Reinforcement alone does not explain increased reproductive isolation in sympatry

Understanding mechanisms underlying the origin of species remains a central goal of biology. Comparative studies of reproductive isolation (RI) have observed increased premating, but not postzygotic, RI between sympatric species pairs (Coyne and Orr 1989a, 1997), supporting a significant role for reinforcing natural selection in speciation. Limited data on patterns of RI have inhibited our ability to extend comparative analyses beyond model systems. Here we use an updated Drosophila dataset and measures of postzygotic RI from lepidopterans and toads ( Bufo ) to assess patterns of postzygotic RI in a comparative framework. Postzygotic RI is higher between sympatric species pairs of all three divergent taxa, including between some recently diverged pairs. Using the updated Drosophila dataset that also includes estimates of premating RI, we demonstrate that comparisons of postzygotic and premating RI depend greatly on divergence time. Both postzygotic and premating RI are strengthened between recently diverged sympatric Drosophila species pairs, as defined by Coyne and Orr (C&O) (Coyne and Orr 1989a) and others (i.e., D Nei < 0.5). However, at slightly lower divergence thresholds only premating RI is significantly increased, highlighting the importance of inclusion criteria when performing comparative analyses of RI. While reinforcing natural selection against maladaptive hybridization can strengthen premating RI in sympatry, it is unlikely to contribute significantly to increases in postzygotic RI that we observe. We discuss mechanisms that may combine with reinforcement to generate increased RI in sympatry.