SEXUAL SELECTION FOR HOMOGAMY IN THE GERRIDAE: AN EXTENSION OF RIDLEY'S COMPARATIVE APPROACH

Ridley (1983) proposed that three factors are necessary and sufficient for the development of positive assortative mating (homogamy) in natural populations. These factors are 1) a correlation between male size and success in competition for mates (intrasexual selection), 2) a correlation between female size and fecundity, and 3) prolonged pairing. In the presence of these three factors, selection will favor males that select large (and thus more fecund) females as mates, thus leading to positive assortative mating. Although this simple hypothesis is generally supported by data across taxa (Ridley, 1983), the evidence for the Insecta is equivocal at best. In this paper, I test this hypothesis using three species of waterstriders (Gerridae; Hemiptera). The three factors and homogamy are measured in several samples from each species. Homogamy is predicted and observed in one species (Gerris remigis) and is not predicted and not observed in the other two (G. comatus and G. buenoi). These results support Ridley's hypothesis and suggest that the processes leading to homogamy in the insects do not, in general, differ from those dominant in other taxa. The possible influence of even low levels of homogamy on genetic variance within populations and the difficulties of assessing such levels on the basis of single samples are discussed.