 |
Glaciers may not have driven modern bird evolution
|
|
Acne Free in 3 Days
A Scientifically Proven Way To Permanently Clear Skin In Just Three Days.
Author: Jeff Poling For decades, neontologists have followed a widely accepted theory about the appearance of many North American animal species. The theory states that toward the end of the last period of widespread glaciation (not to be confused with the end of the last Ice Age, which has not yet ended), the final southward thrust of the North American glacier geographically isolated populations of species. These isolated pockets then evolved separately, evolving into many of the species known today. This theory was challenged in the September 12, 1997, issue of the journal Science.
John Klicka and Dr. Robert Zink, evolutionary biologists at the J. F. Bell Museum of Natural History at the University of Minnesota in St. Paul, studied 35 pairs of bird species considered to be the best examples of species split by the geographical isolation caused by the glaciers. The scientists used mitochondrial DNA sequences to estimate when each pair of species diverged. The results suggest that rather than splitting 100,000 to 250,000 years ago, the time of the last widespread glaciation, they split at times distributed uniformly over a large period of time starting 5 million years ago. The average time for divergence was 2.5 million years ago.
Klicka, a graduate student, and Zink used new data from their own molecular studies as well as data from previous studies done by themselves and other researchers to test their hypotheses. Examining each of the 35 pairs of bird species separately, they counted the number of differences between each pair's mitochondrial DNA sequences. Current DNA molecular-clock theories state that the more recently two species diverged, the more similar their DNA should be. The longer the time since they became isolated as distinct species, the greater the number of genetic differences that should have accumulated between them.
The researchers used other bird and mammal species with dateable fossils to estimate about how many years it took to accumulate a certain number of differences, or the actual rate at which the molecular-clock ticks. Knowing the rate at which differences accumulate on average between two species' DNA, the researchers were then able to estimate how long ago their 70 bird species had split.
The uniform rate of divergence suggests that any number of factors could have been evolved, including other periods of glaciation, climate changes preceding or following the glaciations, or something totally unrelated.
In hindsight, it might seem that it should have been obvious to biologists that more than just the last few glaciations could have been important in the evolution of these suites of species. But Zink suggests that glaciers were attractive as an obvious mechanism by which Cenozoic species could have become isolated (intercontinental seaways are used to explain species divergence in the Mesozoic). Many of the bird species come in east-west pairs. Conventional wisdom was that they were pushed to either coast as glaciers reached down into the frigid center of the continent. The birds often have similar songs and similar plumage and many, like the Baltimore oriole and Bullock's oriole, can still interbreed. Biologists had focused their attention on the most recent glaciation because often the species whose origins biologists sought to explain appeared to be so very newly evolved.
"This calls one of the classic examples into question," said Dr. Michael Rosenzweig, an evolutionary ecologist at the University of Arizona, in Tucson, who wrote a commentary for Science on the paper. "This is a notion that was 100 percent accepted. I never, ever heard anybody challenge it before. Yet they analyze pair after pair after pair, and, in almost every single case, they find evidence that they had separated before the last glaciers came down."
Researchers suggest that the challenge to the theory should encourage biologists to reexamine the evolution of many species of birds, mammals, insects and others thought to have arisen due to the period of glaciation from 2 million to 10,000 years ago. It also suggests that researchers may have to rethink the origins of other species that have been explained by other well-entrenched theories. Dr. Richard Harrison, an evolutionary biologist at Cornell University, said that in the 1940s through the 1960s, "there were a lot of similar stories that were reasonable explanations that were never very critically evaluated. There wasn't any way to get at them. Now we have some of the tools to do that."
Klicka and Zink acknowledge that some may object to the new study as the use of molecular clocks has always been controversial, with many researchers arguing for their usefulness and others arguing that the variability with which molecular clocks tick makes them too unreliable.
Pointing to the new findings, Dr. Sievert Rohwer, curator of birds at the University of Washington's Burke Museum, in Seattle, said "Understanding how species formed in North America is going to be much more difficult. It's not going to be easy to look that much farther back in geologic time and figure out why these populations got isolated."
--------------------------------------------------------------------------------
Copyright © 1997 by Jeff Poling. Quotes are from media sources.
|
Print this page