BioEd Online

Most people tend to think of the tropics as the hottest scene on the planet when it comes to spawning new life. But Canadian zoologists have found that it is actually the world's temperate zones where new species evolve and become extinct the fastest.

The discovery by Jason Weir and Dolph Schluter of the University of British Columbia in Vancouver threatens to overturn the theory that because tropical regions contain the greatest overall species diversity, that they must also have the fastest rates of 'speciation' — the emergence of new species.

"Our findings contradict the conventional view by suggesting that temperate zones, and not the tropics, are the hotbeds of speciation," says Weir.

Sisters

The researchers surveyed 309 pairs of 'sister' species — those that are closely related to one another, much like humans and chimpanzees — from throughout the Americas. They compared their DNA sequences to work out how much the sister species had diverged from one another, and therefore how long ago their split had occurred.

Those in temperate zones tended to have diverged more recently, implying that new species are being thrown up faster in these regions. Near the Equator, sister species were separated by an average of 3.4 million years, whereas at the most extreme latitudes studied, stretching as far as the northern wilds of Canada, the figure was less than 1 million years, the researchers report in Science¹.

The apparently prodigious rate at which new species appear and disappear in temperate regions might be due to the cycle of ice ages and warm periods, which affect extreme latitudes more than the tropics, Weir suggests. "Intense climatic instability at high latitudes has resulted in increased opportunities for extinction, and increased ecological opportunity during the benign periods," he says.

By contrast, the relatively unchanged climate of the tropical region over millennia has meant that once species gain a foothold they are less likely to become extinct.

The current warming of the planet from greenhouse gases is also changing climate conditions more at extreme latitudes. But the results of this study apply only to more dramatic changes that happened over a longer time scale; they do not cast any light on how today's species might be affected by future climate change.

Life aplenty

Although the birth and death of species happens at a more leisurely pace in the tropics, these hot climes might still be considered as biological hotspots: they do contain the greatest number of species at any given time. This is because tropical conditions are so ripe for life, and resources are so abundant, that more species can flourish without crowding one another out.

Weir and Schulter's study focused only on birds and mammals, and it remains to be seen whether similar patterns will be seen for other animals and for plants. But it seems that even on evolutionary timescales, the colder it gets, the more life becomes nasty, brutish and short.