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Both premating and postmating barriers to gene flow can contribute to reproductive isolation but the relative role of these factors, particularly in the early stages of speciation, is not well understood. Evidence suggests that factors contributing to assortative mating and, thus, the development and maintenance of divergent species, can be ecology-dependent. Here, we present results from a study of assortative mating between recently diverged anadromous and freshwater sticklebacks conducted in semi-natural conditions. Sympatric anadromous and freshwater sticklebacks were sampled from a contact zone and multiple male and female morphs were allowed to breed in replicate ponds. Mate choice was determined using genetic markers to assign parents to offspring. Contrary to laboratory-based studies of sticklebacks, after allowing for differences in the propensity of the morphotypes to mate, we found no evidence of assortative mating. Furthermore, there was no evidence of size assortative mating, but rather a general female ‘preference’ to mate with large males was revealed. Analysis of the propensity of different morphotype combinations to produce fry provided indirect evidence that exogenous factors may influence hybrid survival. Our results indicate that hybridization between anadromous and freshwater sticklebacks from the River Tyne, Scotland, occurs readily, and that sexual selection is unlikely to contribute to premating isolation. From these results we infer that postmating isolation (hybrid inferiority) has preceded the evolution of premating isolation (assortative mating) in this population of sticklebacks.


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