Evolution__3rd_Edition

134 PART 2 / Evolutionary Genetics
Summary
1 In the absence of natural selection, and with random
mating in a large population in which inheritance is
Mendelian, the genotype frequencies at a locus move
in one generation to the Hardy–Weinberg ratio; the
genotype frequencies are then stable.
2 It is easy to observe whether the genotypes at a locus
are in the Hardy–Weinberg ratio. In nature they will
often not be, because the fitnesses of the genotypes are
not equal, mating is non-random, or the population is
small.
3 A theoretical equation for natural selection at a single
locus can be written by expressing the frequency of a
gene in one generation as a function of its frequency in
the previous generation. The relation is determined by
the fitnesses of the genotypes.
4 The fitnesses of the genotypes can be inferred from
the rate of change of gene frequency in real cases of
natural selection.
5 From the rate at which the melanic form of the
peppered moth replaced the light-colored form, the
melanic form must have had a selective advantage of
about 50%.
6 The geographic pattern of melanic and lightcolored
forms of the peppered moth cannot be explained
only by the selective advantage of the better
camouflaged form. An inherent advantage to the
melanic form, and migration, are also needed to
explain the observations.
7 The evolution of resistance to pesticides in insects
is in some cases due to rapid selection for a gene at a
single locus. The fitness of the resistant types can be
inferred, from the rate of evolution, to be as much as
twice that of the non-resistant insects.
8 If a mutation is selected against but keeps on arising
repeatedly, the mutation settles at a low frequency
in the population. It is called selection–mutation
balance.
9 Selection can maintain a polymorphism when
the heterozygote is fitter than the homozygote and
when fitnesses of genotypes are negatively frequency
dependent.
10 Sickle cell anemia is an example of a polymorphism
maintained by heterozygous advantage.
11 Subdivided populations have a higher proportion
of homozygotes than an equivalent large, fused
population.
12 Migration, in the absence of selection, rapidly
unifies gene frequencies in different subpopulations;
and it can maintain an allele that is selected against in a
local subpopulation.
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