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Studies on the
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Studies on the
This article is from The Chronicle of Higher Education issue dated May 11, 2001
When Physicists Tried to Explain Evolution, Biologists Cried Foul
By RICHARD MONASTERSKY
At least five times in the history of the planet, life has come close to winking out. It happened most recently 65 million years ago, when three-quarters of the existing species vanished, including the dinosaurs. Some 185 million years before that, nearly 95 percent of all ocean organisms died out, and land life suffered such devastation that fungi briefly took over the continents. One of the biggest problems in evolutionary studies is trying to decipher why life occasionally veers so close to the brink, and at other times motors along with relatively few extinctions.
"The variation in extinction rate is one of the most striking features in the fossil record, and it's really compelling to try to understand that," says Michael Foote, a professor of paleontology at the University of Chicago.
In recent years, some physicists and other adherents of a new field called complexity have argued that the answer involves a simple mathematical theory -- the same one that may explain the collapse of a sand pile or a crash in the stock market. But their suggestions have drawn fire from paleontologists, the group traditionally charged with investigating life's past.
The ongoing debate has ranged from the theoretical to the personal, with jibes flying back and forth in journals and books. At stake are reputations, egos, ideologies, and fundamental insight into how life works.
The story of the debate offers a glimpse into the ecology of science, revealing the behavior of two different species of researchers when they wrestle over the same theoretical territory. Unlike a fight to the death in nature, however, the intellectual struggle in this case has not ended cleanly, but instead has progressed in fits and starts, swinging between competition and compromise as the two camps press their cases.
The latest attack came this March in the journal Paleobiology, which published a paper written by Roy E. Plotnick, a professor of earth and environmental studies at the University of Illinois at Chicago, and the late J. John Sepkoski Jr., of the University of Chicago. The two paleontologists methodically dissected the concept raised by the complexity theorists -- that the panoply of life operates, in essence, just like sand.
The idea emerged 13 years ago, when a medical doctor-turned-theorist named Stuart A. Kauffman, a professor at the Santa Fe Institute, a nonprofit research organization, suggested that life operates in a fundamentally unbalanced way, exhibiting a newly discovered type of behavior called "self-organized criticality."
Per Bak, the physicist who first recognized this phenomenon, known as S.O.C., explains it as "the idea that things do not happen gradually and smoothly but abruptly, in terms of avalanches." Dribble enough sand onto the top of a pile and it will start to give way; sometimes just a few grains will fall, but other times much of the heap will cascade down.
"Such avalanches happen all over: earthquakes, volcanic eruptions, collapses of the stock market, solar flares, and so on," says Mr. Bak, who took Dr. Kauffman's suggestion and started applying the theory to evolution in the late 1980's. He described the idea and its applications in How Nature Works: The Science of Self-Organized Criticality (Copernicus, 1996).
You might say that complexity researchers revise William Blake's famous phrase, looking for a world in many grains of sand.
But paleontologists argue that the S.O.C. theorists are overreaching. These critics point to Mr. Bak's book as Exhibit A. "Modest title," sniffs Mr. Plotnick.
"What hubris, right? Who knows how nature works?" says Anne Weil, a research associate in biological anthropology and anatomy at Duke University, who has also participated in the debate.
No shrinking violet, Mr. Bak chose the title as a symbol of how much he believes the S.O.C. idea can offer. A professor at the Santa Fe Institute and in the department of mathematics at the Imperial College of Science, Technology, and Medicine, in London, he is a founding member of the field of complexity, which seeks to explain how well-understood single entities -- like an atom or a species -- can behave in terribly complicated ways as a group, exhibiting patterns that could not have been predicted by looking at just one unit.
For a human equivalent, think of a mob. Strange and unpredictable things happen when many individuals get together.
A single grain of sand is simple: Blow on it and it will move. The greater the puff, the farther it travels. But those rules evaporate when thousands of grains pile up. Sometimes a small breeze will trigger a small avalanche; other times it will ruin the entire hill. What matters is not the strength of the breeze, but the way that individual sand grains jostle each other, according to S.O.C. theory. The grains are"self-organized" by such interactions and remain poised in a critical state, ready to tumble at the slightest provocation.
