Washington, May 16 : A new study has found that nature can sometimes witness a rapid 'reverse evolution', with the prime example being a little fish called the threespine stickleback in Lake Washington, US.
The study, led by researchers at Fred Hutchinson Cancer Research Center in the US, took the case of the sticklebacks that live in Lake Washington, the largest of three major lakes in the Seattle area.
Five decades ago, the lake was a cesspool, murky with an overgrowth of blue-green algae that thrived on the 20 million gallons of phosphorus-rich sewage pumped into its waters each day.
But, thanks to a 140 million dollar cleanup effort in the mid-'60s - at the time considered the most costly pollution-control effort in the nation, today the lake and its waterfront are frequently visited by boaters and billionaires.
According to senior author Catherine Peichel and colleagues, it's precisely that cleanup effort that sparked the reverse evolution.
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.
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.
According to Peichel, the ability of the fish to quickly adapt to environmental changes such as increased predation by the cutthroat trout is due to their rich genetic variation.
"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," she said.
"Genetic variation increases the chance of overall survival of the species," she added.