The Structure of Evolutionary Theory

by Stephen Jay Gould

  Moreover, if historical data hold such limited promise, then the conse­quences become even more serious for science in general. For if we acknowl­edge that extrapolationism can't suffice in principle because much of macro-evolution proceeds by patterns of differential birth and death among species, and if we cannot generate any theory about such higher-level sorting because we cannot observe the constituent events directly, then much of evolution be­comes unknowable in principle. Fortunately, such pessimism may be firmly [Page 566] rejected (see Gould, 1986, on Darwin's use of historical science, and 1989c, on applications to the history of life at greatest scale). Much of the best sci­ence — inevitably and properly — relies upon inference and insight, not always upon direct sight.

  Young scientists can easily succumb to the thrall of such proclamations by leaders. The stupidest passage I ever wrote occurs in the heart of a contribu­tion to independent macroevolutionary theory — our original piece on punc­tuated equilibrium, where I stated (this excerpt comes from my part of a joint text with Niles Eldredge):

  First, we must emphasize that mechanisms of speciation can be studied directly only with experimental and field techniques applied to living organisms. No theory of evolutionary mechanisms can be generated directly from paleontological data. Instead, theories developed by students of the modern biota generate predictions about the course of evolution in time... We can apply and test, but we cannot generate new mecha­nisms. If discrepancies are found between paleontological data and the expected patterns, we may be able to identify those aspects of a general theory that need improvement. But we cannot formulate these improve­ments ourselves (Eldredge and Gould, 1972, pp. 93-94).

  Stanley (1975) then properly rebuked us for such unwarranted subservi­ence. Our current partnership with “neobiology,” based on the “bonded in­dependence” of macroevolutionary theory — the recognition that we can gen­erate and test novel concepts but cannot come close to a fully adequate account of macroevolution without the vital input of microevolutionary the­ory — produces a better balance of subdisciplines. This mutually sustaining in­teraction must benefit paleontology, but such an enlarged view will also aid anyone, in any evolutionary subdiscipline, who wishes to comprehend the “grandeur in this view of life.”

  From Overstressed Doubt to Overextended Certainty

  A TALE OF TWO CENTENNIALS

  Darwin did all Americans a mnemonic favor by entering the world on the same day as Abraham Lincoln — February 12, 1809. He also made life sim­pler for conference organizers by publishing the Origin of Species in 1859, at age 50 — thus intensifying the force of commemorations and cutting their re­quired number in half. We have indeed celebrated mightily at the requisite times, with the usual array of resulting Festschriften. As others have noted, and as I have stated throughout this chapter, the two celebrations of the 20th century occurred at maximally disparate moments in the history of evolution­ary theory: in 1909 at the heyday of doubt about natural selection as a potent mechanism, and in 1959 at the apotheosis of certainty about the nearly exclu­sive power of selection as an agent of evolutionary change. A comparison of [Page 567] the two centennials therefore provides a striking example and epitome of the success (and rigidification) of the Modern Synthesis.

  Consider two of the leading symposia in 1909: the “official” celebration held in Cambridge (Seward, 1909), and the major American vernacular Fest­schrift, published as a special issue of Popular Science Monthly in 1909. The cardinal message reeks with ambiguity (for a celebration of Darwin's accom­plishments): complete confidence in the fact of evolution; lavish praise for Darwin as midwife of the factual confirmation; admission that no consensus has been reached on mechanisms of evolutionary change; and a general feel­ing that natural selection plays, at most, a minor role.

  A few strong selectionists restated their claims, most notably the two sur­viving members of Darwin's inner circle: Joseph Hooker and Alfred Russel Wallace. But even Wallace, the most ardent of selectionists, could no longer muster the confidence and enthusiasm of former years. The qualifiers in his “triumphalist” statement could not be more revealing — for he can now only assert that selection has been adopted as a “satisfying” solution by “a large number” of qualified experts: “And this brings me to the very interesting question: Why did so many of the greatest intellects fail, while Darwin and myself hit upon the solution of this problem — a solution which this celebra­tion proves to have been (and still to be) a satisfying one to a large number of those best able to form a judgment on its merits” (Wallace, 1909, p. 398).

