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Drought Drove Mexican Bird Species Apart

Researchers have found new proof for the theory that natural barriers, such as mountain ranges or dry areas, can cause the birth of new animal and plant species. In a paper published in today's issue of Science, they present evidence that when the Isthmus of Tehuantepec in Mexico dried out, it split up dozens of bird populations that now live on either side of the dry area. Although these separated populations occupy similar ecological niches, many have diverged into different species.

Biologists since Darwin have argued about how species are born. Recently researchers have looked favorably on a version of Darwin's own idea: that populations of a single species can separate when they change their ecology--for instance, adapting to different temperatures or food resources. But another theory holds that when a new geographical barrier splits a population into two, they may accumulate genetic differences and eventually become unable to interbreed.

A team led by ornithologist A. Townsend Peterson of the Natural History Museum of the University of Kansas, Lawrence, recognized the potential of the dry Mexican lowland to test speciation theories. The 300-kilometer-wide strip was once forested, but climate changes left it scrub-dry by 100,000 years ago, interrupting the ranges of forest species to the north and south.

The team identified 37 pairs of sister species on either side of the isthmus and searched museum records to find out where specimens of each species were collected. They used these location data to define each species' ecological niche based on four conditions: temperature, precipitation, elevation, and vegetation. They then plugged these parameters into a computer program to determine the potential geographic range of each species.

Peterson tested whether the observed ecological niche of one species could predict the niche and range of its sister. For all 37 species pairs, the answer was yes. For instance, the ecological parameters favored by a blue mockingbird also predicted the range of its counterpart south of the isthmus, a blue-and-white mockingbird. This means that each species' niche remained stable throughout the speciation process, showing that "speciation is taking place simply because of geographic isolation," says Peterson.

The idea of geographical speciation is well understood, but well-documented examples are rare. "Showing it the way they did ... is pretty clever," says Robert Zink, curator of birds at the Bell Museum in St. Paul, Minnesota. Even so, he and others warn against dismissing the role of ecology in speciation, because the method here needs refinement and there are counterexamples in which ecological differences have driven populations apart into species.