Evolution Goes Wild In Once-Polluted Lake

May 15, 2008

Release from: Jessica Marshall
Discovery News

The cleanup of a polluted Washington lake appears to have driven evolution backwards for the threespined stickleback fish living there.

Marine-dwelling versions of these fish are covered in bony plates, but as sticklebacks migrated into freshwater, a strong selection pressure caused them to lose their armor.

Findings published online today in Current Biology suggest the trend has reversed: The pinky-sized sticklebacks in Lake Washington, near Seattle, have shifted from being nearly armorless in the 1950s and 1960s to being partially or fully covered in plates today.

Researcher Jun Kitano at the Fred Hutchinson Cancer Center in Seattle arrived at the findings accidentally.

"He was just collecting sticklebacks from a bunch of different locations around the area," said study leader Catherine Peichel, also of the center. "He realized that when he collected sticklebacks from Lake Washington, most of them were completely plated."

"He went back and looked at old papers and found there used to be a higher percentage of low-plated stickleback fish." Only 6 percent of sticklebacks collected in Lake Washington in 1968 and 1969 were fully plated, and 78 percent were "low-plated," with fewer than 12 plates.

By 2005, the researchers found that only 49 percent of the fish were completely plated, 35 percent were partially plated, and only 16 percent fell into the low-plated category.

"Our first thought was maybe sticklebacks are just migrating from Puget Sound into this lake," Peichel said, because a shipping canal was built connecting the two water bodies in the early 1900s, and a fish ladder was added in the 1970s to help salmon migrate.

But comparing genetic patterns in fish from the sound with those in the lake showed migration of plated marine fish into the lake was not enough to account for the shift.

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.