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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Year 10, General Science
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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Year 10, General Science
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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