Reinforcement alone does not explain increased reproductive isolation in sympatry

Comparative studies of reproductive isolation (RI) by Coyne and Orr (1, 2) and others indicate a significant role for reinforcing natural selection in Drosophila speciation. The reinforcement hypothesis predicts increased prezygotic, but not postzygotic, RI between sympatric species pairs in response to maladaptive hybridization. We revisit this hypothesis and others using additional Drosophila, Lepidopteran, and toad (Bufo) data. In contrast to the predictions of reinforcement, we find increased premating and postzygotic RI between sympatric Drosophila species, including between recently diverged species pairs, as defined by Coyne and Orr (1) and others (i.e., DNei < 0.5). However, at slightly lower divergence thresholds increased postzygotic RI in sympatry is not statistically significant, while increased premating RI is, generally in agreement with the predictions of reinforcement. While premating data are unavailable, postzygotic RI is also increased between sympatric Lepidopteran and toad (Bufo) species. We find only modest support for "concordant asymmetries" in premating and postzygotic RI between sympatric Drosophila, described by others as uniquely supporting reinforcement. Finally, the proportion of geographic range overlap shared by species is positively associated with the magnitude of premating RI as predicted by reinforcement, but it is also positively associated with postzygotic RI, which cannot be explained by reinforcement. Taken together, our results demonstrate that comparisons of premating and postzygotic RI in sympatry depend greatly on divergence time, and suggest that fusion, extinction, and/or other mechanisms must combine with reinforcement to generate these patterns. Significance StatementUnderstanding mechanisms underlying the origin of species remains a central goal of biology. By combining laboratory estimates of premating and postzygotic reproductive isolation (RI) with phylogenetic hypotheses, geographic range data, and genetic divergence estimates, the fundamental meta-analyses of Coyne and Orr provided compelling support that reinforcement contributes significantly to Drosophila speciation. Using additional Drosophila data, we demonstrate increased premating and postzygotic RI in sympatry. Postzygotic RI is also increased in sympatry between Lepidopteran and toad species. We also find only modest support for patterns previously described as uniquely supporting reinforcement. Together, our analyses demonstrate that comparisons of premating and postzygotic RI depend on divergence time, and suggest that fusion, extinction, and/or other mechanisms combine with reinforcement to produce these patterns.