Excerpt taken from New York Times, September 30, 1997
New Research Challenges Glaciers as Engine of Bird Evolution
By CAROL KAESUK YOON
For decades, biologists have relied on a well-accepted theory to explain the appearance of a host of North American animal species. As the ice ages were beginning to wind down, so the theory goes, the last rounds of glaciers came creeping into North America, their frozen fingers splitting apart species, creating geographically isolated populations that evolved into many of the species seen today.
Now this explanation, cited countless times over the decades in textbooks, scientific papers, and classrooms, appears to be wrong, according to a study reported on Sept. 12 in 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 in two by the glaciers of the last ice ages. Examining the birds' mitochondrial DNA sequences, the researchers estimated when these pairs of species split apart. Rather than finding that they had all split at the expected times around 100,000 to 250,000 years ago, they discovered that pairs of species appear to have diverged at a wide variety of times, many having split millions of years ago.
"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."
Dr. Sievert Rohwer, curator of birds at the University of Washington's Burke Museum, in Seattle, said: "It's the standard, the thing that's been applied over and over and over again. It's not very often in science that a paradigm can be soundly thrown out like this by data."
Researchers say that the new work should get biologists to rethink the evolution of the many species of birds, mammals, insects, and others thought to have been created by the last glaciers of the Pleistocene, the geological period that began some 2 million years ago and ended about 10,000 years ago. More broadly, researchers say they may need to re-evaluate other species whose origins have been explained by other well-entrenched just-so stories.
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," Harrison said. "Now we have some of the tools to do that."
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 the hypothesis. Examining each of the 35 pairs of bird species separately, they counted the number of differences between each pair's mitochondrial DNA sequences.
The more recently that two species have 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, each difference marking another tick on the so-called molecular clock.
Researchers used other bird and mammal species, those 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 the species they were studying had split.
What they found was that these pairs of birds that had supposedly separated from one another during bouts of glaciation 100,000 or 250,000 years ago had actually separated over a wide period beginning about 5 million years ago. On average, they appear to have separated about 2.5 million years ago.
The species the researchers studied appear to have been generated at a fairly uniform rate over the last 5 million years, suggesting that any number of factors could have driven their evolution, including earlier glaciations of the Pleistocene, climate changes that preceded the glaciations, or perhaps something entirely unrelated to the glaciers.
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 explains that glaciers were attractive as an obvious mechanism by which species could have become isolated. Biologists had focused their attention on the most recent of the glaciers because often the species, whose origins biologists sought to explain, appeared to be so very newly evolved.
Many of the bird species come in east-west pairs, apparently pushed to either coast as glaciers reached down into the frigid center of the continent. They often have similar songs and similar plumage and many, like the Baltimore oriole and Bullock's oriole, can still interbreed.
The authors 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.
But Harrison called the study "convincing," saying that the estimated dates in the new study were conservative.
Pointing to the new findings, Rohwer 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."