  The range of opinions expressed at the Cambridge symposium illustrates the turmoil in evolutionary theory at Darwin's 100th birthday. Participants spanned the full spectrum from August Weismann's defense of selection's Allmacht (all-sufficiency) to Bateson's claims for selection's impotence (ac­companied by lavish praise for Darwin's other achievement in establishing the fact of evolution — see p. 396). More commonly, authors tried to assimi­late Darwin to their own disparate views, thereby turning the profession's hero into a chameleon. For de Vries, Darwin became a closet saltationist (see p. 416 on editor Seward's annoyance at de Vries' false and self-serving reinterpretation of Darwin). For Haeckel, Darwin ranked as a pluralist, a true kin to the speaker who had dedicated volume 2 of his Generelle Morphologie col­lectively to Darwin, Lamarck, and Goethe! (Haeckel, 1866). Haeckel wrote (1909, pp. 140-141), trying to distance Darwin from Weismann's position (called “neoDarwinism” at the time), and to reinvent the symposium's hero as a man in the middle between selectionism and Lamarckism: “It seems to me quite improper to describe this [Weismann's] hypothetical structure as 'NeoDarwinism.' Darwin was just as convinced as Lamarck of the transmis­sion of acquired characters and its great importance in the scheme of evolu­tion ... Natural selection does not of itself give the solution of all our evolu­tionary problems. It has to be taken in conjunction with the transformism of Lamarck, with which it is in complete harmony.”

  The strategy of Henry Fairfield Osborn (in heaping praise on Darwin while denying any substantial power to natural selection) well illustrates the most consistent theme of both 1909 symposia. “There is no denying,” Osborn writes (1909, p. 332), “that there is today a wide reaction against the central [Page 568] feature of Darwin's thought and this leads us to consider the merit of this re­action.” Osborn then invoked a time-honored diplomatic tactic by defining Darwin's achievement as threefold: establishment of the “law of evolution” itself, documentation of the fact of evolution, and development of the theory of natural selection. Since the first two propositions cannot be gainsaid, why fuss over the third, even if Darwin overemphasized the role of natural selec­tion? Osborn notes (1909, p. 332): “There is some lack of perspective, some egotism, much onesidedness in modern criticism. The very announcement, 'Darwin deposed,' attracts such attention as would the notice 'Mt. Blanc re­moved.'”

  Osborn correctly identifies two claims at the core of Darwinian theory: (1) selection operates on undirected variability to cause evolutionary change (legs one and two of my tripod); and (2) gradualism rules in geological time (leg three and the methodological pole): “In the operations of this intimate circle of minute variations within organisms, he was inclined to believe two things: first that the fit or adaptive always arises out of the accidental, or that out of large and minute variations without direction selection brings direc­tion and fitness; second, as a consistent pupil of Lyell, he was inclined to be­lieve that the chief changes in evolution are slow and continuous” (Osborn, 1909).

  But Osborn then gently chides Darwin for putting too much faith in the power of selection: “There can be no question, however, that Darwin did love his selection theory, and sometimes overestimated its importance” (1909, p. 336). Then, as a consummate politician and administrator, Osborn put a positive spin on his criticism — granting ultimate praise with only a faint damn. He emphasized the most common of all anti-Darwinian arguments — that selection can
only operate as a negative force (“judicial” rather than “creative”). But he then converted Darwin's weakness to centennial strength with a remarkable diplomatic move: Darwin's problems arose from his igno­rance of heredity, but he set a great task for us thereby, and we must per­severe:

  Selection is not a creative principle, it is a judicial principle. It is one of Darwin's many triumphs that he positively demonstrated that this judi­cial principle is one of the great factors of evolution. Then he clearly set our task before us in pointing out that the unknown lies in the laws of variation and a stupendous task it is. At the same time he left us a legacy in his inductive and experimental methods by which we may blaze our trail. Therefore, in this anniversary year, we do not see any decline in the force of Darwinism but rather a renewed stimulus to progressive search.

