Evolution's "Driving Force" Shifts Based on Behavior, Study Says

Scott Norris
for National Geographic News
November 16, 2006
Brown anole lizards on tiny islands in the Bahamas were enjoying the good life, untroubled by a lizard predator found on larger islands nearby.

But all that changed when biologist Jonathan Losos of Harvard University in Cambridge, Massachusetts, appeared on the scene.

Losos's team experimentally introduced predatory curly-tail lizards onto six islands where the ground-dwelling anoles had been living free of predators, sparking a see-saw year of natural selection.

For the smaller anole lizards, a trait that was advantageous in November—six months after the introduction—had become a liability by May.

Initially the longer-legged anoles were more likely to avoid being eaten, due to their faster running speed.

But as the anoles increasingly sought safety in trees, where the bulky curly-tails could not pursue them, shorter-legged lizards were favored for their superior climbing ability.

"Once they moved up into the trees, the lizards had to move along narrow surfaces," Losos said. "The long-legged individuals were quite inept."

The evolutionary experiment, reported in tomorrow's issue of the journal Science, reveals that, even though evolution can seem like a slow process, its driving force—natural selection—can shift like the wind.

The study also supports a somewhat controversial idea in biology: Animals' behavior in response to environmental change can spur evolutionary adaptations.

Experimental Islands

Curly-tail lizards already exist elsewhere in the 700 islands that make up the Bahamas, and the species has been known to periodically colonize small islands naturally.

So Losos, a National Geographic Society Committee for Research and Exploration grantee, simply sped up this process on six islands where the brown anoles lived.

Another six islands remained predator-free and were monitored for comparison.

At the outset all lizards on both islands were captured, measured, and marked so that individuals could be identified.

In typical predator fashion, the curly-tails quickly chased down anoles that were slower and easier to capture.

Faster runners were more likely to survive, and these tended to be individuals with longer hind legs (read "Lizards Help Explain Survival of the Not-So-Fittest" [November 24, 2004].)

Accordingly, after the first six months the average hind-leg length of survivors was greater than that of nonsurvivors on the experimental islands.

But short legs came back in style in a big way when the anoles shifted strategies.

Even as selection was favoring longer legs, the researchers noticed that the anoles were spending more and more time in the brush and trees.

This led Losos to predict that selection for longer legs would eventually be reversed.

The change came sooner than expected. Just six months later the anoles were almost exclusively tree-dwelling, and longer-legged lizards had died in disproportionate numbers.

The survivor population had legs that were shorter than the original average.

Losos says the experimental approach—unusual in most evolutionary studies in the wild—is what allowed the researchers to detect the sharp, sudden swings in natural selection.

"This demonstrates that evolutionary biology can be a predictive, experimental science like any other," Losos said.

Window on Evolution

The study adds to increasing evidence that evolutionary change can sometimes occur very quickly when environmental shifts require new survival strategies.

(Related news: "'Instant' Evolution Seen in Darwin's Finches, Study Says" [July 14, 2006].)

Losos and his colleagues' work reported only on changes in the anoles over a single generation.

But that generation's shift to tree dwelling is likely to affect the course of anole evolution on the islands from here on out, as long as the predator curly-tails are present.

"We now expect selection to keep on favoring short legs generation over generation," Losos said.

Cameron Ghalambor, a biologist at Colorado State University in Fort Collins who was not part of the study, says the work provides a unique look at an evolutionary process in its earliest stages.

"In most cases in evolutionary biology, you have to look back and speculate what might have happened," Ghalambor said.

"[Losos's study] captures the changes as they occur. We're getting a window into what is actually happening in the very early stages of evolutionary change."

The new perspective may help biologists better understand evolution in the lizard genus Anolis, which contains about 400 species including the brown anole.

A number of Anolis species are tree-dwellers with small hind limbs.

Losos says that at least some of this diversity may be the result of a process similar to the one his team observed.

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