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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. Sex-specific selection on ecological traits might weaken under strong directional selection in new environments (divergence hypothesis). Competition between the sexes for alternative resources can promote evolution of ESD when interspecific competition is reduced (competition 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 repeatedly reduced in recently derived freshwater populations compared to their contemporary marine ancestor. However, dimorphism declined only weakly with increasing phenotypic distance from the ancestral state. Average dimorphism along major body shape axes was similar between solitary populations and those coexisting with a competitor, contrary to the competition hypothesis. Instead, sympatry with Benthics led to increased sexual size dimorphism in Limnetics, and total shape dimorphism was elevated in the sympatric stickleback ecotypes compared with solitary ones. In contrast to the mechanisms considered by existing theory, species interactions in sympatry might generate novel sex-specific selection pressures, producing elevated ESD