From: LARRY KLAES (ljk4_at_msn.com)
Date: Wed Jul 16 2003 - 14:16:30 PDT
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Sent: Wednesday, July 16, 2003 5:03 PM
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Subject: Cornell News: rapid evolution
Evolutionary 'fast-track,' in which the hunted outwit their hunters,
could explain why human diseases progress so rapidly, Cornell
HOLD FOR EMBARGO: Wednesday, July 16, 2003, 1 p.m. EDT
Contact: Roger Segelken
ITHACA, N.Y. -- In the fishbowl of life, when hordes of well-fed
predators drive their prey to the brink of extinction, sometimes
evolution takes the fast track to help the hunted survive -- and then
thrive to outnumber their predators.
This rapid evolution, predicted by Cornell University biologists in
computer models and demonstrated with Pac-Man-like creatures and
their algae food in laboratory habitats called chemostats, could play
an important role in the ecological dynamics of many predator-prey
systems, according to an article in the latest issue (July 17, 2003)
of the journal Nature.
Physicians, the Cornell biologists say, should keep this rapid
evolution in mind when investigating interactions between diseases
and victims. As one example, they say, it is useful in trying to
understand how HIV, the AIDS virus, manages to evolve so swiftly that
development of improved vaccines is extremely difficult.
"Evolution is not just about dinosaurs and apes, but it can occur
much more rapidly than we previously thought. Rapid evolution is
pervasive, and the list of examples is growing," says Takehito
Yoshida, a postdoctoral research fellow in Cornell's Department of
Ecology and Evolutionary Biology and lead author of the Nature
article. Yoshida demonstrated the evolutionary principle with
near-microscopic, multicelled animals called rotifers that live to
gobble much tinier green algae. He notes, "We humans are part of
complex ecosystems, and if we think we're above the effects of
evolution, we're not looking close enough. If we want to understand
epidemics and outbreaks of insects such as gypsy moths, we should not
ignore the effect of evolution."
Other Cornell authors of the Nature report, illustrated with a cover
photo of a rotifer-eating algae and the headline "Fast Food," are
Laura E. Jones, a postdoctoral researcher, and Stephen P. Ellner, a
professor of ecology and evolutionary biology, who conducted computer
modeling of predator-prey dynamics; Gregor F. Fussmann, a
postdoctoral researcher during the experiments and now a biologist at
the University of Potsdam, Germany; and Nelson G. Hairston Jr.,
professor of ecology and evolutionary biology. The studies were
supported by a grant from the Mellon Foundation.
The rotifers, Brachionas calyciflorus, and the algae, Chlorella
vulgaris, were chosen for the experiment because they are the
standard, well-documented "lab rats of freshwater predator-prey
studies," Hairston says. The eaters and the eaten lived together for
months in transparent glass chemostats stocked with nutrients (for
the algae) and water.
The Hairston research group had noticed that the highs and lows of
predator and prey populations in the chemostats were occurring
completely "out of phase," says Yoshida. When rotifer populations
were very high -- because previously they had plenty of algae to eat,
algal populations hit rock bottom, because they had been consumed
almost out of existence. The opposite occurred when algae were
super-abundant: There were almost no rotifers around to eat them.
Hairston and his collaborators were seeing weeks go by between the
very pronounced oscillations in predator and prey populations.
Computer models developed by Ellner and graduate student Kyle
Shertzer predicted that only evolution on the part of the prey could
account for the out-of-phase, prolonged oscillation effect. Jones and
Ellner refined the models to make detailed predictions about the
effects of prey evolution, and Yoshida and Fussmann ran experiments
in chemostats under two kinds of conditions: In one, all the
single-cell algae were genetically identical clones -- essentially
one-trick ponies that could not evolve their way out of a tough
situation; in the second, the algal population was genetically varied
so that somewhere among their tiny green gene pool might be an
evolutionary innovation or two that could save them.
After running the chemostats for months and counting predator and
prey populations day by day, the computer model's prediction proved
correct. Populations of a single algal clone quickly rose and fell
almost in synchrony with the numbers of rotifers. But the algae with
some genetic variation to draw on enjoyed longer periods when they
were abundant and their predators were few -- along with agonizingly
long periods when they struggled to rebuild their populations.
Instead of millions of years, the algae were evolving in a few weeks.
But exactly how had they changed?
"We're not sure," Hairston says. "We think that somehow they made
themselves indigestible. They figured out how to pass straight
through the rotifer gut without being digested and survived to make
lots more of themselves. Rapid evolution got them out of a tight
In one respect the joke is on the fast-evolving algae, Hairston
notes, because they had to give up something to become indigestible:
They became slow-growing algae relative to their kin. As a result,
the next time they compete for food resources, the slow-growing,
hard-to-eat algae will be at a disadvantage, and the more edible
algae will thrive, allowing the cycle to repeat indefinitely.
Ellner suggests that this cycle of rapid evolution -- between defense
and vulnerability -- could have parallels in human diseases. "There's
hardly anyone left in our [human] population who had resistance or
developed it during the 1918 flu epidemic," he says. "Perhaps the
time is now ripe for a return of those strains or their relatives."
Jones sees some hope that medical researchers will come to recognize
the role of rapid evolution. "HIV is evolving so quickly that
researchers are struggling to make an effective vaccine. As we say in
our report, evolution can substantially alter predator-prey dynamics.
Attempts to understand population oscillations cannot afford to
neglect the potential effects of ongoing, rapid evolution."
The web version of this release, with accompanying photos, may be
Cornell University News Service
Ithaca, NY 14853
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