Countering the widespread view of evolution as a process played out over the course of eons, evolutionary biologists have shown that natural selection can turn on a dime – within months – as a population’s needs change. In a study of island lizards exposed to a new predator, the scientists found that natural selection dramatically changed direction over a very short time, within a single generation, favoring first longer and then shorter hind legs.
“Because of its epochal scope, evolutionary biology is often caricatured as incompatible with controlled experimentation,” says Losos, professor of organismic and evolutionary biology in Harvard’s Faculty of Arts and Sciences and curator in herpetology at the Harvard Museum of Comparative Zoology. “Recent work has shown, however, that evolutionary biology can be studied on short time-scales and that predictions about it can be tested experimentally. We predicted, and then demonstrated, a reversal in the direction of natural selection acting on limb length in a population of lizards.”
Losos and colleagues studied populations of the lizard Anolis sagrei on minuscule islands, or cays, in the Bahamas. They introduced to six of these cays a larger, predatory lizard (Leiocephalus carinatus) commonly found on nearby islands and known as a natural colonizer of small cays. The scientists kept six other control cays predator-free, and they exhaustively counted, marked, and measured lizards on all 12 isles.
Anolis sagrei spends much of its time on the ground, but previous research has shown that when a terrestrial predator is introduced, these lizards take to trees and shrubs, becoming increasingly arboreal over time. Losos and his colleagues hypothesized that immediately following a predator’s arrival, longer-legged – and hence faster-running – Anolis lizards would be favored to elude capture. However, as the lizards grew ever more arboreal in habitat, the scientists projected that natural selection would begin to favor shorter limbs, which are better suited to navigating narrow branches and twigs.
Their hypothesis was borne out. Six months after the introduction of the predator, Losos found that the Anolis population had dropped by half or more on the islands with the predators, and in comparison to the lizards on the predator-free islands, long legs were more strongly favored: Survivors had longer legs relative to nonsurvivors. After another six months, during which time the Anolis lizards grew increasingly arboreal, selective pressures were exactly the opposite: Survivors were now characterized by having shorter legs on the experimental islands as compared with those on the control islands.
The behavioral shift from the ground to higher perches apparently caused this remarkable reversal, Losos says, adding that behavioral flexibility may often drive extremely rapid shifts in evolution.