Fish in Lake Washington rapidly evolved -- in reverse

(Editor's Note: This article has been changed. It was Galapagos Island finches that showed evidence of undergoing reverse evolution.)

For most locals, getting the sewage out of Lake Washington beginning in the late 1960s was a good thing. Not so for the threespine stickleback. For the little fish, the pollution and associated algal murk was good cover to protect it from hungry trout.

When the waste and water cleared, the stickleback faced a genetic scramble to evolve into a more protected, armored and ancestral version of itself.

By dusting off samples of the fish dating to the 1950s, researchers at the Fred Hutchinson Cancer Research Center and University of Washington on Thursday documented the stickleback's "reverse evolution" triggered by the cleanup.

"This idea of reverse evolution has only been documented in a few cases in the wild," said Katie Peichel, an author of the paper in Current Biology and scientist in the Hutch's Division of Human Biology.

Sticklebacks originally were found in marine waters where they grew bony armor and prickly spines to ward off predators. As they migrated into freshwater and over many years, the widespread fish lost much of its skeletal shields, or plating.

That's how they looked in Lake Washington -- at least until recently.

For decades, billions of gallons of sewage were flushed into the lake. The nutrients fueled blooms of algae that made the water unfit for human swimmers, but fine for small fish. The foul flow was redirected to Puget Sound about 40 years ago and the water quickly cleared.

Comparing fish collected in 1957, the late '60s, 1976 and 2005, the researchers documented a shift from most fish having low amounts of plating in early samples, to nearly half the fish being completely plated in recent years.

"As the water transparency increased, that made it easier for predatory cutthroat trout to see the sticklebacks better and they could capture them more easily," Peichel said. "The sticklebacks with the complete set of plates could escape so they were favored."

Through genetic analysis, the scientists -- who included researchers in Texas and Japan -- determined the armoring arose mostly through genetic changes -- not an influx of plated fish from the Sound.

"The idea of an episode of rapid evolution is not unprecedented, but it's an important thing," said Dave Reznick, a University of California-Riverside biologist not associated with the study. "Evolution is much faster than people give it credit for."

Sticklebacks are a good tool for studying genetic traits and evolution. Their populations are large and they reproduce quickly. But many scientists consider them a pest -- at best prey for other fish, at worst a competitor with juvenile salmon for food. So the evolutionary anomaly went unnoticed until now.

Reverse evolution was documented in moths in England changing from white to gray as buildings and trees were covered in soot from the Industrial Revolution. They returned to white as pollution declined. It's also been seen in fossil records of sticklebacks and finches from the Galapagos Islands.

Peichel said that when humans migrated out of Africa about 10,000 generations ago, we too, adapted to new environments. And the gene associated with the fish armoring -- called Eda -- is also found in people. In us, mutations in Eda cause defects in hair, teeth and sweat glands -- also traits that vary environmentally.

Could the quick evolution in sticklebacks be a good omen for species facing dramatic environmental changes, such as global warming? In some cases yes, in others no.

Take the case of polar bears, which this week were added to the Endangered Species list. The population of bears is too small, habitat needs are too specific, reproduction is too slow, genetic variability is too limited and the melting of the polar sea ice too fast for them to adjust, scientists said.

"Whether or not (polar bears) can find some different way of utilizing the environment and adapting to the changes seems improbable," Reznick said.

"Polar bears are doomed," he said. "Sticklebacks may not be."

MARCH 1957, LAKE WASHINGTON

MARCH 2006, LAKE WASHINGTON