— A male masked tityra (Tityra semifasciata), one of the tropical species included in the study, at a nesting hole in a snag. It diverged from its sister species the black-tailed Tityra about 4 million years ago [Image courtesy of Jason Weir]
Contrary to popular belief, the 'hot spots' of evolution are actually quite cool: a study suggests that new species emerge more frequently in temperate regions than in the tropics.
Scientists had assumed that new species develop faster in the tropics, since they are home to greater species diversity than at higher latitudes. But the researchers behind the new analysis say the explanation for this is that fewer species have gone extinct near the equator.
Jason Weir and Dolph Schluter at the University of British Columbia in Vancouver, Canada, compared how animal species across the Americas have evolved. They looked at pairs of 'sister species', which share an immediate common ancestor and so are the most closely related species.
Gene testing allowed them to estimate how long ago each pair of sister species diverged from one another. A small number of genetic differences between two species indicates that they diverged relatively recently, a larger number suggests they diverged longer ago.
Weir and Schluter compared the divergence time of various bird and mammal species in tropical regions of the Americas stretching roughly from the middle of Mexico to the border between Bolivia and Argentina to that of species in higher latitudes.
For example, while the closely related Baltimore oriole and black-backed oriole from North America split about 200,000 years ago, the white-edged oriole and spot-breasted oriole from Central and South America split about 4.3 million years ago. The scientists looked at 191 pairs of bird species and 118 pairs of mammal species.
They found that near the equator, sister species evolved into separate groups about 3.4 million years ago on average. By comparison, those in temperate regions split apart roughly 1.7 million years ago. None of the sister species they reviewed in very high latitudes above the Arctic Circle separated more than a million years ago.
These figures indicate that new species form more frequently in cooler, temperate regions than tropical ones, says Weir.
"It's somewhat unexpected. For the past 50 years people believed species formation was fastest in the tropics because diversity is highest there," says Weir. "It was kind of a shock to find the rate of species splitting into two was actually higher in temperate regions."
Researchers speculate that the dramatic climatic changes in temperate regions over the past hundreds of thousands of years have provided more opportunity for new species to form than the relatively stable climate of tropical regions.
"When the glaciers finished retreating 12,000 years ago there were a lot of 'job openings' in temperate regions," explains James Valentine of the University of California, Berkeley, US, referring to the new niches species could inhabit. When groups of animals break off from the main population to inhabit a new territory they are more likely to develop into a separate species.
Sink or swim
But Weir says that such dramatic climate change has also made survival difficult for species in temperate areas. "We think it's a double-edge sword it drives both speciation and extinction," Weir says.
Valentine notes that the new analysis assumes that sister species evolved in their present-day locations. In fact, he says, some of the animals that now inhabit North America could have evolved in tropical areas. "They don't know exactly where those species are coming from," he explains. This casts some doubt on the idea that more species evolve at higher latitudes.
Also, because many animals in the tropics have not yet been identified, it is possible that there are unknown sister species for some of the tropical birds and mammals included in the analysis. So although Weir and Schluter found that sister species split apart about 3.4 million years ago on average near the equator, this number might be far lower.
Despite these uncertainties experts have welcomed the new analysis. "At this point this is the best that can be done. It's a pretty good sample set," says Kaustuv Roy at the University of California, San Diego, US.
Journal reference: Science (DOI: 10.1126/science.1135590)