SEXUAL SELECTION AND THE EVOLUTION OF ALLOMETRY FOR SEXUAL SIZE DIMORPHISM IN THE WATER STRIDER, AQUARIUS-REMIGIS

Sexual size dimorphism (SSD) typically increases with body size (hyper allometry) in taxa in which males are the larger sex and decreases wit h body size (hypoallometry) in taxa in which females are larger. We de monstrate the commonality of these trends, both of which indicate grea ter evolutionary divergence in male size than in female size and stron g covariation between the sexes. We postulate that both of these compo nents of allometry evolve in response to sexual selection on male size coupled with genetic correlations for size between males and females, and we argue that this hypothesis can be generalized to taxa in which females are the larger sex. For such taxa, we predict hypoallometry f or SSD, sexual selection on male size, and a correlation between the i ntensity of sexual selection and male size. An analysis of total lengt h in 31 populations of the water strider, Aquarius remigis, demonstrat es significant hypoallometry for SSD. Comparisons of mating and single males within 12 populations reveal significant positive univariate or multivariate selection gradients in nine populations and a significan t correlation between the intensity of sexual selection and mean male size, when environmentally based variation in mean size among sites is removed. These results provide the first quantitative evidence that a llometry for SSD may evolve in response to sexual selection favoring l arge males, even in taxa in which females are the larger sex.