nearly recessive mutation will take a very long time to increase in frequency under natural selection. In contrast, a completely or nearly dominant mutation with the same fitness as a homozygote and starting at the same frequency will increase in frequency very rapidly. The examples in Fig. 6.6 where the initial frequency of the A allele is 0.05 are equivalent to a new mutation in a population of Ne = 10.

Heterozygote disadvantage

The results of natural selection acting against the heterozygote phenotype, a situation known as heterozygote disadvantage, underdominance for fitness, or disruptive selection (see Table 6.4), are shown in Fig. 6.7. Starting from an initial allele frequency of p = 0.4, the top panel shows how the aa homozygote eventually reaches fixation over time. The bottom panel requires close attention in this case, since the equilibrium allele frequency depends strongly on the initial allele frequency in the population. Initial allele frequencies above p = 0.5 all lead to fixation of the AA homozygote while all initial allele frequencies below p = 0.5 lead to fixation of the aa homozygote. When the initial allele frequency in the population is exactly p = 0.5, allele frequencies remain constant over time. It turns out that this equilibrium point is not robust to any change in allele frequency, and so is called an unstable equilibrium. Any slight change in allele frequency will result in the allele frequencies changing to alternative stable equilibrium points of fixation or loss. Such an unstable equilibrium is very unlikely to persist in a finite population, since even a slight amount of genetic drift would alter allele frequencies in the population toward one of the stable equilibrium points.

Heterozygote advantage

The results of natural selection acting to increase the frequency of the heterozygous genotype, commonly

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