  This diplomatic theme — that Darwin did not discover an adequate mecha­nism of evolution, but we celebrate his centennial because he opened up a new world of research — became a virtual litany for symposiasts. For exam­ple, William Morton Wheeler virtually threw selection in the ashcan as he praised Darwin: “And even if we go so far as to say that natural selection may eventually prove to be an unimportant factor in evolution, to be consigned to [Page 569] the limbo of defunct hypotheses, together with Darwin's views on Pangenesis, sexual selection and the origin of species from fluctuating variations, we must, I believe, still admit that the great English naturalist opened up before us a vast new world of thought and endeavor” (Wheeler, 1909, p. 385).

  T. H. Morgan, who would later become a strong supporter of natural selection, began his centennial contribution by expressing the standard argument that Darwin's importance transcends the limitations of natural selection: “The loyalty that every man of science feels towards Darwin is something greater than any special theory. I shall call it the spirit of Darwinism, the point of view, the method, the procedure of Darwin” (Morgan, 1909, p. 367). Morgan then ended his article, entitled “For Darwin,” by heaping tangential scorn on natural selection while praising the liberating generality of evolution itself: “We stand today on the foundations laid 50 years ago. Darwin's method is our method; the way he pointed out we follow, not as the advocates of a dogma, not as the disciples of any particular creed, but the avowed adherents of a method of investigation whose inauguration we owe chiefly to Charles Darwin. For it is this spirit of Darwinism, not its formulae, that we proclaim as our best heritage” (1909, p. 380).

  William Bateson, the least Darwinian of the symposiasts, began his article on the same theme, and then stated his own view right up front: “Darwin's work has the property of greatness in that it may be admired from more as­pects than one. For some the perception of the principle of Natural Selection stands out as his most wonderful achievement to which all the rest is subordi­nate. Others, among whom I would range myself, look up to him rather as the first who plainly distinguished, collected, and comprehensively studied that new class of evidence from which hereafter a true understanding of the process of Evolution may be developed” (Bateson, 1909, p. 85). Bateson then added (see p. 596 for more on this quotation and Bateson's general views), in a statement that strikes me as the most appropriately generous, genuine and cogent expression of an argument that could be advanced with equal validity today: “We shall honor most in him not the rounded merit of finite accom­plishment, but the creative power by which he inaugurated a line of discovery endless in variety and extension” (1909, p. 85).

  Finally, consider the dilemma of A. C. Seward, general editor of the “of­ficial” Cambridge celebration, who followed a British tradition of fairness in inviting all sides, but then struggled to find some coherence amidst the Babel of papers he received: “The divergence of views among biologists in regard to the origin of species and as to the most promising directions in which to seek for truth is illustrated by the different opinions of contributors. Whether Dar­win's views on the modus operandi of evolutionary forces receive further con­firmation in the future, or whether they are materially modified, in no way af­fects the truth of the statement that, by employing his life 'in adding a little to Natural Science,' he revolutionized the world of thought” (Seward, 1909, p. vii).

  I can imagine no contrast more stark, no reversal so complete, as the comparison of these doubts in 1909 with the confidence and near unanimity expressed [Page 570] fifty years later at the Origin's centennial in 1959. The success of the Modern Synthesis established the difference. Beginning as a pluralistic mar­riage of Darwin and Mendel in the 1930's, the Synthesis had hardened by 1959 into a set of core commitments that, at least among epigones and aco­lytes, had become formulaic and almost catechistic, if not outright dogmatic.

  Again, I will consider two leading Festschriften of this later centennial, the two major American celebrations in this case: the American Philosophical Society's annual general meeting in Philadelphia, and the elaborate festival held in Chicago in 1959 (published as a three volume compendium, edited by Sol Tax, in 1960). Major speakers at both meetings attributed the remarkable uniformity of opinion on all major issues to the success of the Synthesis, par­ticularly to a consensus on the paramount, virtually exclusive, role of natural selection as the cause of evolutionary change. Ledyard Stebbins, appropri­ately for the City of Brotherly Love, spoke in Philadelphia about the unifying power of natural selection: “The last quarter of the century which has elapsed since the publication of The Origin of Species has seen the gradual spread and an almost universal acceptance by biologists actively working with problems of evolution of some form of the neodarwinian concept of evolutionary dy­namics. This concept may be broadly defined as one which, like Darwin's original concept, maintains that the direction and rate of evolution have been largely determined by natural selection” (Stebbins, 1959, p. 231). Mean­while, in Chicago, Julian Huxley gave a capsule history of Darwinism, ascrib­ing the same binding role to natural selection: “The emergence of Darwinism, I would say, covered the fourteen-year period from 1859 to 1872; and it was in full flower until the 1890's, when Bateson initiated the anti-Darwinian re­action. This in turn lasted for about a quarter of a century, to be succeeded by the present phase of Neo-Darwinism, in which the central Darwinian concept of natural selection has been successfully related to the facts and principles of modern genetics, ecology, and paleontology” (Huxley, 1960, p. 10).

