Genetics, Paleontology and Macroevolution, 2nd edition
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Dwight Davis stated for the Princeton meeting on genetics, paleontology, and evolution , p. Paleontology supplies factual data on the actual rates of change in the skeleton and the patterns of phyletic change in the skeleton. Because of the inherent limitations of paleontological data, however, it cannot perceive the factors producing such changes. Attempts to do so merely represent a superimposition of neobiological concepts on paleontological data.
Such invalid statements in professional publications often follow an unfortunate path towards inclusion in basic textbooks—and errors in this particular medium are almost immune to natural selection, as extinction-proof as a living fossil in the deep ocean. One major, and very fine, introductory text Allen and Baker, , p. Evolution can be studied on the population level only with living organisms.
The fossil record provides too few data to allow such treatment; it merely allows paleontologists to reconstruct the history of animal and plant groups [the restriction of our efforts to descriptive phenomenology]. The population approach makes it possible to ask such questions as: What is the rate of evolution in a given species? What factors influence the course or rate of evolution?
What conditions are necessary for evolution to begin or cease? I would include these three questions within a set most amenable to resolution by the data of fossils and their temporal distribution! As a final illustration of the reductionistic biases that still beset this most comprehensive of fields, and of the usual tendency to ignore or devalue theory based on whole organisms or long times, the assigned reporter for Science magazine presented a remarkably skewed and parochial view of the conference that honored Simpson's Tempo and Mode at its half-century, and formed the basis for this published symposium Cohen, The meeting itself was broad and comprehensive, with talks spanning a full range of levels and durations, from molecules at moments to faunas over geological periods.
Yet the reporter ignored about two-thirds of the presentations, including all from Simpson's own professional domain, and focused entirely upon molecular insights—a central issue to be sure, but surely not the exclusive or even the primary theme of a meeting called to honor Simpson's work and its sequelae. Fifty years ago, the great evolutionary biologist George Gaylord Simpson-… published a classic volume called Tempo and Mode of sic Evolution. Appropriately, the aim of the symposium was to provide a Simpsonian overview of the field, and the conclusion was that its tempo of change is rapid, and one of the main modes of change is the acquisition of new data from molecular biology.
As one presentation after another confirmed, molecular biology is offering researchers a multitude of new genetic clues about evolutionary change. A picture of Simpson, smiling benignly as he did only rarely in life , graces the page. But I can guarantee Science' s reporter that Simpson's ghost is raging at the exclusion of his own field from the primary account of his splendid party.
The conventional view of the Modern Synthetic theory of evolution often called or equated with Neo-Darwinism envisages two sequential stages of development: formulation of the population-genetic core in the s and s through the work of R.
Fisher, Sewall Wright, and J. Haldane; and alignment of more traditional disciplines in natural history with this central theory in a series of books beginning with Dobzhansky in , and continuing with Mayr in for systematics, Simpson in for paleontology, and Stebbins in for botany, among several others. Simpson wrote Tempo and Mode to assert a distinctive theoretical corner for paleontology in evolutionary discussion and to counteract the denigration discussed in the first section of this paper.
That he was not entirely successful will be evident from the fact that most of the deprecatory quotations, cited earlier in this article, postdate the publication of his book. I shall argue, in this section, that Simpson failed because he bowed to the wrong solution in claiming that he could locate nothing distinctive after correctly defining a domain. The Science reporter's error in citing the book as Tempo and Mode of Evolution is important, and not an insignificant difference in a meaningless preposition.
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Simpson was a great and careful writer, who used words with meticulous precision and was an English major in college. He did not write his book to discourse on the tempo and mode of evolution in general, but to advance the key claim that tempo and mode are paleontology's distinctive subjects for winning insight into the causes of evolution. He spoke for paleontology, and against the extrapolationist vision, with some bravado in his introduction , p.
They [geneticists] may reveal what happens to a hundred rats in the course of ten years under fixed and simple conditions, but not what happened to a billion rats in the course of ten million years under the fluctuating conditions of earth history.
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Obviously, the latter problem is much more important. Tempo and Mode , like so many seminal books, lies completely outside the traditions of its profession. To be sure paleontologists had written copiously about "evolution"; but, in the profession, this word referred to the documentation of history, specifically to the establishment of phylogeny, not to a study of processes and mechanisms. Paleontological works on evolution proceeded in descriptive and chronological order.
