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.

7 Comments

1. 1. Bill_Crofut 03:29 PM 8/10/12

   Re: "Finding out exactly which changes...mutation rates...led to this speciation event is tricky."
   
   My understanding is that the vast majority of mutations is neutral at best, deleterious at worst. What is an example of a beneficial mutation which could have been responsible for the "speciation" event herein described?
   

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2. 2. jgrosay 03:46 PM 8/10/12

   It would be good having a freshwater starfish, it will help eliminating or reducing the population of tiger mussels and other pests. Anybody in the genetic engineering field considering working in this line?

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3. 3. quizzical 09:23 PM 8/10/12

   I quote a portion of this article:
   "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."
   
   If the above is true, that the researchers don't even know which genes are involved in gender determination or reproduction, how is anyone expected to believe they have any notion at all about how long ago these species have diverged or, even if they ever did.
   I get very tired of the outrageously extrapolated claims many so called scientists make.
   

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4. 4. Extremophile in reply to quizzical 04:02 PM 8/11/12

   quizzical,
   
   the article responds to your point. All you need to do is read:
   
   The scientists "...analyzed genes in the two species' nuclear and mitochondrial DNA to discover how recently the sea stars diverged."
   
   This method is based on a statistical principle called sampling. It has a sound mathematical foundation, and unless someone has evidence for the opposite, I would generally consider people from the genetic discipline quite capable of doing the maths behind that.
   
   To understand which genes encode the sexual functions and behavior is a task that cannot be done just by sampling; you would need a 100% inspection in order to get a complete sequence of the genetic code.
   
   So, sampling is a way to answer one question - evolutionary relationship - but not enough to answer a second one, encoding genes.
   
   These people are probably right, and you are definitively wrong.

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5. 5. quizzical in reply to Extremophile 09:23 PM 8/11/12

   Thanks for the clarification.

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6. 6. dbasener in reply to quizzical 02:35 PM 8/14/12

   Quizzical, it is important to the ability of society to enact policy based on scientific understanding, that you take a lesson from this. When you say that you are "very tired of the outrageously extrapolated claims many so called scientists make", you are yourself making an 'outrageously extrapolated claim'. This really doesn't happen in the world of peer-reviewed journals. There are other problems with such controls, but the peer-review system tends to be marvelously self-correcting for "outrageous claims".
   
   We live in a world in which a democracy needs to drive changes in policy in the face of inevitable and extreme modifications of our biosphere which are still only visible through the lens of scientific scrutiny. But that democracy is largely controlled by people who believe that scientists engage in, at best wild claims, and at worst active conspiracies to fabricate evidence and crises.
   
   It is gravely important that you understand that, while human and given to the same failings as all humans, scientists in their professional lives perform their jobs in a structure which engages the critical and competitive spirits of other scientists to ensure that their evidence is supported and their conclusions are reasonable.
   
   The fact that this is so is proven by the advanced technological world around us that that very structure has made possible.

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7. 7. quizzical in reply to dbasener 07:31 PM 8/15/12

   dbasener,
   To your first paragraph:
   I am glad to hear that it rarely happens in the world of peer reviewed journals. I trust that Al Gore’s error riddled movie was not part of that process. The trouble is that when too many peers believe the same notion, who is left to challenge any nonsense?
   
   A case in point – evolution itself. Many evolutionists insist that evolution is not a random process. But, no matter how many subsequent parts of the theory are non-random, if the whole house of cards rests solely on random variations, which it must if one is to believe the completely naturalistic concept, then the whole thing must be a random process.
   
   And, of course – abiogenesis. One major reason for Mars exploration is to find the building blocks of life. Seems like everyone forgets that Life is NOT based on a correct recipe alone. But just as importantly, it is based on specific coded instructions that direct the use of the correct recipe.
   
   There is NO scientific knowledge of any information ever being self-generating. Is there?
   
   To your second paragraph:
   Those folks you refer to are usually at the mercy of the media who are possibly the real source of wild claims. What are folks to think? Most folks are not privy to “peer reviewed journals.” Even this SciAm article’s title makes unsupported statements of fact which, to the author’s credit, are more correctly described in the text using terms such as “they estimate”, but their 6000 year estimate is plus 333% and minus 17% - a very strange and lopsided range.
   
   To your third paragraph:
   Don’t worry, I understand that. It is just that articles such as this one are VERY misleading in their thrust.
   
   To your fourth paragraph:
   Close investigation of Biology, Chemistry and Physics has indeed yielded a wonderful array of useful technologies. However, I would like to hear of one item whose development depended critically on speculations of origins. Origins cannot be studied experimentally with the scientific method. No one has millions of years to apply to the effort. Therefore all such speculations remain just that, and should never be sold as fact. I am only asking for truth.
   

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