June 11, 2002 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 funded by the National Science Foundation (NSF) and conducted by University of Chicago scientist David Jablonski.
"It's clear that there is a lot of evolutionary action in the aftermath of mass extinctions," said Jablonski. "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 from 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 journal, Proceedings of the National Academy of Sciences. The research was also supported by the Guggenheim Foundation.
"Because most extinction event survivor organisms rebound so robustly, paleontological studies are generally focused on these evolutionary winners," explains Richard Lane, director of NSF's paleontology program. "Jablonski's research examines why other groups of organisms weakly struggle through these major catastrophic events only to meet their demise somewhat later, geologically speaking."
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.
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. Jablonski creates a special category for them in his article, calling them "Dead Clade Walking," in homage to the 1995 film "Dead Man Walking," about a death-row inmate.
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.
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