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Tuesday, July 01, 2008

Good Science Writers: David Raup

David M. Raup is Avery Distinguished Service Professor (emeritus) of Geophysical Sciences, Evolutionary Biology, and The Conceptual Foundations of Science at the University of Chicago. He retired in 1994.

One of Raup's major interests is extinction. He worked closely with John Sepkoski on patterns of extinction in the fossil record and especially mass extinctions. In the introduction to his 1991 book, Extinction: Bad Genes or Bad Luck, Stephen Jay Gould writes,
... no topic now commands more interest among paleontologists than extinction. The reasons are many, with a prominent root to the impact theory of mass dying. But the principle architect of this shift is my brilliant colleague David M. Raup. Dave may be more at home before a computer console than before a dusty drawer of fossils (and he gets his share of flack from traditionalists for this predilection), but he is the acknowledged master of quantitative approaches to the fossil record. He saw the power of the impact scenario right from the beginning, when most paleontologists were howling with rage or laughter, and refusing to consider the proposal seriously. He has made the most important discoveries and proposed the most interesting and outrageous hypotheses in the field, including the suggestion that mass extinctions may cycle with a frequency of 26 million years. He is also the perennial Peck's bad boy of paleontology—a hard act to maintain past the age of fifty (I am struggling with him), but truly the most sublime of all statuses in science. If Dave has any motto, it can only be: Think the unthinkable (and then make a mathematical model to show how it might work); take an outrageous idea with a limited sphere of validity and see if it might not be extendable to explain everything. This book is a wonderful exposition of this potentially valid iconoclasm.

Raup is not one of the featured authors in The Oxford Book of Modern Science Writing. This shouldn't be a big surprise since not only is Raup a paleontologist—not Richard Dawkins' favorite topic—but his main thesis is the randomness of evolution—also not one of Dawkins' favorite topics.

The following quotations are from Extinction: Bad Genes or Bad Luck.
I have taken the title of this book from a research article I published in Spain some years ago. I was concerned then with the failure of trilobites in the Paleozoic era. Starting about 570 million years ago, these complex, crab-like organisms dominated life on ocean bottoms—at least they dominated the fossil assemblages of that age. But through the 325 million years of the Paleozoic era, trilobites dwindled in numbers and variety, finally disappearing completely in the mass extinction that ended the era, about 245 million years ago.

My question in Spain is the one I still ask: Why? Did the trilobites do something wrong? Were they fundamentally inferior organisms? Were they stupid? Or did they just have the bad luck to be in the wrong place at the wrong time? The first alternative, bad genes, could be manifested by things like susceptibility to disease, lack of good sensory perception, or poor reproductive capacity. The second, bad luck, could be a freak catastrophe that eliminated all life in areas where tilobites happened to be living. The question is basically one of nature versus nurture. Is proneness to extinction an inherent property of a species—a weakness—or does it depend on vagaries of chance in a risk-ridden world?

Of course, the problem is more complex than I have presented it, just as the nature-nurture question in human behavior is complex. But in both situations, nature (Genetics) and nurture (environment) operate to some degree, and the challenge is to find out which process dominates and whether the imbalance varies in time and space. (pp. 5-6)
Raup is known for his "Field of Bullets" scenario. Imagine that individuals in various species are killed at random. If the kill rate is high enough (e.g. 75%) then many species will be wiped out merely by chance while others will survive because some individuals were not struck by bullets. By chance, some genera will disappear because all of its species were wiped out. Sometimes an entire family or class of organisms will go extinct under this scenario: not because they were unfit but just because of bad luck.

Raup also describes The Gamblers' Ruin. Eventually all gamblers will lose all their money as long as they keep playing. That's because their winnings fluctuate up and down by chance but, while there may not be an upper limit to the amount of money they can win, there's definitely a lower limit and once the gambler runs out of money they are finished (extinct).

By recognizing these possibilities, Raup was able to model the life history of species and compare it to what is observed in the fossil record.
In the first chapter, I commented that the title of this book was taken from a research article I had published on the extinction of trilobites. The case provides another example of taxonomic selectivity.

In the rocks of the Cambrian period (570-510 ma BP), somewhat more than six thousand species of trilobites have been found and named. This is three-quarters of the fossil species known in the Cambrian. By the end of the Paleozoic era, 325 million years later, all were gone. On the working assumption that speciation and extinction rates for trilobites were the same as for all other animals of the Paleozoic, my question was whether a group as large as the trilobites could have drifted to extinction by bad luck—as an affluent gambler can drift to bankruptcy, given enough time.

I used mathematical models ... to estimate the probability that the trilobites could have died out because of a chance excess of species extinctions over speciations. The result was a vanishingly small likelihood that chance was operating alone in the trilobite case. The working assumption that trilobites had inherent extinction and speciation rates equal to the Paleozoic average was clearly wrong. It followed that trilobites had (for some reason) either less capability for speciation or a higher risk of extinction. Testing for the latter possibility, one finds the extinction credible only if one assumes life spans of trilobite species 14-28 percent less than the Paleozoic average.

From this, I concluded that the trilobites were indeed doing something wrong. (or that other groups were doing something better). One vote for bad genes. (pp. 102-103)
The trilobites may be the exception to the rule. In other chapters, Raup documents the apparent randomness of extinction. He concludes,
Extinction is evidently a combination of bad genes and bad luck. Some species die out because they cannot cope in their normal habitat or because superior competitors or predators push them out. But, as is surely clear form this book, I feel that most species die out because they are unlucky. They die because they are subjected to biological or physical stresses not anticipated in their prior evolution and because time is not available for Darwinian natural selection to help them adapt.

Having just made an advocacy statement—bad luck not bad genes!—I hope the reader appreciates its uncertainties. Favoring bad luck over bad genes is my best guess. It is shared by many of my colleagues even though a majority of paleontologists and biologists still subscribe to the more Darwinian view of extinction, that of a constructive force favoring the most fit species.

Is extinction through bad luck a challenge to Darwin's natural selection? No. Natural selection remains the only viable, naturalistic explanation we have for sophisticated adaptations like eyes and wings. We would not be here without natural selection. Extinction by bad luck merely adds another element to the evolutionary process, operating at the level of species, families, and classes, rather than the level of local breeding populations of single species. Thus, Darwinism is alive and well, but, I submit, it cannot have operated by itself to produce the diversity of life today.
I don't know why Raup's ideas are not more widely recognized and accepted. But it's not because he's a bad writer. His book is an example of how one can advocate a particular position while still giving other points of view their proper due. It's the hallmark of a good science writer.

1 comment :

  1. Good description. It seems natural that contingency would appear random, when observed over a statistical population of species.

    But not having read Raup, I wonder if ecologists looks at the extinction bit much. Wouldn't they see that species would reclaim much of disappeared niches by adaptation, and, if so, claim that selection favors the fit? (Not that it would detract from the diversity mechanism claim.)

    But it seems you have found your theme here, presumably a bias of Dawkins.