The consequences of fluctuating selection for isozyme polymorphisms in Daphnia.

Temporal sequences of allele frequencies in natural populations of Daphnia are analyzed to obtain the mean and variance of the selection coefficient for both asexual and sexual phases. In general, the alleles at enzyme loci appear to be quasi-neutral. Although significant variation exists for the estimated selection coefficients, the means are in all cases close to zero. Estimates of the variance of selection intensity are applied to existing models to demonstrate the implications of fluctuating selection for the spatial and temporal distribution of gene frequencies in Daphnia. The empirical and analytical results are shown to provide a possible solution to some previously puzzling aspects of Daphnia population genetic surveys. Neither genetic drift nor diversifying selection are necessary conditions for the local diversification of gene frequencies.