January 18, 2001 - In a stunning breakthrough in what was once considered missing evidence to back Darwin's theory, 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 colleagues, Trevor D. Price, a biology professor at UCSD, and Staffan Bensch, a former postdoctoral student at UCSD now at Sweden's Lund University, discovered 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 the songbird coexist without interbreeding.
"This creates a paradox in which the two co-existing forms can be considered as two species and as a single species at the same time," says Irwin. "Such ring species are extremely rare, but they are valuable because they can show all of the intermediate steps that occurred during the divergence of one species into two."
In their paper, the scientists show how they discovered a gradual variation in the song patterns, morphology and genetic markers of 15 populations of the greenish warbler. At each end of the ring of interbreeding populations, which extend around each side of the Tibetan plateau and through the Himalayas, the scientists found that the two distinct, non-interbreeding forms of the bird do not recognize each other's songs, which are critical in the selection of their mates. They determined this from experiments in which they played recordings of male greenish warbler songs and judged the response of other birds in the trees."In the greenish warbler, as in most songbirds, males sing to attract mates and to defend territories," says Irwin. "The greenish warblers living in the Himalayas sing songs that are simple, short and repetitive. As you go north along the western side of Tibet, moving through central Asia, the songs gradually become longer and more complex. On the eastern side of the ring, moving northwards through China, songs also become longer and more complex, but the structure is different than on the western side. Where the birds meet in Siberia, their songs are so different that they do not recognize each other as mates or competitors. They act like separate species, and the genetic evidence supports that conclusion.
"Apparently, as the birds moved north along two pathways into the forests of Siberia, their songs became longer and more complex, perhaps because females in the north rely more strongly on song when choosing a mate. But the forms of complexity differed between west and east Siberia, because there are more ways to be complex than simple.
"The greenish warbler is the first case in which we can see all the steps that occurred in the behavioral divergence of two species from their common ancestor. These results demonstrate how small evolutionary changes can lead to the differences that cause reproductive isolation between species, just as Darwin envisioned."