Contributions of natural and sexual selection to the evolution of premating reproductive isolation: a research agenda

•There are important gaps in our understanding about the role of adaptation in speciation.•To fill these gaps, we clarify and define differences between sexual and natural selection.•We further clarify variation in models of sexual selection, a source of perennial confusion to biologists.•Sexual and natural selection likely interact during the evolution of reproductive isolation.•We provide a conceptual framework for testing hypotheses about the relative significance of sexual and natural selection in speciation.•Progress in speciation research will be made by testing the relative significance of natural and sexual selection in the evolution of premating reproductive isolation. Speciation by divergent natural selection is well supported. However, the role of sexual selection in speciation is less well understood due to disagreement about whether sexual selection is a mechanism of evolution separate from natural selection, as well as confusion about various models and tests of sexual selection. Here, we outline how sexual selection and natural selection are different mechanisms of evolutionary change, and suggest that this distinction is critical when analyzing the role of sexual selection in speciation. Furthermore, we clarify models of sexual selection with respect to their interaction with ecology and natural selection. In doing so, we outline a research agenda for testing hypotheses about the relative significance of divergent sexual and natural selection in the evolution of reproductive isolation.