Rapid Deevolution Creates Lake of Fully-Armored Fish in Just 50 Years

Lake Washington, the largest lake in the Seattle area, has long been home to a soft-skinned fish called the threespine stickleback. But over the past fifty years, the lake's population of sticklebacks has changed dramatically: today, most of the fish sport partial or full body armor, a throwback to their origins as saltwater fish covered in bony plates. (In the picture here, you can see an armored stickleback on top, with plates in red; a non-armored one is on the bottom.) What caused the rapid shift in the fish's morphology? It sounds bizarre, but the mutation is the result of pollution being cleaned up in Lake Washington. A program started in the late 1960s to clear the lake of toxic sludge made the sticklebacks de-evolve.

A study in 2005 at Stanford showed that there is only one gene that controls whether a sticklefish will develop bony plates on its body. So clearly this gene got switched on after the lake was cleaned up. But why? According to PhysOrg:

Back when the lake was polluted, the transparency of its water was low, affording a range of vision only about 30 inches deep. The tainted, mucky water provided the sticklebacks with an opaque blanket of security against predators such as cutthroat trout, and so the fish needed little bony armor to keep them from being eaten by the trout.

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.

The ability of the fish to quickly adapt to environmental changes such as increased predation by the cutthroat trout is due, Peichel believes, to their rich genetic variation. The sticklebacks in Lake Washington contain DNA from both marine (saltwater) fish, which tend to be fully plated, and freshwater sticklebacks, which tend to be low-plated. When environmental pressures called for increased plating, some of the fish had copies of genes that controlled for both low and full plating, and so natural selection favored the latter.

"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 than in the previous one, and so nature selects for it. Genetic variation increases the chance of overall survival of the species," she said.

Researchers document rapid reverse-evolution [PhysOrg]

Send an email to Annalee Newitz, the author of this post, at annalee@io9.com.

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