population size). Frequent forward mutation (U with solid arrow) prevents complete fixation of the wild-type allele, shifting the distribution to the left. A lower rate of reverse mutation (V with dashed arrow) increases the frequency of the wild-type allele a small amount, shifting the distribution to the right. A wider distribution of allele frequencies among independent populations is expected when 4NU and 4NS take intermediate values as shown in the middle panel of Fig. 11.3A. In that case, the forces of selection, mutation, and genetic drift (all solid arrows) are approximately equal to each other so that replicate populations exhibit a range of wild-type allele frequencies. In the right-hand panel of Fig. 11.3A the distribution of allele frequencies is horseshoe-shaped (most replicate populations near fixation or loss) when 4NU and 4NS are small because genetic drift is strong (solid arrows for-) relative to natural selec-

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