I thought that anyone familiar with my _Defense In Depth_ might find
this interesting. -MW
Does Evolutionary History Take Million-Year Breaks?
Science
04-DEC-97
By Richard A. Kerr
The history of life is one continuous upheaval, or so strict Darwinists
would have it. Species come and go continually, as creatures either
adapt to a changing environment and ever-shifting competition and evolve
into new species, or become extinct. But lately, a small group of
paleontologists has been asserting that evolution sometimes takes a
holiday. In the fossil record of hundreds of millions of years ago, they
point to examples of entire communities of marine animals that remain
snared for millions of years in something close to stasis, then plunge
into a brief frenzy of extinction and new species formation.
Claims of such "coordinated stasis" have galvanized the paleobiology
community into a frenzy of its own as researchers try to test the idea
by studying how other animal communities fared over tens of millions of
years. The first results to come in are "a mixed bag," concedes
paleontologist Carlton Brett of the University of Rochester in upstate
New York, who, with Gordon Baird of the State University of New York,
Fredonia, first proposed the concept of coordinated stasis in 1992,
based on 400-million-year-old marine fossils from New York. "The pattern
we have seen is holding up well m our rocks," he says, but "that pattern
is perhaps toward the extreme end of a range." Indeed, most studies of
similar fossil records have found little evidence for prolonged periods
of evolutionary stasis.
Yet confirmation of even occasional episodes of coordinated stasis in
the fossil record could have major ramifications for understanding
evolution. One proposed explanation for the stasis is that the species
in the static ecosystems interacted so tightly that there was no room
for change. If so, the more fluid ecosystems of recent times, in which
individual species react independently to evolutionary pressures, may
not be the evolutionary norm. The brief upheavals of accelerated
evolution said to begin and end the periods of stasis are more widely
accepted, but just as intriguing, hinting at little-understood
evolutionary dynamics (see sidebar "When Evolution Surges Ahead").
The classic case of coordinated stasis comes from the animals that lived
in ocean-bottom muds during the early Silurian to middle Devonian
periods, about 440 million to 380 million years ago. Those muds hardened
into fossil-bearing shales that are now found in Ontario, New York
state, and Pennsylvania. Studying the rocks nearly a century ago,
paleontologist Herdman Cleland noted that the array of fossil species,
including the mollusklike stalked brachiopods, corals, mollusks,
echinoderms such as starfish, and trilobites, seemed to change very
little over many millions of years.
It was not until the early 1990s that Brett and Baird, drawing on fossil
specimens collected over 20 years, quantified the stability that Cleland
had reported. They identified 14 intervals, generally running 3 million
to 7 million years each, during which 60%25 or more of species persisted
with little change. Within each interval, extinction, speciation--the
formation of new species--and immigration of species from outside the
now-vanished ocean basin were all more or less on hold, until the
interval ended in a period of drastic turnover lasting just a few
hundred thousand years.
The herky-jerky pattern hearkens back to the revolutionary concept of
punctuated equilibrium proposed by Niles Eldredge of the American Museum
of Natural History in New York City and Stephen Gould of Harvard
University in 1972. They argued that species tend to persist unchanged
for millions of years before abruptly giving rise to a new species,
instead of evolving gradually. Coordinated stasis "is punctuated
equilibrium at a higher level, the ecological level of the community,"
says Douglas Erwin of the National Museum of Natural History in
Washington, D.C. But while there is finally some strong evidence for the
reality of punctuated equilibrium (Science, 10 March 1995, p. 1421),
many paleontologists have had a hard time swallowing the idea that all
the species in a community could be held in check at once.
Even more startling was the explanation for coordinated stasis advanced
by Paul Morris of the Paleontological Research Institution in Ithaca,
New York, Linda Ivany of the University of Michigan, Ann Arbor, and
Kenneth Schopf of Harvard in 1995. They proposed that the ecological
interactions among species--those that compete, prey on each other, or
depend on each other--might have been so specific and intricate that a
new species or an invader from outside the community could not break in.
This interdependence was so strong, they suggested, that even if the
community came under pressure from, say, climate change, it might not
change--instead, it might shift en masse to a more suitable environment
in deeper or shallower water. That kind of interdependence has no
parallel in modern ecosystems, which constantly reorganize in the face
of environmental change.
Prompted by this provocative hypothesis, paleontologists are now
searching for stasis in their own data sets, with mixed results. During
the past 10 years, Mark Patzkowsky of Pennsylvania State University and
Steven Holland of the University of Georgia, Athens, noted the comings
and goings of brachiopod species in a roughly 20-million-year interval
of the Ordovician period about 450 million years ago, as recorded in
rocks in Tennessee, West Virginia, and Virginia. "Of all the studies out
there, ours is most similar to Brett and Baird's," says Patzkowsky. "Do
we have the same pattern?" he asks. "The answer is no."
