Page 55 - CB1.3 Genetic Varation sciPAD

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CB 1.3 GENETIC VARIATION
GENETICS OF POPULATIONS
Types of Selection
Generally, favourable phenotypes will be more common in a population, while unfavourable
ones will be less common. As a result, when we plot a graph to show the frequency of different
phenotypes, it usually follows a normal distribution.
Populations are rarely stable, and natural selection is always at work. This means the frequency
of different traits is constantly changing. Natural selection can do different things: it may favour
the existing average trait (stabilising selection), push the average trait in a specific direction
(directional selection), or work against the average trait (disruptive selection).
When selection pressure acts against
the two extremes of a trait, the
median phenotypes are retained
and become more common.
When selection pressure acts
against one extreme of the trait, the
population’s trait distribution shifts
toward the other extreme.
When selection pressure acts against
the median phenotype it results
in the two extremes of the curve
creating their own smaller curves.
1. Using the example of the moth population above, discuss why the population is likely to
undergo stabilising selection. 





2. Peahens choose their peacock mates by the size and shape of their tails. This sort of
selection pressure would lead to: (circle the correct answer)
(a) Stabilising selection (b) Directional selection (c) Disruptive selection
3. Human babies with a low birth weight are prone to more disease and medical complications.
Large babies are difficult for mothers to deliver safely. Potentially, both of these can lead
to premature death of the baby. This sort of selection pressure is called: (circle the correct
answer)
(a) Stabilising selection (b) Directional selection (c) Disruptive selection
Number of moths
These moths are very light
coloured and have a higher
chance of being eaten. They
are less likely to survive and
pass on their genes.
These moths are too dark
and have a higher chance
of being eaten. They are less
likely to survive and pass on
their genes.
These moths are a similar colour
to tree bark. They are well hidden
and are less likely to be eaten.
Colour of moths
Median colour
Light coloured
(low frequency)
Dark colour
(low frequency)
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