With millions of life-forms crawling, flying, and slithering about, the world's rainforests seem a cauldron of diversity. Now a study in this week's issue of Science* suggests that many of these species may arise not in the rainforests themselves, but rather along their frayed edges, in transition zones called "ecotones." If so, these oft-forgotten habitats may deserve extra attention from conservationists.
For decades, biologists believed that new species could arise only when there was almost no interbreeding, or gene flow, between populations. Even with natural selection acting in different environments, it was thought that a trickle of gene flow would mix the genetic pool enough to prevent speciation. That could slow speciation in ecotones, where genes can flow between populations that are close neighbors.
To test this idea, evolutionary biologist Thomas Smith of San Francisco State University, with co-workers there and at the University of California, Los Angeles, and the Zoological Society of London, studied Cameroon's little greenbul (Andropadus virens), a small green bird that inhabits both the tropical rainforest and the ecotone. The team trapped birds from six forest and six ecotone sites between 1990 and 1996. They measured a number of physical traits that are important to fitness--weight, the depth of the beak, and the length of the wing, leg bone, and upper jaw. They also drew a few drops of blood for genetic analysis.
Most of the ecotone-dwelling little greenbuls differed markedly in size and shape from their rainforest counterparts. They were heavier and had deeper bills and longer wings and legs. The genetic analysis, however, revealed many similarities between the populations, indicating considerable gene flow: about one to 10 migrants per generation in each population of birds. Taken together, Smith says, the findings mean that natural selection in different environments can offset gene flow and create differences in appearance, even when individuals travel back and forth frequently.
Of course, morphological divergence isn't the same as speciation. But the striking physical differences, says evolutionary biologist John Endler of James Cook University in Townsville, Australia, are "a first step" in that direction. The study is "probably the best piece of work so far" on the question of gene flow and speciation, says Guy Bush of Michigan State University in East Lansing, although Nick Barton of the University of Edinburgh in the United Kingdom points out that the amount of gene flow the team found is quite small.
If gene flow isn't a barrier to new species formation, then the ecotone--often neglected by conservationists--may generate species that find long-term homes in the adjacent rainforest. "The study suggests we need to look at what's happening at the periphery as well" as in the teeming rainforest, says Smith.