Evolution__3rd_Edition

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Populations may be subdivided
CHAPTER 5 / The Theory of Natural Selection 129
higher at high frequencies, where the local birds are well educated about the dangers of
eating the warningly colored forms.
The purpose of Sections 5.11–5.13 has been to illustrate the different mechanisms by
which natural selection can maintain polymorphism. In Chapter 6 we look at another
mechanism that can maintain polymorphism a genetic drift. Then, in Chapter 7, we
tackle the question of how important the mechanisms are in nature.
5.14 Subdivided populations require special population
genetic principles
5.14.1 A subdivided set of populations have a higher proportion of
homozygotes than an equivalent fused population: this is
the Wahlund effect
So far we have considered population genetics within a single, uniform population. In
practice, a species may consist of a number of separate populations, each more or less
isolated from the others. The members of a species might, for example, inhabit a number
of islands, with each island population being separated by the sea from the others.
Individuals might migrate between islands from time to time, but each island population
would evolve to some extent independently. A species with a number of more or
less independent subpopulations is said to have population subdivision.
Let us see first what effect population subdivision has on the Hardy–Weinberg
principle. Consider a simple case in which there are two populations (we can call them
population 1 and population 2), and we concentrate on one genetic locus with two
alleles, A and a. Suppose allele A has frequency 0.3 in population 1 and 0.7 in population
2. If the genotypes have Hardy–Weinberg ratios they will have the frequencies, and
average frequencies, in the two populations shown in Table 5.10. The average genotype
frequencies are 0.29 for AA, 0.42 for Aa, and 0.29 for aa. Now suppose that the two
Table 5.10
The frequency of genotypes AA, Aa, and aa in two populations when A has frequency 0.3 in
population 1 and 0.7 in population 2. The average genotypes are calculated assuming the two
populations are of equal size.
Genotype
AA Aa aa
Frequency (0.3) 2 = 0.09 2(0.3)(0.7) = 0.42 (0.7) 2 = 0.49 population 1
(0.7) 2 = 0.49 2(0.7)(0.3) = 0.42 (0.3) 2 = 0.09 population 2
Average 0.58/2 = 0.29 0.84/2 = 0.42 0.58/2 = 0.29