A comparative test for divergent adaptation: inferring speciation drivers from functional trait divergence

Ecological differentiation between lineages is widely considered to be an important driver of speciation. However, support for this hypothesis is largely derived from the detailed study of a select set of model species pairs. Mounting evidence from non-model taxa, meanwhile, suggests that speciation often occurs with minimal differentiation in ecomorphology, calling into question the true contribution of divergent selection to species richness in nature. To better understand divergent adaptation and its role in speciation generally, researchers require a comparative approach that can distinguish its signature from alternative processes such as drift and parallel selection in datasets containing many species pairs. Here we introduce the first statistical models of divergent adaptation in the continuous traits of paired lineages. In these models, ecomorphological characters diverge as two lineages adapt toward alternative phenotypic optima following their departure from a common ancestor....