The Effect of Plant Size Variation on Genetic Drift in Populations of Annuals

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Abstract
It is shown through a reformulation of an equation developed by Kimura and Crow (1963) that the variance in allele frequency change between generations (the drift variance) can be partitioned into two components: one resulting from random gamete sampling and one from the variation in potential fecundity among adults. Estimates of potential fecundity variances, derived from estimates of size or seedset variances, are presented for populations of 34 species of annual plants. For most of these populations, fecundity variation among adults accounts for the majority of the drift variance. In some cases, fecundity variation increases the magnitude of drift by nearly an order of magnitude relative to gamete sampling alone. Fecundity variation contributes to genetic drift because correlations between genotypes and fecundities are randomly generated in a finite population. The magnitude of these correlations, and therefore of the resulting drift variance depends on the size of the parental population rather than the size of the sample.