Such S.O.C. systems are easy to spot, Mr. Bak says, because they show a mathematical fingerprint called a power law -- basically, the rule that small versions of a phenomenon happen far more frequently than big versions. For instance, magnitude 5 earthquakes strike 10 times more often than magnitude 6 earthquakes, which pack 10 times the punch.
When Mr. Bak traced the lengths of time that ancient groups of animals had survived, he found a power law popping up in those data as well. Much of the time, life was relatively quiescent, and few species were going extinct. Occasionally, though, the bottom dropped out, and most species vanished. "That type of behavior is precisely what is seen with earthquakes, volcanic eruptions, and solar flares," he says. "That's an indication that the system is at a critical state."
The cataclysms emerged not from outside problems -- an asteroid hitting the planet, for example -- but from the ways that species interacted with each other, much like grains of sand, he says. When one species died off, it forced others to evolve or vanish, occasionally producing a cascade of extinctions. Asteroids can provide the initial trigger for such cascades, but so can relatively minor events.
"I do think that's how it works, because it has this fingerprint of this phenomenon," Mr. Bak says. "That's not a proof, but at the very least people should think about [the possibility] that mass extinctions are intrinsic to the way that evolution works and do not need an external cataclysmic effect."
In 1997, another complexity theorist, Ricard V. Sole, extended that work by conducting a more rigorous statistical test of extinction data. A professor at the Santa Fe Institute and at the Polytechnic University of Catalonia, in Barcelona, Spain, he and his colleagues (who included Mr. Bak) found that the data fit a power law. Although that doesn't prove that extinctions are caused by self-organized criticality, it supports the idea that interactions among species play the most important role in determining how life evolves, wrote Mr. Sole and his co-authors in a paper published in Nature.
Ms. Weil shook her head when she first read that work. Recuperating from ankle surgery, she was catching up on her pile of journals. "As I was reading the paper, I thought instinctively that I disagreed with the conclusions, and I wasn't sure why I disagreed. Then I realized that they had done their study without a null hypothesis, which in science is not a safe thing to do."
In other words, Mr. Sole's team had failed to test the method adequately to see if it was reliable, says Ms. Weil. So she and James W. Kirchner, an associate professor of geology at the University of California at Berkeley, ran the required statistical tests and found no power-law pattern in the fossil record that scholars use to study extinction patterns, the two reported in Nature in 1998. Mr. Plotnick and Mr. Sepkoski, the paleontologists, went further in their critique of the theory. "We believe that virtually all paleontologists and evolutionary biologists ...would find S.O.C. not only counterintuitive, but conceptually flawed," they wrote in Paleobiology. The authors took issue with the basic assumptions of S.O.C. models, in particular that species depend on each other so strongly that the extinction of one can drive another to disappear from the globe -- a point that many paleontologists strongly dispute.
The two researchers tested the S.O.C. hypothesis by analyzing a new database of 36,000 genera, compiled by Mr. Sepkoski, showing when the groups appeared and when they vanished. Mr. Plotnick calls it "the best data set that is available at this point in time." They found no evidence of a power-law pattern in the data.
Mr. Bak and others went awry, he argues, in large part because they were unfamiliar with the paleontological data and didn't interact much with the experts who could have helped to avoid problems. For instance, physicists and other complexity scholars have published most of their evolutionary models in physics journals, which are not widely read by paleontologists. Few of the theorists have gone to paleontological meetings to learn more about the field and to encourage collaboration. In fact, Mr. Plotnick is organizing a symposium on evolutionary models for the North American Paleontological Convention this June, although Mr. Bak, Dr. Kauffman, and several other complexity researchers say they will not participate, because of scheduling conflicts, among other reasons.
A lack of familiarity with evolutionary studies emerges repeatedly in the work of complexity researchers, according to paleontologists. "Stemming from the initial papers on S.O.C., the published literature on evolution models often misstates paleontological concepts," write Mr. Plotnick and Sepkoski. Indeed, a reading of Mr. Bak's book and one co-written by Mr. Sole turns up simple but glaring errors. Mr. Bak wrote "Lamarque" for Jean-Baptiste Lamarck, the famous zoologist who developed a version of evolutionary theory before Darwin. Mr. Sole and his co-author wrote "Burguess" for the Burgess Shale, the Canadian rock formation that is hallowed ground for paleontologists because it preserves some of the earliest animal fossils. Such gaffes are on a par with mangling "Troy" in a book on Western civilization.
