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ANSWER SHEET
9
Environmental change and evolution
1. Outline Darwin’s theory of evolution by natural selection.
Darwin suggested that there was variation between members of the same species and those
individuals that were better adapted to an environment survived to breed. Their advantageous
characteristics were passed on to their offspring. The numbers of individuals less well-adapted to
the environment decreased, while those well-adapted increased.
2. Describe some of the selection pressures that affect the sizes of populations.
Competition for the available food and living space between different species and within the same
species occurs. Within species, competition for mates occurs. Other selection pressures could
involve the abiotic factors in the environment, such as the climate, the topography and specific
factors in aquatic environments, such as oxygen concentration. Any advantageous characteristics of
organisms that enable them to be more successful in a particular environment will increase their
population numbers.
3. Explain how the distribution of the peppered moth illustrates natural selection in action.
From the results of the experiments, it appears that there is greater survival of the pale peppered
moth in rural, unpolluted areas, than in the industrialised areas, where the melanic form is more
numerous. The selection pressure is predation. The pale forms are more obvious against the darker
backgrounds in the industrialised areas and so get eaten. The melanic forms survive in greater
numbers here and breed, producing more dark forms. In rural areas, some melanic forms survive
and breed, but not in such large numbers.
4. Suggest what is meant by the term ‘survival of the fittest’.
Fitness means the ability to survive and produce viable offspring. The fittest individuals in a
population will be those that can compete for, and obtain, the resources they need to be able to
survive and reproduce. These individuals may possess advantageous characteristics that enable
them to get more food, avoid predators or get the best choice of mates: they will survive and less
well-adapted individuals will not.
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5. Describe stabilising, directional and disruptive selection.
This can be partly answered by reference to the diagram on page 105 (Figure 8.5) of Genetics,
Evolution and Biodiversity. Stabilising selection favours the mean of the distribution. Directional
selection favours one extreme of the range of characteristics, e.g. the peppered moth. Disruptive
selection favours both extremes and appears to select against intermediates.
6. Explain what is meant by geographical isolation.
Geographical isolation occurs when two demes are separated by a physical barrier, such as a range
of mountains, a river or an ocean. This prevents interbreeding between the two populations and
each then develops its own gene pool, in which allele frequencies may change, resulting in changes
in genotype and eventually changes in phenotype.
7. List three isolating mechanisms that prevent mating (prezygotic), explaining how each operates.
Behavioural isolation: courtship rituals differ so that mating does not occur between different
populations.
Ecological isolation: populations occupy different habitats in one environment.
Seasonal isolation: mating occurs at different times of the year.
Mechanical isolation: differences in genitalia prevent successful copulation.
8. What is meant by postzygotic reproductive isolation?
Postzygotic reproductive isolation refers to the failure of the zygote produced to develop or live to
produce viable offspring. It applies to situations where mating has occurred and fertilisation has
been achieved, but the offspring do not live long enough to reach maturity, fail to produce viable
gametes or their offspring are sterile.
NAS Biology O nline T eac her s ’ G uid e
Nels on T hornes © 2002
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