavioural.
Question 3
Explain how two animals of the same species can be different.
Question 4 Jack rabbits, bilbies and fennec foxes all live in
desert habitats, have very large
ears and are nocturnal. Explain how their adaptations allow them to live in their desert habitats.
Question 5 Match the adaptations below to their survival value and the habitat in which they
are likely to occur:
Adaptation
Survival value
Habitat
body colour that
blends with the
background
avoidance of the
hottest parts of the
day
saltwater
production of small
volumes of
concentrated urine
avoids
dislodgement by
moving fluids
desert
hooks and suckers
on the head end of
the organism
enables waste
removal with
minimal water loss
rainforest
broad, flat, bright
green leaves
avoidance of
predators
intestines of a
sheep
live underground
by day, and are
active at night
maximum
absorption of
sunlight
any
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Theory of evolution
How is the huge variation in life forms explained? Where did they all come from?
Evolution is the gradual development of different species from a common ancestor. The theory
of biological evolution states that life on Earth has changed over time. Although the idea of a
gradual unfolding of life goes back to the ancient Greeks, the modern theory of biological
evolution has only been developed in the past 200 years.
Early theories of evolution
Up until the late 1700s, most scientists believed
that the different types of organisms and their
characteristics had been fixed for all time. This idea of the ‘fixity of species’ was questioned in the
late 1700s by the French naturalist Georges Buffon (1707–88), who suggested that species could
change. Erasmus Darwin (1731–1802), also suggested that one species could change to another,
but he had no evidence to support his ideas.
Lamarck’s theory
French naturalist Jean Baptiste Lamarck (1744–1829) was a
tutor of Buffon’s son and spent many years classifying plants and
invertebrates. He thought that the similarities and differences
between living things made sense only if species were evolving.
Lamarck believed that organisms adapted through their struggle
to survive. In 1809 he suggested that:
Question 1. organs are improved when used repeatedly and
weakened when not used. These changes are called
acquired characteristics. Working out at the gym develops the acquired characteristic of
larger muscles. Playing music or computer games develops the acquired characteristic of
improved hand–eye coordination
Question 2. any improvements (or weaknesses) developed
due to the environment are passed
on through reproduction to the next generation of individuals. Lamarck
suggested
that the long necks of
giraffes, for example,
developed
because
giraffes stretched their
necks to reach food
high in the trees. The
acquired
characteristic of a long
neck
was
then
inherited by their
offspring,
which
would
then be born
with long necks. Lamarck’s theory isn’t true of course, because if it were bodybuilder parents would give birth to
children who would develop equally muscular bodies. Also, an amputee should give birth to
babies lacking the limb the parent had lost.
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Darwin’s theory
In 1831 aged 22, Charles Darwin (1809–82) abandoned his
studies in medicine and theology (religion) to become a naturalist.
He took a position as naturalist on
the HMS Beagle, a ship
commissioned to survey and chart the coast of South America. For
the next five years, Darwin observed the geographical
distribution
of plants, animals, fossils and rocks in various
parts
of the world. He puzzled over the enormous variety and
adaptations of the organisms he saw, and became convinced that
different species of the same animal developed from a common
ancestral type.
On the Galapagos Islands, about 1000 kilometres off the coast of Ecuador, Darwin marveled at
the diversity in the flowers, tortoises, iguana and birds found there. Much of the wildlife differed
in small but significant ways from island to island and to those on the mainland.
The islands were effectively
isolated from one another by
strong ocean currents, and there
were no winds blowing from one
island to another. Darwin found
fourteen species of finches, all
with similar colourings, calls,
nests, eggs and courtship
displays. They differed, however,
in habitat, diet, body size and
beak shape.
Darwin believed these fourteen
species had come from a common
ancestor, and proposed the
process of natural selection to
explain it.
He suggested that a few finches
had arrived at
the islands at
some time in the past. These
finches showed natural variation
in their beak shape. On one island,
those with beaks of one shape
were better
able to feed on the
cacti found there. Finches with
other beak shapes found it
difficult to survive. On other
islands, other beak shapes gave
some finches a feeding advantage.
