It seems like a no-brainer: To find out where most new species arise, see where most of them live. Take the tropics, home of more than half the known organisms on the planet. For a plant or animal to form a new species, something must divide its population so that individuals go their separate ways and develop unique adaptations over time. The barrier needn’t be physical: When the polar bear split from the Grizzly bear about 300,000 years ago, for example, scientists think a change in climate drove them apart. But as climate can create, it also can destroy. Harsh environments can wipe out new species that can’t adapt. Pondering that dual role led zoologists Jason Weir and Dolph Schluter of the University of British Columbia in Vancouver, Canada, to wonder whether Earth’s poles were really anathema to speciation.
The pair studied 309 pairs of bird and mammal sister species (the most closely related pair from a common ancestor) living from the tropics to the poles. DNA analysis revealed that, on average, birds and mammals near the equator diverged from a common ancestor 3.4 million years ago; in contrast, those near the poles diverged less than 1 million years ago. That means new species pop up more frequently at high latitudes than they do at low ones.