http://www.abc.net.au/science/news/stories/s1688507.htm
Darwin's finches evolving fast
Larry O'Hanlon
Discovery News
Monday, 17 July 2006
A Galapagos finch that helped reveal the origins of species to Charles
Darwin has now undergone a spurt of rapid climate-driven evolution,
biologists report.
The medium ground finch ( Geospiza fortis ), of Daphne Island was
nudged, and then shoved, to evolve a smaller beak.
This happened by the combination of competition from another finch that
arrived on the island more than 20 years ago and more recent drought
conditions.
"It happened very fast," says biologist Professor Peter Grant of
Princeton University .
He and Dr Rosemary Grant publish their discovery in the latest issue of
the journal Science .
In fact, it happened in a single bird generation, Grant explains.
The evolutionary nudging began when some larger finches settled on
Daphne during an exceptionally wet El Niño in 1982.
In the years since, the larger G. magnirostris finches have been eating
most of the larger, thorny seeds of the island's puncture vine plants
and steadily pushing the smaller finches to rely on smaller seeds from
other plants.
As a result, G. fortis birds with smaller beaks that did not compete
with the larger birds did better, and were more likely to have
offspring. That essentially enriched the gene pool with small beak
genes and led to more G. fortis with smaller beaks.
But the matter really came to a head in 2003 and 2004, when little rain
fell on the island and seeds of any kind were scarce.
"Most of the birds that had large beaks before the drought
disappeared," says Grant. That included almost all of the recently
arrived G. magnirostris and any remaining G. fortis with especially
large beaks.
The only birds that survived enough to mate and produce offspring in
2005 were the G. fortis with smaller beaks and an ability to exploit
small seeds like those of the drought-tolerant Optunia cactus.
In Darwinian jargon, the small-beaked birds were naturally "selected"
for perpetuating the species, just as a dog breeder might select for
speed in a greyhound.
"For years, this has been the classic textbook example of rapid
evolution," says Professor David Skelly, an ecologist and researcher of
rapid evolution at Yale University , referring to the competitive
pressure on G. fortis by the larger beaked G. magnirostris .
"When I was a student, [Grant's] work was sometimes taught as the
exception to the rule," says Skelly.
That is, normally evolution is thought of as slow and gradual in large
animals like fish, birds, reptiles and mammals.
Beak sizes changing measurably in just decades seems awfully fast.
The Galapagos finches were considered an extreme case of quick
evolution caused by an extreme environment.
"Now it appears that the Grants' work shows a pattern that is likely to
be widespread," says Skelly, who has studied rapid evolution of
amphibians in response to global warming.
"Environmental changes severe enough to cause sharp population
declines, as seen with the finches, are also selection events."
As more and more species undergo the stresses of climate change, more
cases of rapid evolution can be expected, Skelly says.
It's not likely to save most species facing the climatic bottleneck, of
course, but it does give a few a fighting chance, he explains.
It's not unusual
"I don't think there is any reason to suspect this is an unsual
occurrance," says Australian biologist Professor Richard Shine of the
University of Sydney .
Shine has charted the rapid evolution of longer legs in invasive cane
toads in Australia, as well as adaptive changes in native snakes where
the toads have invaded over the past few decades.
We'd see more evidence of rapid evolution if there was more support for
long-term field studies like that of the Grants' 30-year work on the
finches, he says.
"It's incredibly difficult to maintain these long-term studies."
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