Sometimes it's just a case of being a member of the wrong family. Researchers analyzing evidence from 200 million years of fossil records have concluded that some lines of living organisms don't need a cataclysmic event to wipe them out. They just seem destined to go extinct.
The long and varied history of life on Earth is riddled with extinctions. They can signal the end of a species, a genera, or even a whole family of creatures. Sometimes they come suddenly and massively, as in the catastrophe that befell Earth at the end of the Cretaceous period, 65 million years ago. Or they can occur, as in the present day, when single species lose their habitats and quietly fade away. Evolutionary biologists have been studying these phenomena for more than a century, but they seem to have left a third extinction scenario largely unexplored: the possibility that certain types of creatures are simply more prone to dying out, irrespective of outside causes.
That's what evolutionary biologists Kaustuv Roy of the University of California, San Diego, Gene Hunt of the Smithsonian Institution in Washington, D.C., and David Jablonski of the University of Chicago in Illinois decided to explore. The trio examined the extinction rates of marine bivalves--including clams, oysters, mussels, and scallops--whose tough, calcium-carbonate-based shells make them ideal for preservation. Using a global database that includes the evolutionary relationships and fossil record of 1678 types of bivalves spanning about 200 million years, the researchers found a close correlation between the disappearances of some genera and their closest genetic relatives, the researchers report  today in Science. This correlation could not have happened at random, they conclude, so certain categories of creatures must contain genetic traits that make them more vulnerable to extinction.
Perhaps even more striking, the researchers found, was that the Cretaceous event didn't much affect the long-term extinction rates of the bivalve families. Instead, the global cataclysm removed three families, cut back the representatives of two others--which have remained minor players in the bivalve world ever since--and basically left unaffected the long-term extinction rate on another, apparently hardier stock called the Veneridae, which encompasses all modern Venus clams such as cherrystones and quahogs.
"Our beginning hypothesis was that extinction intensity wasn't random relative to evolutionary history," Jablonski says. "But I was really surprised by the pervasiveness of the clustering. History really does matter, and losses intensify according to your position on the Tree of Life."
The findings have relevance to today's conservation efforts, says lead author Roy. "In the current extinction crisis," he says, "we obviously won't be able to save every species that is impacted by human activities." So a major goal would be to design strategies to preserve the genera and families, "even as we lose species," he says. "That way, theoretically at least, we are protecting the potential for these lineages to diversify again in the future."
"This is a very important paper," says evolutionary biologist Charles Marshall of Harvard University. Not only does it quantify the relationship between genetic linkage and extinction rate, he says, but it also shows "the importance of the fossil record as evolution's time machine." Adds Marshall: "Only by analyzing the past do we get a direct sense of the rules by which evolution has worked and will continue to work. I'll certainly be incorporating this into my classes."