[IWAR] Evolution

From: Michael Wilson (MWILSON/0005514706at_private)
Date: Thu Dec 04 1997 - 10:12:48 PST

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    I thought that anyone familiar with my _Defense In Depth_ might find
    this interesting. -MW
    Does Evolutionary History Take Million-Year Breaks?
       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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