Cold fission

Natural History, Nov, 2004 by Stephan Reebs

Zoologists of late have been debating whether many new vertebrate species evolved at the time of the last ice age--the Pleistocene epoch, between 2 million and 10,000 years ago. According to the standard view, the advances and retreats of the ice sheets, and the global cooling trends that accompanied them, caused habitats to become fragmented and populations to be isolated--conditions that typically foster the emergence of new species. Yet most studies, done on faunas that lived south of the ice itself, showed no proliferation of species. Now, by examining birds whose ancestors actually inhabited the icebound regions, Jason T. Weir and Dolph Schluter, both evolutionary biologists at the University of British Columbia in Vancouver, have gathered some strong new support for the old view.

The investigators selected pairs of closely related bird species from the Americas and measured how much the DNA of one species in each pair differed from the DNA of the other species. Those data enabled Weir and Schluter to calculate when each member of the pair branched off along its own path to specieshood. The calculations show that every one of the closely related species now living in boreal (northern coniferous) forests originated during the Pleistocene. By contrast, only about half the paired bird species now living in sub-boreal climes split from each other during that epoch (the other half have more ancient origins). Where glaciation was more intense, the authors suggest, selective pressures were greater and the populations more isolated: a surefire formula for speciation. ("Ice sheets promote speciation in boreal birds." Proceedings of the Royal Society of Loudon B 271:1881-87, September 22, 2004)