Mr. Plotnick does credit Mark Newman, a professor at Santa Fe, with reaching out to the paleontological community in a way that other complexity researchers haven't. And Mr. Newman agrees with the critiques of his fellow theorists by paleontologists: "It's certainly true that some people didn't bother to learn the paleontology involved before jumping into this, and so I think, to some extent, the [paleontologists] are right about it, that there is a problem there."
What's more, Mr. Newman takes issue with Mr. Bak and others, saying that a power-law pattern cannot be regarded as a fingerprint of S.O.C., because other phenomena give rise to similar patterns.
"It's not enough to say that self-organized critical models give power laws, and we see power laws in the fossil record, and therefore they must be self-organized critical. That's logic on the same level as saying, 'Bears like honey, my wife likes honey, therefore my wife is a bear.'"
For his part, Mr. Sole says, the criticism and analyses of his work have led him to modify his ideas. "We have to realize that external events -- asteroid impacts or climate change -- are clearly a very important part of the whole picture," he acknowledges.
While S.O.C. doesn't seem to emerge in the fossil data, he says, there is still strong evidence that interactions among species play a major role, combining with external events to cause cascades of extinctions. He is now collaborating with Douglas H. Erwin, a paleontologist at the Smithsonian Institution, to see if complexity theory can help explain the appearance of new species after major biological crises.
Mr. Plotnick welcomes the input of physicists, although he wishes that they would respect the expertise of paleontologists. "We're not dumb. Give us the credit in paleontology of knowing our field and knowing our data."
The bad blood between the two groups goes back to the mid-1980's, when the late Luis Alvarez, a Nobel laureate in physics, equated paleontologists with stamp collectors -- a slight that rankled many fossil experts.
Mr. Bak has carried on that tradition by consistently goading paleontologists. In How Nature Works, he put down David M. Raup, a mathematical paleontologist at the University of Chicago who was a graduate adviser to Mr. Plotnick, Mr. Foote, and other top researchers. "Unfortunately, Raup is not a good mathematician," wrote Mr. Bak.
Ever the provocateur, he later wrote a book review for Nature that started off with the question: "Is biology too difficult for biologists?"
Published in 1998, that line continues to irk Duke's Ms. Weil: "It is insulting. ... There's an element here of, 'You need a physicist to come in and tell you what was going on.'"
Mr. Raup and other paleontologists acknowledge that their field was less quantitative and less motivated by theory in the past, but they say it has advanced tremendously in the past two decades. "The prejudice that [this] is a nonquantitative field is really unfounded," says Ms. Weil.
Even so, she values the work of complexity theorists precisely because they are outsiders, with their own perspective. "I think that's really important, the meeting of the minds," she says. "You can't be a scientist and not be open to new ideas -- even if you don't think they're right."
In fact, the problems of S.O.C. studies have spurred paleontologists to do parallel research on their own. Ms. Weil and Berkeley's Mr. Kirchner are searching for other mathematical signals in fossil data. Meanwhile, Mr. Plotnick and Mr. Sepkoski (who cowrote a first draft of the paper before he died in 1999) presented a new theoretical model to explain the pattern of evolution. Instead of self-organized criticality, they envision something more like Murder on the Orient Express. Major bouts of extinction happen, they suggest, when life gets assaulted by many problems that compound each other's power.
For Santa Fe's Dr. Kauffman, who helped launch the debate, the vitality of the research proves that complexity theorists have made a significant contribution to understanding life's past.
"That means that the S.O.C. model is doing its job of generating alternative hypotheses," he says. "It's part of the march of science."
So even if the physicists' S.O.C. theory isn't fit to survive in the world of paleontology, it has left behind some intellectual progeny that continue to compete in the harsh world of evolutionary studies.
Provided by Metanexus: The Online Forum on Religion and Science http://www.metanexus.net.