Surprising Pace of Evolution and Extinction Revealed

A male Masked Tityra (Tityra semifasciata) at nesting hole in dead snag is one of the tropical species included in this study of sister species ages. Its sister species, the Black-tailed Tityra, diverged from it about 4 million years ago. (Image credit: Jason Weir)

New species of birds and mammals evolve faster at high latitudes than in the tropics, but they also go extinct faster, a new study suggests.

The finding, detailed in the March 16 issue of the journal Science, could help explain why biodiversity in the tropics is so much greater compared with other parts of the world.

Researchers Jason Weir and Dolph Schluter of the University of British Columbia in Canada mapped the genetic family tree of more than 300 mammal and bird species in the Americas over the past 10 million years.

The researchers focused on so-called sister species [image], which are closely related pairs of species that share an immediate common ancestor. They found that the process by which one species splits into two, called speciation, occurs faster in temperate zones than in the tropics.

“It would take one species in the tropics 3 to 4 million years to evolve into two distinct species, whereas at 60 degrees latitude, it could take as little as one million years,” Weir said.

Higher extinction rates Yet, there are more than 10 times more species in the tropics than in the Arctic, and several times as many more species in the tropics than in temperate zones. If speciation occurs so rapidly at high latitudes, the number of different species should also skyrocket over time. But that’s not the case: The increased speciation rate at higher latitudes is counteracted by higher extinction rates due to more frequent and severe climate changes, Weir said.

Over the past 2 to 3 million years, changes in glacial coverage have drastically altered North America’s landscape, and thus the ecosystem. Organisms that couldn’t survive the abrupt changes died out, leaving an empty niche for other species to populate. Plus, the changing landscape provided a mosaic of habitats that might have promoted speciation by splitting up species, forcing them to diverge.

“These more intense pressures are occurring on species at high latitudes because of all this climatic fluctuation, and this might drive faster evolution,” Weir told LiveScience.

Moving in

The overall picture at higher latitudes is one of high species turnover. The climate changes drive extinction, but they also promote speciation, Weir said. “What we’re suggesting is that there’s a lot of extinction up here at high latitudes, and when species go extinct, there’s opportunities for new species either to form or move in from the tropics.”

James Valentine, a University of California, Berkeley, biologist who was not involved in the study, says the new results are “plausible,” but he points out an alternative explanation for why more species are found in the tropics versus other parts of the world.

“I think it’s a matter of preclusion,” Valentine said in a telephone interview. “It’s just hard to invade higher latitudes because the guys already there are very well-adapted, and it’s a bad environment, so moving in is tough.”

In other words, species looking to move to higher latitudes from the tropics face stiff competition from resident species already well-adapted for the harsh climate.

Not contradictory

The new study is the latest to examine evolution rates in the tropics versus other parts of the world. One study last year found that evolution is actually occurring faster in the tropics than at high latitudes, at least for plants, while another that found evolution rates are equivalent in tropic and temperate environments, at least for tree frogs.

All of these findings are not necessarily contradictory, however, since they involve different groups of organisms and look at different slices of geologic time, said John Wein, an evolutionary ecologist at Stony Brook State University of New York involved in the tree frog study.

The current study, for example, only looked at species that diverged up to 10 million years ago, while the tree frog study looks back about ten times further, Wein said.