St

where ST is twice the variance in allelic sizes in the total population and SW is twice the average of the within subpopulation variance in allelic sizes (Slatkin 1995; Goodman 199 7). The states of the alleles then influence the perceived amount of population subdivision. Alleles with states further apart (greater variance in state) are counted more heavily in the estimate of population structure since they are less likely to be recently identical by descent (multiple stepwise mutations would be required to make a large change in state). In contrast, alleles with very similar states (less variance in state) make a smaller contribution to the estimate of population subdivision since they are more likely to be recently identical by descent but changed in state due to mutation. Using the stepwise mutation model and RST accounts for high rates of mutation that can give the appearance of more or less gene flow than has actually occurred. Table 5.5 gives hypothetical genetic data from two subpopulations, illustrating the degree of population subdivision under the infinite alleles and stepwise mutation models.

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