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Week of 7 January 2000

Vol. III, No. 18

Feature Article

The wonderful lizard of Aus.

Skink research shows evolutionary importance of rain forest edges

By Eric McHenry

A small lizard has thrown a large monkey wrench into traditional evolutionary theory.

With a study of Carlia rubrigularis, a skink common to both Australia's rain forests and dry open forests, CAS Assistant Professor of Biology Chris Schneider has helped upend a long-standing belief that geographic isolation fosters genetic diversity. Published recently in the Proceedings of the National Academy of Science, the study further clarifies an emerging view that evolution happens more rapidly in ecotones, the peripheries of rain forests, than at their centers.

Schneider and investigators from two other universities observed marked physical differences between skinks found deep in the Australian rain forest, where populations have long been isolated, and those found on the forest edges, where populations interbreed. They then tested their theory that natural selection was driving this divergence by placing plastic skink decoys in various deep forest and ecotone locations. Marks on the retrieved decoys revealed that those in the ecotones were about five times as likely to be attacked by birds.

"Since perhaps the 1940s," Schneider says, "biologists have viewed gene flow -- the exchange of genes between populations -- as a force that tends to homogenize. If populations are exchanging a lot of genes, they're not going to diverge very much. The idea of geographic isolation is so attractive because it simply eliminates gene flow. Once you don't have this exchange of genes homogenizing populations, they're free to diverge, whether in response to selection or through random changes in the genome.

"Our research points to the fact that geographic isolation alone does essentially nothing," he says. "The key, really, is divergent selective regimes -- natural selection pushing populations in different directions. Geographic isolation is not even necessary, because we're seeing these populations diverging in response to natural selection even though there's gene exchange between them and neighboring populations. Selection is strong enough to overwhelm whatever homogenizing effects gene flow might have."


Chris Schneider and his coinvestigators placed plastic decoy skinks, like the one on the right, in various deep rain forest and ecotone locations. By observing the frequency with which birds attacked the decoys, they were able to determine that real skinks, like the one on the left, face greater predatory pressure at the edges of rain forests than at their centers.

The research team, which included scientists from San Francisco State University and the University of Queensland, looked at adult skink specimens from paired rain forest and ecotone locations in the Wet Tropics World Heritage area of North Queensland, Australia. They found that the ecotone skinks were smaller, had shorter limbs, had bigger heads, and became sexually mature earlier than their counterparts in the deep rain forest.

Reasoning that these differences were likely the results of different predatory patterns, the investigators placed 480 dummy skinks of various sizes throughout both regions. Returning, they found unmistakable beak marks on 21 of the ecotone decoys, and on only 4 of the rain forest models. This wasn't surprising, Schneider says; the kookaburra, a kingfisher that dines on skinks, is much more common in the open forest.

"This goes right back to Darwin," says Schneider. "In Origin of the Species, he talked explicitly about how natural selection acting differently among isolated populations will push them to different places and create new species.

"Selection seems to be driving the divergence of these ecotone populations from their rain forest ancestors," he says. "The rain forest proper is a reservoir of primarily ancient lineages -- species that have stood the test of time. But it's new habitat and geographic complexity, mountain ranges and edges, that seem to be critically important for generating new diversity."

This conclusion jibes with earlier work done by one of the study's co-authors, Thomas Smith of SFSU. In a 1997 Science article, Smith brought rain forest edges to the center of the scientific community's conversation by showing that West African ecotones were agents of evolution.

"The skink work strengthens our idea that this is not just something happening in a few bird species in Cameroon," says Smith. "It's happening in another rain forest, with different taxa [taxonomic groups]."

The National Science Foundation has recognized the importance of this recent ecotone research with a $2.6 million grant, which will support a three-year, three-continent study of biodiversity in and around rain forests. Based at SFSU's Center for Tropical Research, where Smith is director and Schneider is a principal investigator, the study will involve scientists from the three universities that produced the skink study, as well as UCLA, the University of San Francisco at Quito, Ecuador, the World Resources Institute, and NASA. Scientists at NASA's Jet Propulsion Laboratory, Schneider says, have valuable remote sensing capabilities, as well as strong existing data on rain forests because of previous work on carbon sequestration -- the greenhouse effect and global warming.

"We're trying to see if we can identify regions that are important for generating diversity," Schneider says. "And if those regions can be remotely sensed, then we could target them for rapid assessment programs and try to incorporate them into reserve design."

The overarching goal of the NSF grant-funded study, which will deploy researchers to Africa, Australia, and South America, is to inform the creation of more effective conservation policy.

"This is a terrific opportunity to do some good work, and I'm very excited about it," Schneider says. "We've got a great chance to work with students and organizations in several countries, to develop resources there, and to improve conservation planning.

"Conservation efforts have traditionally been focused on preserving areas of high diversity -- preserving the products of evolution. We've been asking what processes result in patterns of high diversity. And it appears that altitudinal gradients, habitat gradients -- edges -- may be very important engines that generate new species. If that's the case, it sort of changes our priorities when we think about what we want to preserve."