21 January 2005
Genetic Gradient Theory Challenges Evolutionary Ideas
by Kate Melville 
Research published in Science is the first in the world to demonstrate a genetic gradient - or path of gradually changing genetic traits - between two distinct species that have been isolated by distance. The research challenges the prevailing theory among evolutionary biologists that species evolve only when separated by a geographical barrier. The research team, led by Darren Irwin from the University of British Columbia, say the results could have broad implications for preserving biological diversity and endangered species. "The process for how one species evolves into two is a subject of intense research interest and debate and is fundamental to understanding diversity of life," says Irwin, who spent ten months studying greenish warblers in Asia. "Until now, no one has been able to show continuous gene flow between reproductively isolated species via geographically connected populations - a process of evolution called 'speciation by distance.'" Part of the difficulty in proving the theory has been that few examples of such species are known today. The greenish warbler, living throughout Asia, and the Ensatina salamander found in mountains in North America's west coast, are the only known clear examples of species that may have evolved across distance.
Two distinct forms of greenish warblers co-exist in central Siberia but do not interbreed there, making them distinct species in that region. Irwin, along with co-authors Staffan Bensch, Jessica Irwin and Trevor Price used a new genetic analysis technique called amplified fragment length polymorphism to trace a genetic gradient from one Siberian species to the other via a long chain of geographically connected populations to the south, surrounding the Tibetan Plateau. Irwin believes the findings have broad implications for current approaches to conservation. "Much of endangered species law relies on identifying distinct groups that are reproductively isolated from other groups, and only those distinct groups are targeted for protection," says Irwin. "Our findings show that in some cases there are not well-defined groups, but rather a gradient of forms. In such cases the whole gradient of forms needs to be conserved. With massive habitat destruction being caused by humans, these gradients are being destroyed, as are the stories they tell about evolution and biodiversity. This is happening in much of Asia, where there is a tremendous loss of habitat from humans. Ten years from now, I'm not sure I would be able to find this same evidence."
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