News: Replaying evolution.

Replaying evolution
By Patrick Barry
June 2nd, 2008

http://www.sciencenews.org/view/generic/id/32801/title/Replaying_evolution_

Scientists show that happenstance mutations matter

If Stephen Jay Gould were alive today, he would be smiling. Maybe even
gloating.

New research suggests that the famous evolutionary biologist was right
when he argued that, if the evolution of life were ?wound back? and
played again from the start, it could have turned out very
differently.

In experiments on bacteria grown in the lab, scientists found that
evolving a new trait sometimes depended on previous, happenstance
mutations. Without those earlier random mutations, the window of
opportunity for the novel trait would never have opened. History might
have been different.

?It?s a wonderful experiment, a wonderful set of observations,?
comments Geerat Vermeij, an evolutionary biologist at the University
of California, Davis.

Though not firmly conclusive, the new research adds a real-world case
study of evolution in action to the decades-old debate stirred by
Gould?s thought experiment. British paleontologist Simon Conway Morris
and others argued that only a few optimal solutions exist for an
organism to adapt to its environment, so even if the clock were wound
back, environmental pressures would eventually steer evolution toward
one of those solutions ? regardless of the randomness along the way.

Scientists obviously can?t turn back the hands of time, but Richard
Lenski and his colleagues at Michigan State University in East Lansing
did the next best thing. Lenski?s team watched 12 colonies of
identical E. coli bacteria evolve under carefully controlled lab
conditions for 20 years, which equates to more than 40,000 generations
of bacteria. After every 500 generations, the researchers froze
samples of bacteria. Those bacteria could later be thawed out to
?replay? the evolutionary clock from that point in time.

After about 31,500 generations, one colony of bacteria evolved the
novel ability to use a nutrient that E. coli normally can?t absorb
from its environment. Thawed-out samples from after the
20,000-generation mark were much more likely to re-evolve this trait
than earlier samples, which suggests that an unnoticed mutation that
occurred around the 20,000th generation enabled the microbes to later
evolve the nutrient-absorption ability through a second mutation, the
researchers report in the Proceedings of the National Academy of
Sciences.

In the 11 other colonies, this earlier mutation didn?t occur, so the
evolution of this novel ability never happened.

?I would argue that this is a direct empirical demonstration of
Gould-like contingency in evolution,? Lenski says. ?You can?t do an
exact replay in nature, but we were able to literally put all these
populations in virtually identical environments and show that
contingency is really what had occurred.?

The next step will be to determine what that earlier mutation was and
how it made the later change possible, Lenski says. If the first
mutation didn?t offer any survival advantage to the microbes on its
own, it would make the case airtight that Gould was right. That?s
because a mutation that doesn?t improve an organism?s ability to
survive and reproduce can?t be favored by evolution, so whether the
microbe happens to have that necessary mutation when the second
evolutionary change occurs becomes purely a matter of chance.

?I don?t think they?ve necessarily shown? that the first mutation gave
the microbes no survival advantage, comments Christopher Dascher, a
microbiologist at Mount Sinai School of Medicine in New York. ?But
they certainly point very strongly in that direction.?

Lenski notes that the growth rate and the density of bacteria in the
colony jumped up after the second mutation, but not after the first
one.

--
Bob.

.

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