What can 'ring species' teach us about evolution?

Ten thousand years ago, at the end of the last ice age, a species of greenish warblers lived in a forest south of the Tibetan Plateau. As the ice receded, the forest grew to form a ring around the plateau—and so did the songbird's habitat. Two thousand years later, birds living on the eastern edge of the expanding ring once again met those living in the western edge, only now they couldn't mate. Although still members of the same species, something had changed. What led to their genetic incompatibility?

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