Sexual dimorphism and speciation on two ecological coins: patterns from nature and theoretical predictions.

Adaptive divergence of phenotypes, such as sexual dimorphism or adaptive speciation, can result from disruptive selection via competition for limited resources. Theory indicates that speciation and sexual dimorphism can result from identical ecological conditions, but co-occurrence is unlikely because whichever evolves first should dissipate the disruptive selection necessary to drive evolution of the other. Here, we consider ecological conditions in which disruptive selection can act along multiple ecological axes. Speciation in lake populations of threespine sticklebacks (Gasterosteus aculeatus) has been attributed to disruptive selection due to competition for resources. Head shape in sticklebacks is thought to reflect adaptation to different resource acquisition strategies. We measure sexual dimorphism and species variation in head shape and body size in stickleback populations in two lakes in British Columbia, Canada. We find that sexual dimorphism in head shape is greater than interspecific differences. Using a numerical simulation model that contains two axes of ecological variation, we show that speciation and sexual dimorphism can readily co-occur when the effects of loci underlying sexually dimorphic traits are orthogonal to those underlying sexually selected traits.

Parallel evolution of sexual isolation in sticklebacks.

Mechanisms of speciation are not well understood, despite decades of study. Recent work has focused on how natural and sexual selection cause sexual isolation. Here, we investigate the roles of divergent natural and sexual selection in the evolution of sexual isolation between sympatric species of threespine sticklebacks. We test the importance of morphological and behavioral traits in conferring sexual isolation and examine to what extent these traits have diverged in parallel between multiple, independently evolved species pairs. We use the patterns of evolution in ecological and mating traits to infer the likely nature of selection on sexual isolation. Strong parallel evolution implicates ecologically based divergent natural and/or sexual selection, whereas arbitrary directionality implicates nonecological sexual selection or drift. In multiple pairs we find that sexual isolation arises in the same way: assortative mating on body size and asymmetric isolation due to male nuptial color. Body size and color have diverged in a strongly parallel manner, similar to ecological traits. The data implicate ecologically based divergent natural and sexual selection as engines of speciation in this group.