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where if equals the frequency of G2 at equilibrium. This final equation tells us that the allelic frequency at equilibrium is determined solely by the forward and reverse mutation rates.

Summary of effects When the only evolutionary force acting on a population is mutation, allelic frequencies change with the passage of time because some alleles mutate into others. Eventually, these allelic frequencies reach equilibrium and are determined only by the forward and reverse mutation rates. When the allelic frequencies reach equilibrium, the Hardy-Weinberg law tells us that genotypic frequencies also will remain the same.

The mutation rates for most genes are low; so change in allelic frequency due to mutation in one generation is very small, and long periods of time are required for a population to reach mutational equilibrium. For example, if the forward

23.9 Change due to recurrent mutation slows as the frequency of p drops. Allelic frequencies are approaching mutational equilibrium at typical low mutation rates. The allelic frequency of C1 decreases as a result of forward (C1 :C2) mutation at rate ^ (.0001) and increases as a result of reverse (C2 : C1) mutation at rate v (.00001). Owing to the low rate of mutations, eventual equilibrium takes many generations to be reached.

and reverse mutation rates for alleles at a locus are 1 X 10—5 and 0.3 X 10—5 per generation, respectively (rates that have actually been measured at several loci in mice), and the allelic frequencies are p = .9 and q = .1, then the net change in allelic frequency per generation due to mutation is:

= (1 X 10—5)(.9) — (.3 X 10—5)(.1) = 8.7 X 10—6 = .0000087

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