The Structure of Evolutionary Theory

by Stephen Jay Gould

  But the prevailing tenor of these symposia does not display pugnaciousness towards opponents (which would imply an existing and meaningful conflict of uncertain resolution), but smugness in the confidence that a total victory has, at last, been achieved after a long battle (cigars and a drink around the [Page 572] fireplace at night, to cite an androphilic metaphor of past and privileged generations). Evolutionary theory is now essentially complete; we know how the process works and now only need to supply some details. G. G. Simpson had written in a 1950 essay (reprinted in Simpson, 1964, p. 14): “This general theory is now supported by an imposing array of paleontologists, geneticists, and other biological specialists. Differences of opinion on relatively minor points naturally persist and many details remain to be filled in, but the es­sentials of the explanation of the history of life have probably now been achieved.”

  The Chicago symposiasts continually asserted their agreement with this confident consensus. Tinbergen spoke of “the all-pervading power of selec­tion” (1960, p. 609). Huxley (in Tax and Callender, 1960, volume 3, p. 45) defined the future task of evolutionary biology as filling in the blanks: “We are no longer having to bother about establishing the fact of evolution, and we know that natural selection is the major factor causing evolutionary change. Our problems now concern working out in detail how natural selec­tion operates, defining what we mean by 'increase of organization,' tracing the general trends that appear in the course of evolution, and so on.” He then described the range of phenomena that selection can fashion — in short, every­thing that might happen in evolution! “It produces branching; it produces in­creasing adaptation, improvement, progress, or whatever you like to call it; and it produces horizontal persistence of branches, or stabilization” (in Tax and Callender, 1960, volume 3, p. 139).

  I argued in the last section that “hardening” of the Synthesis gains clearest expression in an increasing faith that adaptation must be both the impetus and result of nearly any evolutionary change. The 1959 symposia continually stress this theme. Panadaptationism became a premise for Chicago's major panel discussion on “the evolution of life,” not an issue to be adjudicated by participants. Panelists received a list of assumptions, including the statement that “transformation always leads to adaptive or, better, teleonomic results” (in Tax and Callender, 1960, volume 3, p. 109).

  Confidence in adaptation grew so great that many symposiasts presented their arguments in a “can't fail” manner, by delimiting a set of supposedly in­clusive outcomes, each validating adaptation for any conceivable result.* Mayr, for example, argued that the general ecological rules of Bergmann and others enjoy good adaptive explanations, but that the numerous exceptions also affirm adaptation because local (and opposite) factors can override the [Page 573] general trend: “In recent years the analysis of these rules has shown that, as we stressed earlier, all phenotypes are compromises among a variety of con­flicting selection pressures. As a result, there are many so-called exceptions to such rules, where a new selection pressure takes over and adjusts an organism or a local population in a different way” (in Tax and Callender, 1960, volume 3, p. 138).

  In another example of victory by virtual definition, Tinbergen acknowl­edged that randomness might provide a theoretical alternative to adaptation in the evolution of a behavior. But since he construed randomness only as an absence of evidence for selection, and since he regarded the variety of con­ceivable adaptationist explanations as effectively inexhaustible, how would one ever validate randomness in any particular case? “This task [of explain­ing the results of evolution] really amounts to an assessment of the relative importance of the contribution made by random variation, on the one hand, and by adaptation directed by selection, on the other. Since randomness is, per definition, detectable only by elimination of every conceivable directed-ness, it is natural that this approach should lead to a quest for directedness” (Tinbergen, 1960, p. 602).

