In 1968, after the cleanup was complete, the lake’s transparency reached a depth of 10 feet. Today, the water’s clarity approaches 25 feet. Lacking the cover of darkness they once enjoyed, over the past 40 years about half of Lake Washington sticklebacks have evolved to become fully armored, with bony plates protecting their bodies from head to tail. For example, in the late ‘60s, only 6 percent of sticklebacks in Lake Washington were completely plated. Today, 49 percent are fully plated and 35 percent are partially plated, with about half of their bodies shielded in bony armor. This rapid, dramatic adaptation is actually an example of evolution in reverse, because the normal evolutionary tendency for freshwater sticklebacks runs toward less armor plating, not more.

“We propose that the most likely cause of this reverse evolution in the sticklebacks is from the higher levels of trout predation after the sudden increase in water transparency,” said Peichel, whose Hutchinson Center lab has established the stickleback as a new model for studying complex genetic traits. By examining multifaceted traits in the fish, such as body type and behavior, Peichel and colleagues shed light on the genetic networks at play in other complex traits, such as cancer and other common human diseases.