  Michael Lerner's development of the argument (in the Philadelphia symposium) may be viewed as typical for this confident time. He begins with the venerable (if cryptic) motto of the Greek poet Archilochus: “The fox knows many things, but the hedgehog knows one big thing.” As a profession, Lerner states, we marched along the path from Darwin to the Modern Synthesis as urchins, following the “one big thing” of natural selection, and ultimately re­jecting the major alternatives as “sins against Occam's razor.” Lerner wrote:

  Their one big thing, natural selection, set at rest the doctrine of special creation. In combination with our knowledge of Mendelian inheritance acquired since Darwin's day, it rendered obsolete such alternative theo­ries of evolution as were based on extra-mechanical agencies, or on di­rect adaptation of organisms to their immediate environment (that is, on inheritance of acquired characters), and exposed them as sins against Occam's razor. Natural selection furnished the binding principle for a general or unified theory of historical change in the living world (Lerner, 1959, p. 173). [Page 571]

  Lerner felt so confident that he proclaimed natural selection necessarily dominant a priori, not merely validated by evidence: “There is no longer any doubt that natural selection is more than a theoretical possibility — it is un­questionably a logically imperative necessity in any accounting for evolution” (1959, p. 174). He acknowledges, of course, that selection cannot manufac­ture, but can only shape, the physical material of organisms, but he compares selection's role to Michelangelo's claim that a great sculptor works to liberate beautiful forms from the blocks of stone that begin as their raw material — a lovely, poetic r
endition of the standard argument for selection's creativity (see Chapter 2). In so doing, Lerner trivializes the role of potential constraints, even suggesting that a sow's ear might not represent an impossible starting point for a silk purse:

  In the same way, natural selection does not originate its own building blocks in the form of mutations of genes. But from them it does create complexes; it solves in a diversity of ways the great variety of problems that successful individuals and populations face; it builds step by step, even if by trial and error, entities of infinite complexity, ingenuity, and be one inclined to say so, beauty. Granted that it needs appropriate raw ma­terials, that it may not necessarily be able to make a silk purse out of a sow's ear; yet, interacting with other evolutionary mechanisms, it has created the human species out of stuff which in its primordial stage may have looked no more promising (1959, p. 179).

  Lerner's conclusion bears more than a whiff of similarity to the apostolic creed, suitable for multiple repetition by the faithful: “Evolution is the most fundamental biological law yet discovered. Natural selection is the basic mechanism implementing it. The principle of descent with modification, creatively, albeit opportunistically, husbanded by natural selection, is as firmly established as any concept in biology” (1959, pp. 181-182). (I don't disagree with the content; I just don't feel fully at ease with the triumphalist presen­tation.)

  If Lerner verges on the overconfident, some centennial expressions treated any conceivable alternative with disdain. I have already cited Mayr's as­sertion of “complete unanimity” in competent professional opinion and of the “colossal ignorance” of remaining doubters. In Chicago, Mayr even re­sorted to theological language in citing “the opposing evils of Lamarckism and saltationism” (Mayr, 1960, p. 350). Others noted, but with some sense of unfairness, the vilification of Lamarck. C. H. Waddington regretted that “Lamarck is the only major figure in the history of biology whose name has become, to all intents and purposes, a term of abuse” (1960, p. 383); while Marston Bates noted that “Lamarck remains some kind of horrible example of wrong thinking in the introductory textbooks” (1960, p. 548).