Two sea star species thrive beneath waves off the coast of Australia. Upon cursory examination, they are very similar. Both are cushion stars—a group of slightly plump sea stars—and both are colored light green with hints of slate blue. In fact, they are sister species, a term that means the same in evolutionary biology as it does in genealogy: they share a parent. But a closer look at their genes reveals that these two stars separated just a few thousand years ago—an incredibly short period of time.
One twist makes this evolutionary eye-blink even more interesting. The two species, Cryptasterina pentagona and C. hystera, enjoy dramatically different sex lives. The first reproduces in a fairly typical starfish way: males and females spurt sperm and eggs into the water. The gametes meet, fertilize and grow into larvae that drift in the current before finding a place to settle as adults. The second, C. hystera, is a hermaphrodite that self-fertilizes and keeps the young inside its body until they are ready to venture forth as tiny fully-formed stars.
Tracing this rapid divergence is one example of how biologists use closely related species to decipher evolutionary history. The story of any speciation event—the point when a species branches into two—can reveal the importance of genetic changes and how they relate to functional changes in biochemistry or behavior. It can also tell researchers how changes over time led to the biodiversity that fills the world today.
To understand the sea stars' example, a team of researchers in Australia, Canada and the U.S. analyzed genes in the two species' nuclear and mitochondrial DNA to discover how recently the sea stars diverged. They estimate that the split occurred around 6,000 years ago, with some uncertainty due to mutation rates that gives a range of as recent as 1,000 years ago or as distant as 20,000 years ago. "Even 20,000 years is still stupidly fast" for the extensive change in life history, says Richard K. Grosberg, an evolutionary biologist at the University of California, Davis, and a co-leader of the work. The group's findings were published online in July in Proceedings of the Royal Society B.
The two sea stars' common ancestor was likely a broadcast fertilizer like C. pentagona, Grosberg says. He imagines that after fertilization, some larvae drifted south along the Queensland coast with the current. Low population density in the new area meant gametes looking to encounter partners would have been out of luck. A rare few larvae might have been hermaphrodites through accidental mutation and that gender quirk started the species. "It is better to fertilize yourself than not fertilize at all," Grosberg says. A fluctuating current may have isolated the population and colder waters of the south favored smaller body size and internal fertilization. C. hystera has incredibly low genetic diversity from individual to individual, a telltale sign that the whole species descended from one or just a few migrants.
Finding out exactly which changes led to this speciation event is tricky. The researchers haven't yet pinpointed which genes are most important for gender determination and reproduction in starfish, which is relatively unstudied, Grosberg says. The changes that distinguish C. pentagona from C. hystera may be just a handful of genes—or thousands. Understanding the divergence between species is not just a matter of counting the differences in their genes, however, but also a matter of unraveling how those genes interact with one another.
Still, the study offers a fascinating look at the varying rate of evolution. "I'm amazed that quite a change occurred in such a short period of time," says Jerry Coyne, an evolutionary biologist at the University of Chicago who was not involved in the study.
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.