When a small group of sea star larvae got swept away from their parents off the coast of Australia thousands of years ago, they proved more resourceful than Tom Hanks in Cast Away. Rather than befriending a volleyball, the short-legged sea stars—called "cushion stars" for their plump shape—developed the ability to mate with themselves. Their evolution into live-bearing hermaphrodites is one of the fastest known examples of speciation among marine animals , say the authors of a study published online today in the Proceedings of the Royal Society B. To pinpoint when and where the sea stars broke away from their kin, the team analyzed DNA from the tissue of nearly 400 animals, half belonging to the ancestral species, Cryptasterina pentagona, and half to the new species, C. hystera. By analyzing the evolutionary relationships between the two species' DNA sequences, they were able to infer that C. hystera had broken away from the southern range of C. pentagona near the Great Barrier Reef at most 22,000 years ago. By about 6000 years ago, C. hystera had become a distinct species—lightning-quick adaptation, by evolutionary standards.
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