Not All Survivors Are Winners in the Game of Evolution

A significant number of organisms that survived the five greatest mass extinctions in Earth's history subsequently failed to achieve evolutionary success, according to a new study published by the University of Chicago's David Jablonski.

"It's clear that there is a lot of evolutionary action in the aftermath of mass extinctions," said Jablonski, a Professor in Geophysical Sciences at Chicago. "During the rebound from mass extinctions, it's not an all-or- nothing thing. The shape of the post-extinction world comes not only from who goes extinct, but by which survivors are successful or instead become extinct or marginalized in the aftermath."

Jablonski lays out his evidence in the June 11 issue of the Proceedings of the National Academy of Sciences. The research was supported by the National Science Foundation and the Guggenheim Foundation.

"Everybody knows that some groups are so affected by mass extinctions that even though they may survive they are but a shadow of their former selves and succumb soon after the main event," said Karl Flessa, a professor of geosciences at the University of Arizona. A major problem with this bit of common scientific knowledge, he said, is that it had never been successfully demonstrated.

"Jablonski's contribution is to add long-needed rigor to documenting and understanding the phenomenon," Flessa said.

To test the idea that many survivors go on to lose the evolutionary game, Jablonski turned to the paleontological literature and to his own work on the aftermath of mass extinction at the end of the Mesozoic Era. In a global analysis of marine genera, he determined how many lineages survived each of the largest mass extinctions in Earth's history, only to die off within the first five or 10 million years thereafter. Genera are groups of closely related species. For example, the genus Canis today includes dogs, wolves and coyotes.

Jablonski found post-extinction losses among survivors of 10 to 20 percent, a number lower than during the mass extinction times themselves, but significantly greater than seen during "normal" times.

"So there's an extra ripple of extinction in the aftermath of each of the big events, sorting out the survivors into winners and losers," Jablonski said. "Contrary to a popular television show, not every survivor is a winner."

Patterns at higher levels of biological organization-for example, orders that include a large number of genera-often play out differently. However, Jablonski also found a 17 percent extinction rate for orders following three of the five big mass extinctions.

This result surprised Jablonski, who had assumed that survival of a mass extinction would be good news for most major groups. "It wasn't good news for everybody, even at this level," he said.

These sets of doomed survivors are the last representatives of their clades, a technical term for an evolutionary group of organisms that includes an ancestor and all of its descendants. And so Jablonski creates a special category for them in his Proceedings article, calling them "Dead Clade Walking," in homage to the 1995 film "Dead Man Walking," about a death-row inmate.

"Some clades manage to stagger on for tens of millions of years," Jablonski said. Those that survive to the present day are sometimes called "living fossils." For example, the coelacanth and the pearly nautilus managed to survive major extinction events in Earth's history, but have failed to thrive in the post-extinction world.

Paleontologists still poorly understand the process that sorts the winners from the losers after a major extinction, Jablonski said. His statistical analysis ruled out one of the most straightforward of possible causes- that lineages that have suffered a major blow to their numbers during a mass extinction might be especially extinction-prone in the aftermath because they contain fewer species to buffer against the hard times.

Instead, Jablonski found that many of the biggest post-extinction winners had passed through a diversity bottleneck as narrow as the Dead Clade Walking groups.

Other possible causes include environmental change and increased competition between species. Both issues need further study, Jablonksi said, and there are probably examples of each in the fossil record.

"We've now established that the post-extinction world is littered with losers," he said. Paleontologists already study the rules that apply to evolutionary success. The next step is to find the rules behind the post-extinction sorting.

It's clear now that post-extinction recoveries are a mixed bag, Jablonski said. Some groups diversify in the most exuberant way, while others simply go nowhere.

"It's those two things together that actually govern the composition of the post-extinction world. It's why we're here and the dinosaurs aren't," Jablonski said. "Not only did our ancestors have to survive the last great extinction, but they also had to flourish in the big scramble afterwards."

The answers to such questions could help conservation biologists develop guidelines for restoring the health of ecosystems and biodiversity in areas that have suffered significant losses.

"The only way we can see into the future is to try to draw lessons from the past," Jablonski said.