Biologists at the University of California, San Diego have demonstrated, in a study of the songs and genetics of a series of interbreeding populations of warblers in central Asia, how one species can diverge into two.
Their description of the intermediate forms of two reproductively isolated populations of songbirds that no longer interbreed is the "missing evidence" that Darwin had hoped to use to support his theory of natural selection, but was never able to find.
"One of the largest mysteries remaining in evolutionary biology is exactly how one species can gradually diverge into two," says Darren E. Irwin, a biologist at UCSD who headed the study, detailed in the January 18 issue of the journal Nature. "This process, known as speciation, is very difficult to study because it can take a great deal of time to occur."
Biologists have generally learned about the divergence of species by comparing many different species at various stages of speciation. But in their study of the greenish warbler, a songbird that breeds in forests throughout much of temperate Asia, Irwin and his colleaguesTrevor D. Price, a biology professor at UCSD, and Staffan Bensch, a former postdoctoral student at UCSD now at Swedens Lund
Universitydiscovered a rare situation known to biologists as a "ring species.""Ring species are unique because they present all levels of variation, from small differences between neighboring populations to species-level differences, in a single group of organisms," says Irwin, a former student of Price who is in the process of beginning his postdoctoral work with Bensch at Lund University.
In the case of the greenish warbler, Phylloscopus trochiloides, the scientists discovered a continuous ring of populations with gradually changing behavioral and genetic characteristics encircling the Tibetan Plateau, which is treeless and uninhabitable. This ring is broken by a species boundary at only one place, in central Siberia, where two forms of t
Contact: Darren Irwin
University of California - San Diego