This is the text of a news release from the U. of Massachusetts. Interesting
findings on rapid adaptation and reproductive isolation. Notice particularly
the next-to-last paragraph where the researcher cautions against reading too
much into his findings. Good science, IMHO. Here's the URL:
http://www.umass.edu/newsoffice/archive/2000/101900salmon.html
ralph
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Release: Immediate
Contact: Elizabeth Luciano
Oct
Oct. 19, 2000
MAIN PAGE |
MONTH-IN-REVIEW
New Species Arise More Quickly Than Previously
Believed, UMass Researcher Finds
Details are published in the prestigious
journal, Science
AMHERST, Mass. - The splitting of a species into two new
species may occur in far fewer generations than scientists
previously believed, according to a study led by University of
Massachusetts postdoctoral researcher Andrew Hendry.
Hendry, an evolutionary ecologist, conducted his study on two
populations of sockeye salmon in the Pacific Northwest. The
findings are published in the Oct. 20 issue of the journal,
Science. His co-authors are Paul Bentzen and Thomas
Quinn, both of the University of Washington, John Wenburg of
the University of Montana, and Eric Volk of the Washington
State Department of Fish and Game.
"There is a widely-held perception that when one population
splits into two different
environments, traits evolve quickly and, as a result, the
two new populations become less
likely to interbreed. That is, they become reproductively
isolated. This process, called
ecological speciation, may be one of the easiest and
fastest ways that new species arise.
Our results suggest that this perception may not only be
correct, but in spades," said
Hendry. "The classic examples of ecological speciation are
for groups that have existed for
10,000 years. Even the fastest examples are for some
insects over 200-400 generations. In
these cases, we know reproductive isolation evolved
sometime in the past, but we don't
know how quickly."
In contrast, Hendry's team found evolutionary adaptations
and reproductive isolation in
salmon after only 12-14 generations: some 60-70 years.
Specifically, scientists studied
salmon introduced into Lake Washington, in Washington
State, during the 1930s and 1940s.
Soon after the initial introductions, two populations
became established, one spawning in a
river and one along a lake beach. "Sockeye salmon bury
their eggs and spawn in different
kinds of locations, and in a variety of environments, even
in a small system such as this,"
Hendry explained. "When new populations become established
at different sites, you'd
expect them to evolve different adaptations, and that's in
fact what happened."
Some of the differences between river salmon and beach
salmon included:
Body depth of males - male beach salmon are "deeper"
from back to belly than their
river counterparts. This trait influences mating
success, Hendry says. "Deeper-bodied
males gain access to more females during mating but,
in a river with a strong current,
a deep-bodied male would be inefficient
hydrodynamically, and would have a more
difficult time, so river fish have adapted by becoming
slimmer, more streamlined."
Size of females at breeding age - river females are
larger than their beach
counterparts. Hendry points out that the river
female's larger size enables her to bury
her eggs deeper in gravel, decreasing the chances that
the eggs will be disturbed or
destroyed by high water flows.
"Rapid evolution has also been documented for other
organisms," Hendry points out. "The
unique twist to our study is that we were able to
demonstrate these differences resulted in
reproductive isolation." Scientists examined tiny ear bones
known as otoliths, which have a
sort of bar-code identifying each fish as to where it had
been born, the river or the beach.
"We can look at breeding adults and know who is a river
resident, who is a beach resident,
and who is a beach immigrant - a fish born in the river but
now breeding at the beach," said
Hendry. "We found that a large percentage of the adults
spawning in the beach came from
the river - almost 39 percent each generation," he said.
"If those fish were successful in
producing offspring, the two populations would homogenize."
Genetic analysis of "beach immigrants" relative to
"residents" revealed just the opposite:
immigrants are considerably less successful at producing
offspring. This difference may have
arisen because immigrants have reduced mating success or,
if a beach immigrant does
breed, the resulting hybrid offspring will be less likely
to survive, Hendry said, because
they're less than ideally suited to either environment.
This evidence demonstrates for the first
time how rapidly adaptation can lead to reproductive
isolation, and it is about 10 times faster
than the previously accepted maximum. "This should really
make us rethink the importance
of natural selection and adaptation to the rapid generation
of new species and the generation
of biological diversity."
Hendry does offer a word of caution: "Despite our findings
of rapid adaptation and
reproductive isolation, I don't necessarily presume these
two salmon populations will evolve
into what would be recognized as separate species. We have
simply used new populations
to demonstrate the same processes that lead to new species."
Hendry is a Darwin postdoctoral fellow, and conducts
research in the biology department's
Organismic and Evolutionary Biology program. He conducted
his graduate work at the
University of Washington in Seattle.
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