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 nearly a century, researchers have assumed that new species are constantly popping up here, while speciation is far more stagnant at Earth's relatively deserted poles. But a new study claims the opposite: Species evolve much more readily at higher latitudes. It's just that the new arrivals die off so fast that most of them never get counted.
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, the team reports tomorrow in Science. Biologists had assumed that the temperate weather near the equator was a boon to speciation, the authors note, but these mild conditions may just be keeping old species around longer. Harsh conditions near the poles, in contrast, kill off many of the new species that arise, creating the illusion of less speciation.
"It's a surprising result," says phylogenetic biologist John Wiens of Stony Brook University in New York. "I think it'll get people talking."