Contacts:
Robert Zink, department of ecology, evolution and behavior, (612) 624-7207 Deane Morrison, University News Service, (612) 624-2346
U of Minnesota research puts glacier theory of evolution on ice
How did our continent end up with two species of bluebirds, eastern and western? Or eastern and western species of orioles, buntings and several other songbirds that grace our lives with their songs and, in some cases, glorious plumage? For a long time, glaciers were credited. As the theory went, the glaciers that invaded the Midwest during the late Pleistocene epoch (the Pleistocene lasted from about 2 million to 10,000 years ago) separated the songbird populations into eastern and western groups that went on to develop into different, but closely related, species while the glaciers were keeping them geographically isolated.
Similarly, glaciers have been regarded as catalysts for the multiplication of species around the globe. Now, new research at the University of Minnesota casts doubt on this general theory and points to a more complicated evolutionary history for birds and other animals. Working with DNA from 35 pairs of songbird species--each pair being "sister species," or each other's most closely related species--evolutionary biologists John Klicka and Robert Zink have found evidence that new bird species have been evolving at a rather constant rate for at least 5 million years. The work will be published in the Sept. 12 issue of Science.
"Songbirds evolved in a protracted series of splittings of ancestral species, not a recent 'pulse,'" said Zink, a professor in the department of ecology, evolution and behavior and curator of birds at the university's Bell Museum. "Glaciers were likely suspects--convenient environmental perturbations--but they're not the whole story. Lots of species are thought to have originated when their ancestors were split into separate populations by glaciation. We think a few textbooks might have to be rewritten to reflect the more complicated nature of the evolutionary process."
To determine how long the pairs of bird species had been separated, the researchers examined DNA from the birds' mitochondria, the tiny membrane-enclosed engines that generate most of the energy used by cells. Mitochondrial DNA (mtDNA) is believed to mutate at a relatively rapid but steady rate, and mutations spread throughout a species through breeding. If two populations of a given species have become cut off and don't interbreed, they will eventually accumulate different patterns of mutations in their mtDNA. By counting the number of such differences between two species, scientists estimate how long the species have been going their separate ways.
Klicka and Zink found that the 35 pairs of songbird species they examined had been separated for a large range of times, back as far as 5 million years. That is, ancestral species seem to have been splitting to form modern species pairs at a steady rate, not in sharp bursts tied to the appearance of glaciers. For example, blue jays (eastern) and Steller's jays (western) were thought to have emerged as separate species sometime during the ice ages that afflicted the continent during the late Pleistocene, between about 100,000 and 12,000 years ago. But, say Klicka and Zink, the two jays went their separate ways about 5 million years ago. Other pairs have been around as separate species for periods ranging from 200,000 to nearly 5.5 million years. The researchers found only one instance of a bird that seemed to have been separated from its closest relatives in the late Pleistocene. The next step is to search for more species of songbird that originated during that time.
"Glaciers don't tell the whole story, but we want to see what effects they did have," said Zink.
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