In the case of the sticklebacks, returning to an older model made good sense.
When Washington Lake was polluted, visibility was only about 30 inches (76 centimeters).
Sticklebacks didn't need much armor to protect them, because the muck hid them from predators.
In 1968, after the cleanup was complete, the lake's transparency reached a depth of 10 feet (3 meters). Today it approaches 25 feet (7.6 meters).
Before the cleanup, only 6 percent of the fish were completely plated. Now 49 percent are fully armored, with bony plates protecting their bodies from head to tail.
Another 35 percent are partially plated, with about half of their bodies shielded.
This rapid, dramatic adaptation is an example of evolution in reverse, the study team says, because sticklebacks usually evolve toward less armor plating, not more.
The speed of the adaptation is what most impressed Peichel.
"The biggest change occurred between 1968/1969 and 1976," she said. "This is really rapid!"
The sticklebacks in Lake Washington contain DNA from both saltwater species, which tend to be fully plated, and freshwater sticklebacks, which tend to be less so, she said.
"Having a lot of genetic variation in the population means that if the environment changes, there may be some gene variant that does better in that new environment, and so nature selects for it," she said.
"Genetic variation increases the chance of overall survival of the species."
A similar number of generations—about 10,000—separate today's Lake Washington sticklebacks and human beings from their respective ancestors, she pointed out.
And as a species, humans have faced selection pressures that call for similar kinds of adaptation.
For example, "humans in northern latitudes have light skin, and now those people are predisposed to things like skin cancer," Peichel said.
The stickleback study does not reveal a cure for such a predisposition to cancer in humans, she said.
But humans and sticklebacks share a gene, called Eda, that is responsible for the sticklebacks' armor, she said. In humans, mutations to Eda can alter skin, teeth, and hair, she explained.
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Andrew Hendry, a biologist with McGill University in Montreal, said the study is a valuable lesson in evolutionary biology.
"To my mind, it shows how humans can dramatically affect the rate and trajectory of evolution in organisms with which we interact," he said.
Michael A. Bell, an ecologist at Stony Brook University in New York, said the new paper "hangs together as a pretty good story."
The role of the Eda gene is well established, he said, and the researchers did all they could to demonstrate that the sticklebacks' quick shift was a direct result of predation.
But there's a fly in the ointment, he said.
"In western North America, there are other clear lakes with stickleback populations and predators, and they're not completely plated," he said.
"Maybe you've got it right, or maybe there's some other environmental effect that's important that you didn't measure."
For her part, Peichel sees additional opportunities for study within Lake Washington.
"We would next like to look at other traits that appear to have changed in the sticklebacks, like body size, and to investigate the genetic basis for these," she said.
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