Yseult Héjja-Brichard, Julien P Renoult, Tamra C Mendelson

A preference for mating with conspecifics over heterospecifics is fundamental to the maintenance of species diversity in sexually reproducing organisms. This type of positive assortative mating results in sexual isolation, a reduction in gene flow between species due to differences in mate choice. Sexual isolation is often stronger between closely related sympatric species as compared to allopatric species, because of processes such as reinforcement. In animals, this pattern has been demonstrated across the genus Drosophila, however, most empirical evidence for stronger species-level assortative mating in sympatry comes from studies of two or only a few species. In addition, traditional theories of sexual selection suggest that because reproduction is more costly to females, they should be the choosier sex. This prediction was supported in Drosophila, but it remains unclear whether this is a general trend. We therefore conducted a comparative analysis of mate preferences in sympatric and allopatric species of darters (genus Etheostoma). We performed a meta-analysis of 16 studies, encompassing 21 species. As expected, we found stronger preferences for conspecifics over heterospecifics across studies and species. Contrary to studies in Drosophila, however, we did not find an effect of sympatry or sex on the strength of preference for conspecifics. We offer several testable hypotheses to explain the variation we observed in the strength of assortative preference.