Comparing rates of molecular and morphological evolution identifies multiple speciation trajectories in a diverse radiation of skinks

There is increasing recognition that the process of species divergence is not uniform across the tree of life, and that newly diverged taxa may differ in their levels of phenotypic and genetic divergence. We investigate the relationship between phenotypic and genetic differentiation across the speciation continuum using sister pairs from a large ecologically diverse radiation of Australian skinks, the Tribe Eugongylini, a high-quality alignment of genomic sequence data, and morphometric data for 90 lineages across the radiation. Based on the framework proposed by Struck et al. (2018) for comparative study of species divergence, we used latent class regression to test for multiple speciation "trajectories." We found evidence for multiple relationships between genetic divergence and morphological disparity for recently diverged sister taxa, which we summarize into 2 broad patterns. One of these patterns is characterized by relatively rapid morphological differentiation for pairs with greater disparity in environmental variables, consistent with expectations of ecological speciation. The second pattern shows accumulation of both morphological and genetic differences in proportion to each other, consistent with gradual speciation. Our study shows how heterogeneity in speciation processes can be captured in a comparative framework.