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.