Prolonged geographic separation in ecologically similar environments can drive new species

Trait data from more than 1,000 different vertebrate species suggests that new species often arise not because of pressures from different environments, but because of prolonged geographic separation in ecologically similar environments. The findings contrast the classical view of divergent adaptation as the dominant driver of early speciation. While it is widely agreed upon that most speciation events require an allopatric phase – a period in which a species population becomes geographically separated long enough to have it diverge into a distinct new species – whether ecological divergence drives this critical allopatric phase is unknown. To better understand the role divergent ecological adaptation plays in allopatric speciation, Sean Anderson and Jason Weir combined new data from 129 allopatric sister pairs of bird species (speciating or recently speciated taxa) with 14 other published allopatric sister pair datasets for various other birds, mammals and amphibian species. They used the trait data to model the relative contribution of divergence adaptation to allopatric divergence. Anderson and Weir discovered that divergent ecological adaptation is a relatively minor force during allopatric divergence and that most species tend to evolve under similar selective pressures. “Using new models to analyze sister-pair trait differences, we find that adaptive ecological divergence to be the exception rather than the rule in vertebrates,” write the authors. “This result contradicts the classical idea that divergent adaptation initiates the earliest stages of speciation, and it supports an emerging picture in which new species commonly arise despite minimal ecological divergence.”