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

Sexual size dimorphism (SSD) is often attributed to sexual selection, particularly when males are the larger sex. However, sexual selection favoring large males is common even in taxa where females are the larg er sex, and is therefore not a sufficient explanation of patterns of S SD. As part of a more extensive study of the evolution of SSD in water striders (Heteroptera, Gerridae), we examine patterns of sexual selec tion and SSD in 12 populations of Aquarius remigis. We calculate univa riate and multivariate selection gradients from samples of mating and single males, for two sexually dimorphic traits (total length and prof emoral width) and two sexually monomorphic traits (mesofemoral length and wing form). The multivariate analyses reveal strong selection favo ring larger males, in spite of the female-biased SSD for this trait, a nd weaker selection favoring aptery and reduced mesofemoral length. Se lection is weakest on the most dimorphic trait, profemoral width, and is stabilizing rather than directional. The pattern of sexual selectio n on morphological traits is therefore not concordant with the pattern of SSD. The univariate selection gradients reveal little net selectio n (direct + indirect) on any of the traits, and suggest that evolution away from the plesiomorphic pattern of SSD is constrained by antagoni stic patterns of selection acting on this suite of positively correlat ed morphological traits. We hypothesize that SSD in A. remigis is not in equilibrium, a hypothesis that is consistent with both theoretical models of the evolution of SSD and our previous studies of allometry f or SSD. A negative interpopulation correlation between the intensity o f sexual selection and the operational sex ratio supports the hypothes is that, as in several other water strider species, sexual selection i n A. remigis occurs through generalized female reluctance rather than active female choice. The implications of this for patterns of sexual selection are discussed.