New species can arise in just a few thousand years, say scientists who have tracked the process in action.
They say a DNA analysis has established that one type of Australian sea star has separated off in as little as six thousand years.
"That's unbelievably fast compared to most organisms," says Rick Grosberg, professor of evolution and ecology at UC Davis.
He and his colleagues studied two closely related 'cushion stars', Cryptasperina pentagona and C. hystera, living on the Australian coast. They're identical in appearance but live in different regions, with Hystera occuring on a few beaches and islands at the far southern end of the range of pentagona.
And their sex lives are very, very different. Pentagona has male and female individuals that release sperm and eggs into the water where they fertilize, grow into larvae and float around for a few months before settling down and developing into adult sea stars.
Hystera, by contrast, are hermaphrodites that brood their young internally and give birth to miniature sea stars ready to grow to adulthood.
"It's as dramatic a difference in life history as in any group of organisms," says Grosberg.
The researchers looked at the diversity in DNA sequences from sea stars of both species and estimated the length of time since the species diverged.
And the results show that the species separated about 6,000 to 22,000 years ago - ruling out some ways in which new species evolve. For example, they clearly didn't diverge slowly with genetic changes over a long period of time, but were isolated quickly.
Over the last 11,000 years, the boundary between cold and warm water in the Coral Sea has fluctuated north and south. A small population of the ancestral sea stars, perhaps even one individual, might have colonized a remote area at the southern end of the range and then been isolated by one of these changes in ocean currents, Grosberg says.