Long-term climate changes have led to a higher turnover of species in northern regions like Canada than in more diverse regions like the Amazon, according to a new study by Canadian researchers.

The genetic diversity in tropical climates such as rainforests is likely due to a lower extinction rate and not, as commonly thought, because of a faster rate of evolution in those regions, according to University of British Columbia zoology PhD candidate Jason Weir and professor Dolph Schluter.

Polar bears are most closely related to grizzly bears, with the two species having split apart 300,000 years ago.
(Jonathan Hayward/Canadian Press)

Weir and Schluter charted the genetic family tree of 618 mammal and bird species across the Americas, looking for clues into why the tropics have so much greater genetic diversity.

It's been assumed this diversity was because of a faster rate of speciation — the process whereby one species splits into two — Weir told CBC News Online. But it turns out the opposite is true, he said.

A DNA analysis of sister species — pairs of closely-related species in a genetic family tree, like the polar bear and grizzly bear — found that species studied actually evolved faster as they moved away from the equator.

"It takes about three and a half million years on average for one species to split into two in the tropics," said Weir. "But that process can happen in as little as one million years in the Arctic."

The polar bear and grizzly bear, for example, split apart 300,000 years ago, while South America's bush dog and maned wolf — another pair of sister species — diverged some eight million years ago.

"The results suggest there's a much quicker turnover as you move away from the equator, and we suspect it's because of climatic fluctuations," he said. "In tropical regions, there's more stability and so less of a need to diversify."

The same climatic fluctuations that spur genetic diversity also contribute to higher extinction rates, which, Weir said, is the more likely reason for the greater number of species in the tropics.

Weir cautioned the results did not suggest the tropical regions are immune to fluctuations in climate. Human-induced impacts on previously stable regions could lead to higher extinction rates and global warming would likely lead to an even quicker turnover of species in Arctic regions, he said.

The study will appear in Friday's issue of the journal Science.