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[Fwd: Evolution of Birds - NY Times]

Look! Up in the sky! It's a bird, it's a plane! No! It's Larry!

At his request, I'm forwarding this to the list.

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     dinosaur@interport.net wrote:
     <<An interesting article appears in today's New YOrk Times, Tuesday, 
     30 September 1997 page F3
     "March of Glaciers Challenged as Engine of Bird Evolution" DNA Casts 
     doubt on long-held view on when species split.
     In brief, it is about the work of Klicka nd Zink who examined 
     mitochondrial DNA sequences which apparently contradict the existing 
     paradigms about bird species splits which heretofore have been 
     attributed to physical separations as a result of recent ice ages.
     Larry Dunn? Where are you when we need you!>>
     Wherever there's a guy beating' up on another guy, I'll be 
     there . . . "
     Here's the URL to the Times Online story:
     People who are not signed up to Times Online will need to do so (it's 
     free -- there will be a clickable sign-up link on the page just 
     given).  Then just go into the story.  You must do this today, as the 
     news goes up(9/30/97 NY Time), then comes down at the end of the day.
     For people in Asia, and Australia, I'll post the story below.  But 
     please everyone sign up so you can look up the stories yourselves in 
     the future!  There's likely to be several "Sue" stories when the 
     auction takes place.
     Bird story follows.
                                *   *   *
     September 30, 1997
     New Research Challenges Glaciers as Engine of Bird Evolution
      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 
     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 
     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 
     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 
     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." 
     ______________________________ Reply Separator 
     Subject: Evolution of Birds - NY Times
     Author:  MIME:dinosaur@interport.net at INTERNET
     Date:    9/30/97 12:14 PM
     ichat PAGER - dinogazette
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