The birds most suited to their island survived to produce offspring, which inherited that beak
shape. This is sometimes called survival of the fittest. The ‘fittest’ were the birds that were able
to feed and reach breeding age. The beak type that gave particular birds on a particular island an
advantage was ‘selected for’. Over many generations, the birds on different islands became
sufficiently different from each other to be recognised as a different species.
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Darwin spent the next twenty years collecting and sorting evidence
for his natural selection theory of evolution. Meanwhile, another
naturalist, Alfred Russel Wallace (1823–1913), was developing a
similar theory. Wallace realised that natural selection would
‘improve’ the species, later stating that ‘the inferior would inevitably
be killed off and the superior would remain’. Wallace had reached a
conclusion similar to Darwin’s: that evolution occurs by natural
selection.
In 1858, Darwin and Wallace together presented
a paper on their theories of evolution and natural
selection. A year later Darwin published “On the
Origin of Species by Natural Selection or Preservation of Favoured Races in the
Struggle for Life” in England. Religious leaders throughout England
denounced his work as against the word of God since the Bible said that man
was formed in the image of God, so how could man have apes as ancestors?
Darwin’s explanation that evolution occurs through natural selection is
regarded as one of the most important theories of science and is still regarded as being essentially
correct. Darwin was not the first to suggest evolution, but
he was the first to give it a scientific
explanation. At that time there was no understanding of genetic
inheritance. Darwin was, therefore, unable to explain the source
of the variation in species that his theory was based on.
Darwin’s theory can be restated in terms of modern genetics.
Evolution is natural
selection based upon
the natural genetic
variation that appears
in all populations.
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Questions
Theory of evolution
Question 1.
Explain what is meant by the term ‘evolution’.
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Question 2.
Evolution can only ever be considered a theory. Explain why.
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Early theories of evolution
Question 3.
How does Lamarck explain the evolution of the giraffe’s long neck?
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Question 4. It is easy to prove Lamarck’s theory is wrong. Give an example that proves
Lamarck’s theory wrong.
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Darwin’s theory
Question 5. Darwin observed fourteen species of finches on the Galapagos Islands. Give two
possible explanations for this large number of species.
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Question 6.
True or false?
a Darwin was the first to think of the idea of evolution.
b Darwin’s theory depended on the developments of acquired characteristics.
c Darwin suggested that differences in finches were due to mutations.
d Darwin published his theory many years after his return on HMS Beagle.
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Question 7. Explain how the long- legged, tree-grazing animal shown in Figure 3.1.14 might
have evolved according to:
a Lamarck’s theory
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b Darwin’s theory.
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Question 8.
Are the following characteristics acquired or inherited?
a a suntan
b black hair
c the athletic ability of a gymnast
d high resistance to a bacterial infection
e blue eyes.
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Year 10, General Science
2.2 Evolution Explained
Natural selection
We have discussed Natural selection, which is the process by which the environment ‘selects’
favourable characteristics, reducing the chance of unfavourable characteristics. This means that
after many generations of selection, a species becomes better adapted to its environment.
Individuals will become highly adapted if their environment does not change. Except for the effect
of mutations, each individual will be very similar since the amount of variation will have declined.
However, environments do change, imagine if it suddenly got colder. Some individuals within
the
species may naturally be more tolerant to the cold, having thicker coats or some other favourable
characteristic. In other words they are better suited to the new, colder conditions than the rest of
their species and over time, natural selection would increase the proportion of individuals with
this tolerance of the cold.
Natural selection takes several generations to become evident and so it is extremely difficult to
observe in large plants and animals. It is more obvious in organisms that reproduce quickly.
Bacteria and insects are two organisms in which natural selection can occur quickly enough to be
observed.
Selection of peppered moths
Over the last 150 years, dramatic changes have
been seen in the
populations of peppered moths in England. Originally,
populations of the peppered moth, were mostly light-coloured. In
the mid-1800s, however, scientists noticed that populations were
changing to mostly dark- coloured forms.
