Divergent sexual selection via male competition: ecology is

hybridization; intrasexual aggression; reproductive isolation; speciation. Sexual selection and ecological differences are important drivers of specia-tion. Much research has focused on female choice, yet the role of male com-petition in ecological speciation has been understudied. Here, we test how mating habitats impact sexual selection and speciation through male compe-tition. Using limnetic and benthic species of threespine stickleback fish, we find that different mating habitats select differently on male traits through male competition. In mixed habitat with both vegetated and open areas, selection favours two trait combinations of male body size and nuptial colour: large with little colour and small with lots of colour. This matches what we see in reproductively isolated stickleback species, suggesting male competition could promote trait divergence and reproductive isolation. In contrast, when only open habitat exists, selection favours one trait combina-tion, large with lots of colour, which would hinder trait divergence and reproductive isolation. Other behavioural mechanisms in male competition that might promote divergence, such as avoiding aggression with heterospe-cifics, are insufficient to maintain separate species. This work highlights the importance of mating habitats in male competition for both sexual selection and speciation