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.