Embargoed until 5 p.m. EDT
NSF PR 02-52 - June 10, 2002
In Evolution Game, Survival Doesn't Equal Success
Finding has implications
for future of biodiversity
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
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.