Ecological divergence exhibits consistently positive associations with reproductive isolation across disparate taxa

Abstract

To what degree is the divergent adaptation responsible for life's phenotypic variety also responsible for generating the millions of species that manifest this variation? Theory predicts that ecological divergence among populations should promote reproductive isolation, and recent empirical studies provide support for this hypothesis in a limited number of specific taxa. However, the essential question of whether ecology plays a truly general role in speciation has yet to be systematically evaluated. Here we address this integral issue using an approach that adds an ecological dimension to comparative studies investigating the relationship between reproductive isolation and divergence time. Specifically, we quantify ecological divergence for >500 species pairs from eight plant, invertebrate, and vertebrate taxa and statistically isolate its association with reproductive isolation. This approach demonstrates a highly consistent and significant positive association between ecological divergence and reproductive isolation across taxa. This relationship was also observed across different aspects of ecological divergence and components of reproductive isolation. These findings are highly consistent with the hypothesis that ecological adaptation plays a fundamental and taxonomically general role in promoting reproductive isolation and speciation.

Fig. 1.

Method illustration. (A) A hypothetical data set, illustrating the distribution of species comparisons (data points) with respect to the three aspects (axes) of evolutionary differentiation analyzed here. GD is used as a surrogate for time. (B) A hypothetical best-fit line illustrating the predicted positive association for the relationship between ED and residual RI upon statistical removal of the contributions of time by regression of RI on GD. (C) An individual analysis of actual data that illustrates the predicted pattern, in this case, of a positive association between habitat divergence and postmating isolation for the angiosperm data set (see Table 1).