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<<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
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
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
"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."
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Subject: Evolution of Birds - NY Times
Author: MIME:email@example.com at INTERNET
Date: 9/30/97 12:14 PM
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