Figure 6.8 The change in the genotype and allele frequencies when there is overdominance for fitness and natural selection favors individuals with Aa genotypes. From any initial allele frequency the population converges on a maximum frequency of heterozygotes. This corresponds to equal allele frequencies with random mating. The colored, dashed line in the bottom panel corresponds to the allele frequencies in the top panel. In this illustration wAA = wa. Genotype frequencies assume random mating.

dominant allele and near loss of the recessive allele. Selection against a heterozygote also results ultimately in fixation of one allele and loss of the other allele. These three forms of natural selection all produce an equilibrium with little or no genetic variation, known as a monomorphic equilibrium. In contrast, when heterozygotes have the highest fitness natural selection maintains both alleles in the population at equilibrium, resulting in a polymorphic equilibrium. Thus, overdominance for fitness is one type of natural selection that is consistent with the permanent maintenance of genetic variation in populations.

The allele frequencies expected at equilibrium with overdominance can be obtained from equation 6.23, as shown in Math box 6.2. The equilibrium allele frequencies are where s and t are the selection coefficients against the AA and aa homozygotes, respectively (see Table 6.4). The equilibrium allele frequency is higher for the allele in the homozygous genotype that has the smaller selection coefficient (or higher relative fitness).

The strength of natural selection

The strength of selection against a genotype can vary from weak, such as a viability 0.1% less than the most fit genotype, to very strong, such as 50% viability or even zero viability (lethality) of a genotype. Allele frequencies over time (starting from the same initial allele frequency) are plotted in Fig. 6.9 for a wide range of selection coefficients in the case of natural selection against a homozygous recessive genotype. Notice that the shapes of the curves in the top and bottom panels of Fig. 6.9 are very similar but that the time scale of each plot is very different. Selection coefficients of 10% or greater bring the dominant allele to high frequencies within 100 generations. In contrast, reaching these same allele frequencies takes 10,000 generations when the

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