So what makes 6,000 years so extraordinarily fast? Bacteria can split into two species in much less time, but they swap packages of genes even outside of their species. For more complex, multicellular life, scientists often adhere to the biological definition of a species—two groups become so genetically distinct they cannot interbreed. Sometimes different species mate and produce young, but those hybrid offspring are usually sterile, as in the mule's case.
Some of the most studied speciation rates are among the more than 600 species of cichlid fish in Africa's Lake Victoria and neighboring lakes, which evolved sometime in the last 250,000 years (pdf). Even small color changes are enough to create red and blue fish that become different species.*
Speciation time depends on generation length and selection pressures, including limited habitat and food or threats such as disease and predation. Still, the two sea star species would win in an evolutionary race, at least among animals. Coyne gives a few examples: fruit flies in the wild can diverge in 200,000 to three million years and some North American song birds may have split some 170,000 years ago.
Plants are a different case, and claim spots at both ends of the spectrum. A doubling of their genome can overcome the typical genetic dead-end of hybrids, as in the recent case of a Scottish flower, and new species could theoretically arise in three generations, Coyne says. Yet populations of Liriodendron tulipifera, the tulip tree, remain one species despite being isolated for millions of years.
As surprising as the sea stars may be, evolutionary theory does predict such rapid speciation, Coyne says. The two sister stars offer a prime opportunity to study the rate of genetic change, not only because they share a recent common ancestor, but also because the changes are so dramatic.
*Editor's note (8/10/12): To increase its clarity, this paragraph has been edited since posting.