The record does include some intervals when evolutionary churning
slowed, but even then the rates of speciation and extinction "are much
higher than what Brett and Baird report," says Patzkowsky. Fewer than
10%25 of the late Ordovician species persisted through an interval,
compared with 60%25 in the Devonian. Stephen Westrop of Brock University
in St. Catherines, Ontario, found similar patterns in the trilobites
late in the Cambrian period, 520 million years ago.
But the news for coordinated stasis is not all bad. Mark Harris of the
University of Wisconsin, Milwaukee and Peter Sheehan of the Milwaukee
Public Museum inventoried species in a brachiopod-dominated community
from about 430 million years ago, early in the Silurian period just
following the Ordovician. "The story's very similar to that in the
Devonian of New York," says Sheehan.
To Schopf, the mixed findings suggest that "different parts of the
fossil record may show this pattern more clearly than others. Some parts
may not show it all." Some paleontologists see an exciting implication:
The rules of the ecological game that enforced stasis may have changed
over time, locking up ecosystems at some times but not others. Westrop,
however, thinks the sporadic appearance of stasis implies that it is an
artifact, appearing when the ecosystem is dominated by a group of
animals in which individual species happen to be unusually resistant to
extinction. For example, a species whose larvae disperse widely because
they are free-floating in the sea should endure longer than one whose
less mobile offspring could be wiped out by a local disaster. Westrop
thinks he sees examples of a species duration effect in the Cambrian:
The trilobite community, made up of typically short-lived species, was
relatively unstable while brachiopods, which had longer species
durations, were more stable.
No one expects paleontologists to agree anytime soon on how common
coordinated stasis is or what accounts for it. "Everybody's data sets
aren't quite comparable," says Patzkowsky. "They're from different
times, the rocks are different, the rock exposures are different, the
animals are different. It makes it difficult to compare patterns." And
repeating any one of these massive studies would take a decade of a
researcher's time and would mean infringing on another paleontologist's
field area. The best hope, says Patzkowsky, is "a more consistent
comparison of data sets," using procedures that limit the number of
variables, including the inevitably subjective process of defining
species in the fossil record.
Brett, who started it all, thinks there will be at least one sure
payoff. "I think the idea of coordinated stasis has stimulated a lot of
people to look at the record more closely," says Brett. "Maybe that's
the main benefit of it all."
RELATED ARTICLE: When Evolution Surges Ahead
While paleontologists poring over their records of ancient sea creatures
debate whether the evolutionary turnover of species ever slows toward
stasis (see main text), they do agree that every few million years, it
can speed up dramatically. All three studies done to test the idea of
stasis in 400- and 500-million-year-old communities of mollusklike
brachiopods showed these evolutionary "events." So did studies of
500-million-year-old trilobites and 300-million-year-old
crinoids--stalked marine animals that look like flowers.
These events fall well short of the species turnovers that follow global
mass extinctions, every hundred million years on average: They are more
frequent and may be limited to a single ocean basin. But in each one,
upwards of 60%25 of species seem to be replaced over a period of a few
hundred thousand years. At least some of these boundary events could be
artifacts of the geologic recording process, says Steven Holland of the
University of Georgia, Athens. At times when sea level falls rapidly, he
says, a dearth of sediment deposition can compress the geologic record
in shallow-water sediments, making the evolutionary clock seem to speed
up. But by examining the events in sediments from deep water as well as
shallow, he and Mark Patzkowsky of Pennsylvania State University have
identified at least one event about 455 million years ago that is
clearly real.
Now the question is what triggered this and other surges. Holland and
Patzkowsky identify several possibilities in the environment. Their
studies of the sediments that record the evolutionary surge show a drop
in sea level, a change in ocean circulation, and perhaps a change in
water temperature. In general, "it looks like the coincidence of several
rapid environmental changes is what undoes the system," says
paleontologist Carlton Brett of the University of Rochester in New York
state. Together, he suggests, the changes push organisms so hard that
they cannot adapt or move to more suitable conditions fast enough. As a
result, leisurely change--or outright stasis-gives way to upheaval.
Additional Reading
Special issue on "New Perspectives on Faunal Stability in the Possil
Record," L. C. Ivany and K. M. Schopf, Eds., Palaeogeography,
Paleoclimatology, Paleoecology 127, 1-359 (1996).
Article Dated 03-DEC-97
COPYRIGHT 1997 American Association for the Advancement of Science
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