Losos and his team studied Anolis sagrei--a common small lizard that has a signature throat flare, reproduces quickly and tends to live on the ground in the absence of predators. The scientists found 12 islands with natural populations of the reptiles, which occur in short- and long-limbed versions. Six of these islands they left undisturbed, with the exception of marking the lizards for the experiment. On the other six they introduced a common predator--Leiocephalus carinatus, the curly-tailed lizard--to test the hypothesis that in the short term this introduction would favor long-limbed A. sagrei (the better to run away) but in the long term it would favor shorter-limbed A. sagrei (the better to climb) as they left the ground and took to the shrubs for protection.
Of course, such real-world conditions make this experiment both tremendously valuable from the perspective of scientific proof and tremendously difficult from the perspective of scientific rigor. "Most of this kind of experimental work, as I said, is under artificial conditions," says biologist Butch Brodie of the University of Virginia. "They're doing it in the wild, and in a replicated way--this allows them to look at evolution with all the warts and demons like hurricanes and climatic variables."
But "to measure selection, you need to compare survivors with nonsurvivors," Losos notes. By May 2004 the original 12 islands were reduced to just five--three controls and two experimentals; on the others either all the lizards had survived or all had perished. Nevertheless, limb measurements of the males on the two remaining experimental islands (the females could not be included because of their propensity to lay an egg a week, swelling their abdomens and ruining leg measurements) showed that during the first six months of the year more longer-limbed A. sagrei survived the introduction of the predator L. carinatus.
The second six-month period, however, revealed that their shorter-limbed peers--by taking to the bushes--had come to predominate. "Over the much longer term, we expect that in the continued presence of L. carinatus the initially long-legged A. sagrei would remain on high, thin perches, eventually evolving substantially shorter legs," the team wrote in the paper presenting the finding in the November 17 issue of Science. Unfortunately, the team never got to see if the leg shrinkage would continue: Hurricane Frances swept that possibility away.
But A. sagrei is resilient. "The population does not go extinct," Losos says. "The eggs in the ground subsequently hatch out. The Anoli are repopulating right now." And the scientists persevere; Losos plans to relaunch the experiment yet again and hope for a good run of five to 10 years before the next hurricane wipes out A. sagrei adults. Plus science, like the tide, rolls on, revealing new possibilities for the future. "The genome sequence of an Anolis lizard is just now being sequenced," Losos adds. "Maybe we can go in there and find the genes that are actually being selected on." Only if the climate--and the lizards--cooperate over the long term.