Sexual size dimorphism (SSD) typically increases with body size (hyperallometry) in taxa in which males are the larger sex and decreases with body size (hypoallometry) in taxa in which females are larger. We demonstrate the commonality of these trends, both of which indicate greater evolutionary divergence in male size than in female size and strong covariation between the sexes. We postulate that both of these components 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 for SSD, sexual selection on male size, and a correlation between the intensity of sexual selection and male size. An analysis of total length in 31 populations of the water strider, Aquarius remigis, demonstrates 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 significant 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 allometry for SSD may evolve in response to sexual selection favoring large males, even in taxa in which females are the larger sex.
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