Reproductive isolation, reproductive mode, and sexual selection: empirical tests of the viviparity-driven conflict hypothesis.

A central goal in evolutionary biology is to elucidate general mechanisms and patterns of species divergence. The viviparity-driven conflict (VDC) hypothesis posits that intense mother-embryo conflict associated with viviparity drives rapid reproductive isolation among viviparous species, is intensified by multiple paternity, and reduces female reliance on precopulatory cues in mate choice. We tested these predictions using comparisons of oviparous and viviparous fishes. Consistent with the VDC hypothesis, we found that, relative to oviparous species, only closely related viviparous fishes are known to hybridize. Also in support of the VDC hypothesis, we found that (1) elaborate male sexual ornamentation may be more common in viviparous species with relatively low levels of maternal provisioning of embryos compared with those with high levels of provisioning and (2) the degree of multiple paternity is higher in viviparous species than in oviparous species. In contrast to a prediction of the VDC hypothesis, we found no relationship between the degree of multiple paternity and elaborate male sexual ornamentation, although statistical power was quite low for this test. Whereas overall our results strongly support the central tenet of the VDC hypothesis-that reproductive mode affects rates of evolution of reproductive isolation and the strength of sexual selection-they cannot rule out two alternative models we propose that may also explain the observed patterns.