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Frontiers
Scientists Unveil Lake Victoria's Secret
May 1997
Evolutionary biologists have long regarded Africa's
Lake Victoria as an aquatic goldmine, home to more than 300 species of
cichlid fishes found nowhere else in the world. But while biologists have
studied the fishes in this largest of Africa's lakes for years, it took
an NSF-funded team of climate researchers to uncover Victoria's most stunning
evolutionary secret.
While probing the lake bottom for clues about long-term climate changes,
an international team of scientists, led by Thomas C. Johnson of the Large
Lakes Observatory of the University of Minnesota in Duluth, discovered
evidence that the lake's cichlids evolved in the unthinkably short span
of about 12,000 years.
The researchers were trying to determine how wet or dry it had been in
the past by documenting how large or small Lake Victoria had been, using
remote sensing to map out the ancient edges of the lake in long-buried
sediments. Setting out in a Ugandan fisheries research vessel, scientists
discharged an air gun into the water, sending sound waves down to the
lake bottom. Detectors captured the returning sound waves, which researchers
then analyzed, in a process known as seismic reflection profiling. Just
as an ultrasound reveals the hidden contours of a baby below the surface
of a pregnant woman's abdomen, seismic profiling can reveal the nature
of ancient sediments below the surface of a lake.
What researchers were hoping to find was a long, continuous record of
the lake shrinking and growing. Instead, they found that prior to 12,000
years ago, the lake disappeared completely. In its place was a dry, grassy
plain.
"We got out there and initially we were disappointed to find out that
there was no continuous sequence of lake sediments," said Johnson. That
disappointment turned to excitement when it became clear that their discovery
meant the hundreds of cichlid fishes found only in Lake Victoria must
have evolved in under 12,000 years. Their study was published in the journal Science.
For evolutionary biologists, the implications are enormous. "It's a world
record, no question," says Axel Meyer, evolutionary biologist at the State
University of New York at Stony Brook, of the speed with which cichlid
species formed. In fact, Lake Victoria's speedy fishes make laggards of
other groups typically celebrated for their rapid evolution, like Darwin's
finches on the Galápagos Islands.
Still reeling from the finding, researchers say that more questions have
been raised than answered. How did these cichlid fishes manage to spin
off so many species in so short a time? And what is it that makes them
prone to such speed? In the meantime, biologists continue to marvel.
Amy McCune, an evolutionary biologist at Cornell University in Ithaca,
New York, said, "It's amazingly exciting. We're talking about rates of
speciation that have not even been imagined."
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