GENETIC SLIPPAGE IN RESPONSE TO SEX

It is widely held that sexual reproduction enhances the potential for evolution by expanding the range of variation on which selection can act. However, when nonadditive gene action contributes to the expression of traits under selection, sexual reproduction can also result in slippage of the mean genotypic value in the direction contrary to selection. We show how the magnitude of genetic slippage depends on the extent to which segregation and recombination create maladapted genotypes. We also show how random mating can induce a change in the expressed genetic variance for a quantitative trait by eliminating Hardy-Weinberg disequilibria and reducing gametic-phase disequilibria. Depending on whether genes of like effects are in repulsion or coupling disequilibrium, this change will be positive or negative. Thus, depending on the mode of gene action and the form of the selection function, sexual reproduction can either enhance or impede the short-term response of quantitative characters to selection. Although this issue is relevant to all sexual populations, it is most easily investigated in species that infrequently engage in sex, since prolonged phases of clonal propagation can greatly exaggerate genetic disequilibria. We describe a population of the cyclical parthenogen Daphnia pulex in which sexual reproduction induced average changes in the means of life-history characters equivalent to approximately one-tenth of a phenotypic standard deviation. Contrary to the usual expectation, sex also caused a significant reduction in the expressed genetic variance for several traits in this population. A large fraction of the genetic variance in Daphnia appears to be due to dominance, and in the study population, clonal selection appears to cause a buildup of coupling disequilibrium between genes and gene combinations of like effects.