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Tempo and Mode in the Macroevolutionary Reconstruction of Darwinism STEPHEN JAY GOULD Museum of Comparative Zoology, Harvard University, Cambridge, MA 02138

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Tempo and Mode in the Macroevolutionary Reconstruction of Darwinism STEPHEN JAY GOULD Museum of Comparative Zoology, Harvard University, Cambridge, MA 02138 Proc. Nadl. Acad. Sci. USA Vol. 91, pp. 6764-6771, July 1994 Colloquium Paper This paper was presented at a coloquium ented "Tempo and Mode in Evolution" organized by Walter M. Fitch and Francisco J. Ayala, held January 27-29, 1994, by the National Academy of Sciences, in Irvine, CA. Tempo and mode in the macroevolutionary reconstruction of Darwinism STEPHEN JAY GOULD Museum of Comparative Zoology, Harvard University, Cambridge, MA 02138 ABSTRACT Among the several central nings of Dar- But conceptual complexity is not reducible to a formula or winism, his version ofLyellian uniformitranism-the extrap- epigram (as we taxonomists of life's diversity should know olationist commitment to viewing causes ofsmall-scale, observ- better than most). Too much ink has been wasted in vain able change in modern populations as the complete source, by attempts to define the essence ofDarwin's ideas, or Darwin- smooth extension through geological time, of all magnitudes ism itself. Mayr (1) has correctly emphasized that many and sequences in evolution-has most contributed to the causal different, if related, Darwinisms exist, both in the thought of hegemony of microevolutlon and the assumption that paleon- the eponym himself, and in the subsequent history of evo- tology can document the contingent history of life but cannot lutionary biology-ranging from natural selection, to genea- act as a domain of novel evolutionary theory. G. G. Simpson logical connection of all living beings, to gradualism of tried to combat this view of paleontology as theoretically inert change. in his classic work, Tempo and Mode in Evolution (1944), with It would therefore be fatuous to claim that any one legit- a brilliant argument that the two subjects of his tide fall into a imate "essence" can be more basic or important than an- unue paleontological domain and that modes (processes and other. Yet I wish to focus on a Darwinism that is more causes) can be inferred from the qumtitative study of tempos pervasive than some ofthe other meanings-a status won by (pattern). Nonetheless, Simpson did not cash out his insight to its role as the fundamental operational, or methodological paleontology's theoretical benfit because he followed the strict postulate of all Darwin's theorizing and experimentation. doctrine of the Modern Synthesis. He studied his domain of Charles Lyell was Darwin's guru and intellectual father potential theory and concluded that no actual theory could be figure. Darwin commented, in a statement that (for once in found-and that a full account of causes could therefore be his writing) does not reek offalse modesty in proper Victorian located in the microevolutionary realm after all. I argue that taste, "I always feel as if my books came half out of Lyell's Simpson was unduly pe c and that modernism's beliefin brain" (2). Much of Lyell's thinking did not contribute to reductionistic unction (the conventional view of Western Darwin's evolutionism and may have acted as an impediment intellectuals from the 1920s to the 1950s) needs to be supplated to transmutation-in particular, Lyell's steady-state vision of by a postmodernist commitment to pluralism and multiple change without direction. But we can scarcely doubt that levels of causation. Macro- and microevolution should not be Lyell's major working postulate and philosophical premise- viewed as opposed, but as truly complementary. I describe the his uniformitarian vision-becamejust as firmly embedded in two major domain where a helpful macroevolutionary theory Darwin's thought and scientific action. may be sought-unsmooth causal boundaries between levels (as Lyell's uniformitarianism held that the full panoply ofpast illustrated by punctuated equilibrium and mass extinction) and events, even those of greatest extent and apparent effect, hierarchical exp n of the theory of natural selection to must be explained as extrapolations from causes now oper- levels both below (gene and cell-line) and above organisms ating at their current observable rates and intensities. In other (demes, species, and clades). Problems remain in operationally words, and invariably, the small and immediate may be defining selection at non-organismic levels (emergent traits vs. extended and smoothly accumulated-drop by drop and emergent fitness approaches, for example) and in specifying the grain by grain-through time's immensity to produce all nature and basis oflevels, but this subject should be the central scales of historical events. Time is the great enabler. No focus in formulating a more ample and satisfactory general uniquenesses should be attributed to events oflarge scale and theory of evolution on extended Darwinian principles. long times; no principles need be established for the great and the lengthy; all causality resides in the smallness of the observable present, and all magnitudes may be explained by Darwin's Uniformitarianism and the Downgrading extrapolation. of Macroevolution Darwin accepted and promulgated Lyell's uniformitarian vision in all its uncompromising intensity. Extrapolationism We yearn to capture the essence of complexity in a line. (the methodological side of uniformity) underlies and unites Rabbi Hillel (ca. 30 B.C.