To understand how new species form and what causes their collapse, we examined how reproductive isolation evolves during the speciation process, considering species pairs with little to extensive divergence, including a recently collapsed pair. We estimated many reproductive barriers in each of five sets of stickleback fish species pairs using our own data and decades of previous work. We found that the types of barriers important early in the speciation process differ from those important late. Two premating barriers-habitat and sexual isolation-evolve early in divergence and remain two of the strongest barriers throughout speciation. Premating isolation evolves before postmating isolation, and extrinsic isolation is far stronger than intrinsic. Completing speciation, however, may require postmating intrinsic incompatibilities. Reverse speciation in one species pair was characterized by significant loss of sexual isolation. We present estimates of barrier strengths before and after collapse of a species pair; such detail regarding the loss of isolation has never before been documented. Additionally, despite significant asymmetries in individual barriers, which can limit speciation, total isolation was essentially symmetric between species. Our study provides important insight into the order of barrier evolution and the relative importance of isolating barriers during speciation and tests fundamental predictions of ecological speciation.

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Sexual selection and ecological differences are important drivers of speciation. Much research has focused on female choice, yet the role of male competition in ecological speciation has been understudied. Here, we test how mating habitats impact sexual selection and speciation through male competition. 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 combination, 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 heterospecifics, are insufficient to maintain separate species. This work highlights the importance of mating habitats in male competition for both sexual selection and speciation.

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