The change occurred during the Industrial Revolution when coalburning factories produced large amounts of black soot, which
blackened nearby trees. When sitting on the soot-darkened trees,
the light- coloured form of the moth was easily seen by birds, their
main predator. The dark-coloured moth blended with the
blackened background, increasing its chances of survival. The
dark colour is an inherited characteristic. Hence, more dark-coloured moths survived to produce
dark-coloured offspring.
After
clean
air
regulations
were
implemented,
lichen
began to regrow on tree
trunks and the
trees
returned to their original
paler colouring.
Moth
populations in many of
these areas have shifted
back towards the lightcoloured forms. Natural selection seems to have taken the
moths from pale to dark and back to pale again.
Selection and rabbit control
Rabbits overran the land in Australia for many years, digging
burrows, stripping vegetation and causing erosion. In December 1950, the myxoma virus was
released in Australia to control the booming rabbit population. Carried by fleas and mosquitos,
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the virus caused the disease myxomatosis and within two months, 90 per cent of rabbits in certain
areas had died. Less than 1 per cent of rabbits infected with the virus survived. Ten years later,
only 25 per cent of rabbits in those same areas died as a result of the virus, and around 40 per
cent of those infected with the virus survived. These dramatic changes were the result of natural
selection acting on both the rabbits and the virus.
It is probable that a few rabbits had a natural, genetic resistance to the myxoma virus. These
resistant rabbits would have survived the initial myxoma spread and produced offspring with an
inherited resistance. A healthy rabbit may produce seven or more litters of young per year and so
the number of resistant rabbits would have increased dramatically within a few years. The
myxoma- resistant rabbits were selected for.
Selection and Viruses
Many bacteria are now resistant to certain types of antibiotics. When penicillin was first
introduced it was very effective in treating infections caused by the bacteria Staphylococcus
aureus, known as golden staph. Now, MRSA (methicillin resistant Staphylococcus aureus) is
resistant to penicillin and around twenty other substances, including other antibiotics, antiseptics
and disinfectants. Recently, several strains of MRSA have become resistant to the drug of lastresort—vancomycin. If vancomycin fails, the death rate from MRSA will rise dramatically.
Speciation
A species is defined as a group of organisms that
normally interbreed in nature to produce fertile
offspring. The formation of a new species is called
speciation. It can happen through natural selection
combined with other factors such as isolation and
genetic mutations. Speciation occurs over long
periods of time and generally cannot be seen in a
human lifetime or even through the recorded
history of humans. A possible mechanism for
speciation is shown in Figure 3.2.6.
Step 1: Geographic isolation
Geographic isolation is the first step in speciation.
Figure 3.2.6 shows a population of rabbits being
split into two physically and geographically
isolated groups. Each group will now experience a
different set of circumstances—food type and
availability might differ, as might climate and the predators that live there.
Step 2: Natural selection at work
Although initially the same species, each population will change over many generations through
natural selection and the occasional genetic mutation. Eventually the two rabbit populations will
have their own characteristics, sufficiently different from each other to be called a variety, or
subspecies. Subspecies appear different but are still capable of interbreeding.
Step 3: Reproductive isolation
If the populations are isolated long enough, the change might be sufficient to make them incapable
of interbreeding. They will then have reproductive isolation. At this point a new species has
emerged.
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Factors that might cause reproductive isolation are:
• a change in colour patterns so that mates are no longer recognised
• a change in mating habits so that mates are no longer recognised
• seasonal differences in mating times
• a changed chromosome which prevents the sperm of one group from fertilising eggs of the other.
Questions
Natural selection
Question 1. Give an example of natural selection and explain how it makes a species ‘stronger’
in its environment.
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Question 2. Use the peppered moth to explain what is meant by: a natural variation
b natural
selection.
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Question 3.
Describe how it is thought the MRSA bacteria developed.
