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Evolution Goes Wild in Once-Polluted Lake

Jessica Marshall, Discovery News

Instead, the researchers surmise that this evolution resulted from the $140 million cleanup of the lake in the 1960s, which transformed the overgrown, polluted lake fed by 20 million gallons a day of sewage into a body of clear water. This transformation removed the murky blanket of cover for the sticklebacks that hid them from predators like cutthroat trout.

Once the fish became easier to spot in the water, having armor became an advantage. Peichel believes that marine fish coming through the ship canal helped seed the population with genes for bony plates, allowing the evolution to happen quickly.

Using information about marine stickleback migration rates in an evolutionary model, the team estimated that from 1969 to 1976, completely plated sticklebacks had a 58 to 72 percent greater chance of surviving and reproducing than low-plated fish. This matches the cleanup period well, Peichel noted.

Since then, the competitive advantage for fully plated fish has been 1 to 3 percent, perhaps reflecting an adaptation on the part of cutthroat in how they hunt that has reduced the selection advantage, Peichel said.

"It all fits together in a very nice story, and it's probably right," said Michael Bell of the University of Stony Brook in New York. But without any fish samples from the lake before it became polluted, the researchers don't know what proportion of the fish were plated before the water clouded.

"We would love to have fish from the 1900s before the ship canal was built," Peichel said.

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How Stuff Works: Evolution