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Modern Genetic Analysis Chapter 17. Population and Evolutionary Genetics Selection
Genetics In Process17-6: The effect of selection on allele frequency

Suppose that a population is mating at random with respect to a given locus with two alleles and that the population is so large that (for the moment) we can ignore inbreeding. Just after the eggs have been fertilized, the zygotes will be in Hardy-Weinberg equilibrium:

andp2 + 2pq + q2 = (p + q)2 = 1.0, where p is the frequency of A.

Further suppose that the three genotypes have probabilities of survival to adulthood (viabilities) of WA/A:WA/a:Wa/a. For simplicity, let us also assume that all selective differences are differences in survivorship between the fertilized egg and the adult stage. Differences in fertility give rise to much more complex mathematical formulations. Then among the progeny once they have reached adulthood, the frequencies will be

These adjusted frequencies do not add up to unity, because the W's are all fractions smaller than 1. However, we can readjust them so that they do, without changing their relation to each other, by dividing each frequency by the sum of the frequencies after selection (

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