Evolution__3rd_Edition

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5 The
Theory of Natural
Selection
T his chapter introduces formal population genetic models.
We first establish what the variables are that the models
are concerned with, and the general structure of population
genetic models. We look at the Hardy–Weinberg equilibrium,
and see how to calculate whether a real population fits it. We
then move on to models of natural selection, concentrating
on the specific case of selection against a recessive
homozygote. We apply the model to two examples: the
peppered moth and resistance to pesticides. The second
half of the chapter is mainly about how natural selection
can maintain genetic polymorphism. We look at
selection–mutation balance, heterozygous advantage,
and frequency-dependent selection; and we finish by
looking at models that include migration in a geographically
subdivided population. The theory in this chapter all
assumes that the population size is large enough for random
effects to be ignored. Chapters 6 and 7 consider how random
effects can interact with selection in small populations.