The role of divergent ecological adaptation during allopatric speciation in vertebrates

After decades of debate, biologists today largely agree that most speciation events require an allopatric phase (that is, geographic separation), but the role of adaptive ecological divergence during this critical period is still unknown. Here, we show that relatively few allopatric pairs of birds, mammals, or amphibians exhibit trait differences consistent with models of divergent adaptation in each of many ecologically relevant traits. By fitting new evolutionary models to numerous sets of sister-pair trait differences, we find that speciating and recently speciated allopatric taxa seem to overwhelmingly evolve under similar rather than divergent macro–selective pressures. This contradicts the classical view of divergent adaptation as a prominent driver of the early stages of speciation and helps synthesize two historical controversies regarding the ecology and geography of species formation. Description Similar but separate species Speciation often requires a period of allopatry, when populations are separated long enough to diverge into distinct species. Sister species may occupy different niches, but whether ecological divergence occurs during or after allopatric speciation is poorly understood. Anderson and Weir used trait data on more than 1000 pairs of sister taxa, including birds, mammals, and amphibians, to model trait divergence over time. They found few examples of clear divergent adaptation, with greater support for a model of sister taxa evolving under similar selective pressures toward similar trait optima. —BEL Trait data suggest that sister taxa of birds, mammals, and amphibians that diverged in isolation mostly experienced similar selection pressure.