Figure 3.7 Probability distributions for binomial random variables based on samples of N = 20 from populations where the allele frequency is 0.50, 0.75, or 0.95 (dark blue, white, and light blue bars, respectively). The range of probable outcomes with sampling depends on the allele frequency. As allele frequencies approach the boundaries of fixation or loss, there is a decreasing number of outcomes other than fixation or loss that are probable due to sampling error.

drift in changing allele frequencies is also smaller. This explains the tendency of populations to go to fixation or loss under genetic drift. The sampling effect is greatest when the genetic variation is greatest but also weakest when genetic variation is least (Fig. 3.8). A population is most likely to experience larger changes in allele frequencies, toward fixation or loss, due to drift when both alleles are near equal frequencies. However, a population with strongly unequal allele frequencies is less likely to experience genetic drift of a magnitude that would equalize allele frequencies.

Genetic drift effect

Genetic drift effect

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