Geoscience Reference
In-Depth Information
of the American Museum of Natural History and Stephen Jay Gould
called this model "punctuated equilibrium," known to aficionados as
"punc eek," and when they proposed it in 1972, most evolutionists
scoffed.
23,2
4
Today, however, many believe that not only species, but
whole ecosystems, remain stable for long periods of time, until some-
thing disturbs them enough to cause a multitude of extinctions. The
motor of evolution then revs up and gives rise to new species that are
adapted to the postextinction conditions. Eldredge believes that
"Nothing much happens in evolution without extinction first dis-
rupting ecosystems and driving many preexisting, stable species ex-
tinct. And extinction is almost always the result of the physical envi-
ronment's changing beyond the point where species can relocate by
finding familiar habitat elsewhere."
2
5
If punc eek is the rule, then something punctuates evolution.
That something produces extinctions that in turn open up ecological
niches into which the pressure of natural selection propels a new set
of organisms. According to this notion, the driving force behind evo-
lution is the punctuator itself. What could it be? If we follow where
Raup, Gould, and Eldredge would lead us, we see that the punctua-
tor must be unfamiliar to species over long periods of geologic time
and must disrupt the environment beyond the ability of species to
adapt or migrate. Though mass extinction may have more than one
cause, however we come at the question, we find hints, if not corrob-
oration, that impact may have played a more important role in the
history of the earth than almost anyone has appreciated.
