Schneider, who is a principal investigator for CTR, says: "When we think about the processes that generate biodiversity in rainforests, we need to move beyond the traditional view of geographic isolation and focus on the ecological opportunities that are provided by habitat gradients and newly formed habitats."

In their study, the researchers found that open-forest lizards were smaller, had shorter limbs and a bigger head, and became sexually mature earlier than their rainforest counterparts.

To test for the selective forces influencing the rapid changes in the skinks' appearance and reproductive maturity, the researchers looked at predation. Because theoretical studies suggest that natural selection caused by predation favors the evolution of smaller bodies and earlier reproduction, they suspected that lizard-eating birds hunting in open forests were the agents of natural selection.

The researchers placed 480 plastic lizard decoys, painted to match the striped, reddish skink, throughout the dense rainforest and the open dry forest. By looking for the telltale bite marks created by bird bills, the researchers identified how many models were attacked.

According to Schneider, it was no contest between the two sites. Twenty-one models were targeted in the open, transitional forest, versus only four in the closed rainforest habitat.

"We have identified a potential selective mechanism that would explain the difference in size between rainforest and open forest habitats," says Schneider. "The changes in morphology across habitats, in spite of high levels of gene flow, suggest rapid adaptive evolution in response to natural selection."

The evolutionary mechanisms that have fostered the rich tapestry of plant and animal species found in tropical rainforests, which harbor roughly half the Earth's species, have been hotly debated for decades.
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Contact: Merrik Bush-Pirkle
merrik@sfsu.edu
415-338-6747
San Francisco State University
21-Nov-1999