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Question 4. Why it is easier to observe natural selection in action with bacteria and insects
rather than larger animals?
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Speciation
Question 5. Give two reasons why isolated populations of a species may evolve differently
from one another.
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Question 6. Two animals are different, but very similar to each other. How can you tell if the
animals are of the same species?
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Types of evolution
Speciation illustrates how new species
might arise from a common ancestor. Does it
account for the huge variety of life forms
that exist now, and those that have come
before us? Different evolutionary paths and
mechanisms have been suggested.
Divergent evolution
The Galapagos Island finches and the
geographically isolated rabbits illustrate the
idea that new forms
can evolve from a
single ancestor. This is known as divergent
evolution. The idea is that new species will
evolve in new environments.
Divergent
evolution
results
in
a
phenomenon known as adaptive radiation.
Ancestral organisms became adapted to their new environments, evolving into new forms suited
to them. Australia’s marsupial ancestors have evolved and radiated into many different forms,
from tree-dwelling, fruit-eating possums into everything from blind, meat-eating underground
moles to the more familiar kangaroos and koalas.
Convergent evolution
Evolution can produce similar structures in organisms of quite
different origins. For example, Australia’s different marsupials
show resemblances to cats, wolves, moles, mice and squirrels
even though they are not closely related and do not have a
common ancestor.
Convergent evolution, or convergence, occurs when
organisms evolve and develop similar adaptations due to:


living in similar environments and habitats
having similar lifestyles and food sources.
In similar habitats, the same types of characteristics are selected Convergent evolution: Despite having
for, resulting in organisms that look similar despite having very quite different ancestors, the shark,
different genes passed ichthyosaur and dolphin have evolved
down
from
very a similar set of characteristics—
streamlined body, bilobed tail, fins
different
ancestors. and flippers
These organisms may
even have analogous
structures; that is, specific body parts that appear
similar. One example is the gliding membrane found
between the front and
rear limbs of Australia’s gliding
possums. Similar membranes are found in the flying
squirrels of North America, Europe and Asia, and in the
flying lemurs of South-East Asia. These three animals
Convergent evolution: Australian
marsupials and placental mammals of
are similar in their lifestyle—they are all nocturnal
other continents have many similarities,
herbivores.
but are not closely related.
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Parallel evolution
Another type of evolution is parallel evolution, which
occurs where related species evolve similar features
while separated from each other. The result is organisms
that look alike and have common ancestry, but are found
in different locations. For example, Old and New World
monkeys share many features because they come from
common ancestors. Their tails, however, indicate
different evolutionary paths: New World monkeys live in
the trees and have prehensile tails that hold onto the
branches; Old World monkeys evolved to live on the
ground and do not have prehensile tails.
Questions, Types of evolution
1 State three different types of evolution.
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2 What is divergent evolution and what conditions are
required for it to occur?
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3 Give analogous structures that are shared by:
a gliding possums, flying squirrels and flying lemurs
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b sharks and dolphins.
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4 Explain how Old and New World monkeys are examples of parallel evolution.
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Thinking Questions
1
Mosquitoes carrying the disease yellow fever have developed a resistance to chemical
pesticides once sprayed to kill them. Explain how this may have happened.
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2
Explain why a population that is very homogeneous is more likely to be at risk of
extinction than a population that has considerable variation.
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2 New breeds or subspecies of domestic animals are regularly created by breeders through
artificial selection. It takes far longer for natural selection to do the same. Suggest why.
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3 Suggest three events that might lead to geographic isolation of a population.
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5
Rearrange the following events in the order in which they occur in the process of
speciation: reproductive isolation natural selection formation of a species further natural
selection geographic isolation formation of a subspecies.
6
Which of the following diagrams could represent the processes of:
a. divergent evolution
b. convergent evolution
c. parallel evolution?
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7
The African aardvark and the South American anteater have similar feet and tongues, but
they are not closely related.
a What type of evolution best describes these similarities?
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b Suggest why the these similarities developed.
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