Species interactions, divergence, and the rapid evolution of ecological sexual dimorphism in threespine sticklebacks

Variation in ecological sexual dimorphism (ESD), defined as differences between the sexes in ecologically-relevant traits, is a common feature of adaptive radiation, yet its causes remain unclear. Competition between the sexes for alternative resources can promote evolution of ESD when interspecific competition is reduced (competition hypothesis). Alternatively, sex-specific selection on ecological traits might weaken under strong directional selection in new environments (divergence hypothesis). We tested these hypotheses via their expected evolutionary outcomes in threespine stickleback populations from southwestern Canada. We found striking among-population variation in magnitude of ESD. Consistent with the divergence hypothesis, dimorphism along the main axis of body shape variation was reduced in recently derived freshwater populations compared to their contemporary marine ancestor. However, dimorphism only weakly declined with increasing phenotypic divergence. Average dimorphism along major freshwater body shape axes was similar between solitary populations and those coexisting with a competing species, contrary to the competition hypothesis. Instead, sympatry with a Benthic ecotype led to increased sexual size dimorphism in Limnetics, and total shape dimorphism was elevated in the sympatric stickleback species compared with solitary populations. In contrast to the mechanisms considered in existing theory, interactions in sympatry might produce elevated ESD by generating novel sex-specific selection pressures.