Warblers make species in a ring.

A new look at warblers in habitat that surrounds the Tibetan Plateau revives an old hope that the birds represent a long-sought evolutionary quirk called a ring species.

In theory, a ring species spreads around some obstacle, such as a mountain, lake, or plateau. In the originating region, members of the species interbreed freely. As the species spreads around the obstacle, neighbors within each branch interbreed.

However, by the time the branches meet on the far side of the obstacle, the creatures in the opposite branches have diverged so much that they no longer interbreed. Such rings--if they truly exist--would provide clues to how one species splits in two.

Some 60 years ago, when researchers first described the small, forest-dwelling warbler called Phylloscopus trochiloides, they proposed that in Tibet the bird meets the criteria for a ring species. Now, DNA comparisons and analyses of the warblers' songs show that the birds indeed have characteristics of the hypothetical ring species, say Darren E. Irwin, now at the University of Lund in Sweden, and his colleagues at the University of California, San Diego.

The birds' similar DNA markers and songs suggest that greenish warblers to the south of the Tibetan Plateau interbreed, the researchers report in the Jan. 18 NATURE.

The birds don't live on the treeless Tibetan Plateau but populate both eastern and western flanks. As researchers worked their way northward, birds on both sides of the plateau revealed increasingly complex songs. North of the plateau, where the ring appears to close, various greenish warblers with different songs share territory.

There, singers with different styles showed little reaction when the researchers played them each other's songs. That indifference suggests the birds didn't recognize the sound as coming from another greenish warbler, say the investigators.

Several scenarios could have produced such a pattern, they note. However, the researchers highlight the possibility that greenish warblers spreading around the plateau fancified their songs in a furor of competition to increase their attractiveness to mates. If that's true, the ring of greenish warblers suggests that sexual selection has a great deal of power to make new species.

--S.M