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"There are not many documented examples of reverse evolution in nature,"said senior author Catherine "Katie" Peichel, Ph.D., "but perhaps that's justbecause people haven't really looked."
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Peichel and colleagues turned their gaze to the sticklebacks that live inLake Washington, the largest of three major lakes in the Seattle area. Fivedecades ago, the lake was, quite literally, a cesspool, murky with anovergrowth of blue-green algae that thrived on the 20 million gallons ofphosphorus-rich sewage pumped into its waters each day. Thanks to a $140million cleanup effort in the mid-'60s -- at the time considered the mostcostly pollution-control effort in the nation -- today the lake and itswaterfront are a pristine playground for boaters and billionaires.
It's precisely that cleanup effort that sparked the reverse evolution,Peichel and colleagues surmise. Back when the lake was polluted, thetransparency of its water was low, affording a range of vision only about 30inches deep. The tainted, mucky water provided the sticklebacks with an opaqueblanket of security against predators such as cutthroat trout, and so the fishneeded little bony armor to keep them from being eaten by the trout.
In 1968, after the cleanup was complete, the lake's transparency reached adepth of 10 feet. Today, the water's clarity approaches 25 feet. Lacking thecover of darkness they once enjoyed, over the past 40 years about half of LakeWashington sticklebacks have evolved to become fully armored, with bony platesprotecting their bodies from head to tail. For example, in the late '60s, only6 percent of sticklebacks in Lake Washington were completely plated. Today, 49percent are fully plated and 35 percent are partially plated, with about halfof their bodies shielded in bony armor. This rapid, dramatic adaptation isactually an example of evolution in reverse, because the normal evolutionarytendency for freshwater sticklebacks runs toward less armor plating, not more.
"We propose that the most likely cause of this reverse evolution in thesticklebacks is from the higher levels of trout predation after the suddenincrease in water transparency," said Peichel, whose Hutchinson Center lab hasestablished the stickleback as a new model for studying complex genetictraits. By examining multifaceted traits in the fish, such as body type andbehavior, Peichel and colleagues shed light on the genetic networks at play inother complex traits, such as cancer and other common human diseases.
The ability of the fish to quickly adapt to environmental changes such asincreased predation by the cutthroat trout is due, Peichel believes, to theirrich genetic variation. The sticklebacks in Lake Washington contain DNA fromboth marine (saltwater) fish, which tend to be fully plated, and freshwatersticklebacks, which tend to be low-plated. When environmental pressures calledfor increased plating, some of the fish had copies of genes that controlledfor both low and full plating, and so natural selection favored the latter.
"Having a lot of genetic variation in the population means that if theenvironment changes, there may be some gene variant that does better in thatnew 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.
The researchers' findings challenge a widely held theory behind rapidevolutionary change, the idea of "phenotypic plasticity" -- when an organismcan take on different characteristic