If they attempted any closing statements on theoretical generalities, they tried to portray such conclusions as inductions in the enumerative mode from the facts of phylogeny—hence, the various "laws"—Cope's, Williston's, Dollo's—of the classical literature. Simpson turned this procedure around. Instead of an exhaustive tome in documentation, he wrote pages of stimulating suggestions. He started from the principles of neontological Darwinism as he saw the theory emerging. He then asked if major features of the fossil record could be reconciled to this modern version of Darwinism, without postulating any special macroevolutionary theory.
Tempo and Mode contains 36 figures, but only one portrays an animal—actually only the lower second molar and fourth premolar of the Eocene condylarth Phenacodus , cribbed from Osborn Figure 9, p. The rest are graphs, frequency distributions, and pictorial models. No paleontological innovation could have been more stunning than this. But the most innovative feature of all resides in Simpson's well-chosen title, for he properly selected tempo and mode as the two paleontological subjects that might provide novel theory, and not just phenomenology, to the evolutionary sciences.
His title is, therefore, a statement about paleontological relevance, a defense of the theoretical importance of those billion rats. There is much, of course, that paleontology cannot do—based on imperfection of the record, and our imposed inability to observe or manipulate past processes directly.
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But, in specifying tempo and mode, Simpson sought to isolate and feature the theoretically tractable subjects of paleontology. His argument is both simple and elegant: paleontology has unique access to questions of evolutionary tempo, which require the direct data of long durations. These paleontological tempos can and should be quantified to attain a testable generality transcending the. Scientific theory is, essentially, the attempt to explain nature's processes. By using uniquely paleontological data about pattern to infer the unseeable processes of long temporal spans, paleontology may be an active purveyor of evolutionary theory.
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This strategy of using uniquely paleontological data about tempo to infer mode, and thus to develop theory directly from the domain of macroevolution, pervades Simpson's book and underlies all his examples. To cite just two cases:. Among leaders of the second phase of the synthesis, only Simpson was well trained mathematically, and only he could read the primary source material of the first phase with full understanding. Dobzhansky, for example, often stated that he adopted a "father knows best" approach in his collaborations with Sewall Wright—that is, he simply accepted Wright's verbal interpretation because he could not understand Wright's equations in their own joint papers!
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Simpson was mathematically adept and a particularly fine statistician. His textbook, Quantitative Zoology , written with his wife Anne Roe, was a standard source for decades, and remains unmatched for clarity and well-chosen examples. How ironic that words built the bridge to the second phase, while formulae constructed the pillars and anchor of the first phase—so that, with Simpson's exception, the crucial linkage rested upon faith.
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Simpson based this hypothesis upon a fascinating treatment of data on generic longevity. He contrasted extant with extinct genera by plotting longevities as conventional survivorship curves. Extinct genera fit the ecological models without anomaly, but extant genera yielded a hump of "too many" values at extended longevities—in other words, too many living bivalve genera had inhabited our planet for too long according to random models of survivorship.
Simpson called this hump the bradytelic distribution. The tachytelic distribution then emerged as a theoretical concept for a spectrum of rates too rapid to be recorded in most geological circumstances, and therefore responsible for the notorious gaps of the fossil record, even in relatively complete stratigraphic sections.
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As a testimony to his proper restriction of science to the operational, Simpson said little about tachytely which can rarely be measured and must be inferred from gaps in the record , basing his entire conception upon an attempt to identify and quantify the tractable bradytelic distribution through analysis of survivorship data, as described above. Simpson's general argument is both illuminating and correct: tempos are a unique paleontological domain; modes may be inferred from them, and status as a source for theory thus conferred upon paleontology.
Why, then, did Simpson's work fail to establish such a role for the fossil record and not lead to an independent body of macroevolutionary theory—as the deprecatory quotes cited in section one of this paper, all postdating Tempo and Mode , demonstrate? Two reasons can resolve this only apparent paradox:. Thus, although Simpson did enunciate a methodology—modes from tempos—for discovering uniquely macroevolutionary theory, he applied the procedure to deny this possible outcome. In other words, he developed a method that might have yielded theory, and then claimed that none was to be found.
And this conclusion was no passive or subsidiary result of other purposes, but the central goal—and, in Simpson's view, the intellectual triumph—of his work. Paleontology became a dutiful son to the synthesis, and no longer an unruly child. Simpson concluded, with evident satisfaction , p. The materials for evolution and the factors inducing and directing it are also believed to be the same at all levels and to differ in mega-evolution only in combination and in intensity.