-A.D. 10) wrote: "What is hateful to the otherwise disparate pieces and opinions in the Origin of you do not do to your neighbor. That is the whole Torah. The Species. What other principle could coordinate, for example, rest is commentary." And Marcus Aurelius, a century later Darwin's hostility to mass extinction (ref. 3, pp. 317-329), his and a culture apart, stated: "Look to the essence of a thing, brilliant section on graded structural transition in the evolu- whether it be a point of doctrine, of practice, or of interpre- tion ofcomplex and "perfect" organs like the eye (ref. 3, pp. tation." 186-189), his initial case of pigeon breeding as a model for change at all scales (ref. 3, pp. 20-28), and even his choice of The publication costs ofthis article were defrayed in part by page charge the phrase "natural selection" as an analogy to small-scale payment. This article must therefore be hereby marked "advertisement" changes produced by breeders and called "artificial selec- in accordance with 18 U.S.C. §1734 solely to indicate this fact. tion." 6764 Downloaded by guest on October 1, 2021 Colloquium Paper: Gould Proc. Natl. Acad. Sci. USA 91 (1994) 6765 Considerjust two statements from the Origin ofSpecies on Evolution: the power of geological time to build small and present Paleontology supplies factual data on the actual rates of changes into any observed or desired effect. First, on na- change in the skeleton and the patterns of phyletic change in ture's greater power based on time and fuller scrutiny: the skeleton. Because of the inherent limitations of paleon- As man can produce and certainly has produced a great result tological data, however, it cannot perceive the factors pro- by his methodical and unconscious means of selection, what ducing such changes. Attempts to do so merely represent a may not nature effect? Man can act only on external and superimposition of neobiological concepts on paleontological visible characters: nature cares nothing for appearances. .. data. She can act on every internal organ, on every shade of Such invalid statements in professional publications often constitutional difference, on the whole machinery of life... follow an unfortunate path towards inclusion in basic text- How fleeting are the wishes and efforts of man! how short his time! and consequently how poor will his products be, com- books-and errors in this particular medium are almost pared with those accumulated by nature during whole geo- immune to natural selection, as extinction-proof as a living logical periods. (ref. 3, p. 83) fossil in the deep ocean. One major, and very fine, introduc- tory text (ref. 6, p. 524) states: Second, on time's promotion of the infinitesimal to great magnitude: Evolution can be studied on the population level only with living organisms. The fossil record provides too few data to It may be said that natural selection is daily and hourly allow such treatment; it merely allows paleontologists to scrutinising, throughout the world, every variation, even the reconstruct the history of animal and plant groups [the re- slightest; rejecting that which is bad, preserving and adding up striction of our efforts to descriptive phenomenology]. The all that is good; silently and insensibly working, whenever and population approach makes it possible to ask such questions wherever opportunity offers, at the improvement of each as: What is the rate of evolution in a given species? What organic being in relation to its organic and inorganic condi- factors influence the course or rate of evolution? What tions oflife. We see nothing ofthese slow changes in progress, conditions are necessary for evolution to begin or cease? until the hand of time has marked the long lapse of ages. (ref. 3, p. 84) Funny. I would include these three questions within a set most amenable to resolution by the data The pure extrapolationism of Darwin's uniformitarian per- of fossils and their spective creates an enormous, if not fatal, problem for temporal distribution! paleontology. We would like to be a source of meaningful As a final illustration of the reductionistic biases that still evolutionary theory, for this discipline explains the pattern- beset this most comprehensive of fields, and of the usual ing of the objects we study. But if every event at our scale tendency to ignore or devalue theory based on whole orga- may be built by extrapolation from a present that contains all nisms or long times, the assigned reporter for Science mag- causes, then we have no theoretical contribution to make. We azine presented a remarkably skewed and parochial view of are still needed in a lesser role, of course, for history is the conference that honored Simpson's Tempo and Mode at massively contingent, as Darwin well knew, and theory must its half-century, and formed the basis for this published therefore underdetermine actual events.
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