  Adaptation pervades Tinbergen's discourse and world of thought. He even proposes a turnabout from Darwin's own, eminently sensible, view that non-adaptive features of conservative inheritance (deep homologies) provide opti­mal characters for taxonomic definition, since more recent adaptations tend to be homoplastic (as easily convergeable with similar features in indepen­dent lineages) and nondistinctive. Tinbergen, on the other hand, states that his paper will focus upon “the extent to which taxonomic characters must be assumed to be due to natural selection” (1960, p. 595). He then carries his adaptationist paean even further by arguing that evolutionists (as opposed to other scientists who might need to classify for different reasons) must divide organisms and designate their characters in terms of adaptive complexes, thus assuring his preferred interpretation by predefining the structure of ob­servation itself:

  The conclusion that adapted features are systems of functionally related components forces us to reconsider once more the question What is a taxonomic character? The answer is, of course, that it depends on the aims, which the scientist has in mind. The classifier is fully entitled to use, e.g., the tameness of the kittiwakes, their nest-building behavior, the black neckband of their young, and their nidicolous habits as four sepa­rate characters. But the evolutionist is not entitled to treat them as four independent characters. To him, the correct description of the characteristics of the species would be in terms of adapted systems, such as (1) cliff breeding; (2) pelagic feeding; (3) orange inside of the mouth and related characteristics of posturing (Tinbergen, 1960, p. 609).

  These symposia not only featured adaptation as the centerpiece of the biological world, but also extended the concept to all other fields included within their program of lectures. Robert McC. Adams, then a young anthropology [Page 574] professor at Chicago, but later the Secretary of the Smithsonian Institution, confessed an initial skepticism about the symposium, based on “uncertainty about finding anything in common to talk about with representatives of other disciplines” (in Tax and Callender, 1960, volume 3, p. 268). But he dis­covered relevance in learning to view human societies as “adaptive mecha­nisms,” and in using this idea to grant an “evolutionary role” to culture, thus equating adaptation with the entire realm of potential evolutionary insight:

  As man evolves, he superadds culture to his genetic equipment, and by this new addition he is enabled to adapt in a whole series of much more effective and complex ways — to spread himself over the entire globe, to construct very complex societies, and, in fact, frequently to direct the evolution of species all around him. Human societies are adaptive mechanisms; they have to be understood as having an evolutionary role rather than as uniquely human creations that are not to be compared with the evolutionary development of other organisms (Adams in Tax and Callender, 1960, volume 3, p. 268).

  Only one “interloper,” historian Ilza Veith, dared to suggest that nonadaptive phenomena might be important in evolution, but Julian Huxley firmly dismissed these worries:

  Veith: In my field, perhaps the most rewarding line would be to find those moments or those evolutionary processes that will present weak­nesses, where maladaptation will occur, and where the mind will not continue to function in its normal manner.

  Huxley: I am sorry you wish to concentrate on maladaptation. I should think it would be much better to concentrate on adaptation from the positive angle (Tax and Callender, 1960, volume. 3, p. 269).

  Sweetness surely triumphed in Chicago, but perhaps at the expense of light. The panel discussions ended in a virtual orgy of agreement, with Darwin as hero and adaptation as king. Even Sewall Wright, who had approached selec­tion with ambiguity for years but had finally made his peace with the hard­ened consensus (though in his own idiosyncratic way — see pp. 522–524), ended his paper by writing: “From a more general standpoint, all of this is merely an elaboration in terms of modern genetics of the conception of evolu­tion by natural selection advanced by Darwin in the Origin of Species a hun­dred years ago” (Wright, 1960, p. 471). Wright became even more accom­m
odating in his role on the “evolution of life” panel. As the discussion wound down, Wright presented a simple comment as a last word before Julian Huxley's summary: “I agree with everybody.”

  Yet a bit of rain, as our mottoes proclaim, must fall on any long parade. One skeptic and whistle blower did speak out in Chicago, unsurprisingly a paleontologist who doubted the sufficiency of synthetic adaptationism as a complete explanation for the fullness of events in geological time. The Ameri­can vertebrate paleontologist E. C. Olson had become disturbed by the in­creasingly dogmatic, peremptory, and exclusivist tone that many synthesists [Page 575] had adopted in this period of hardening. He spoke, with some irony, of the consensus that “has come to be known as the 'synthetic theory of evolution' but has also been variously termed 'selection theory,' 'neo-Mendelian theory,' and 'neo-Darwinian theory.' It is unfortunate that occasionally it is called 'the theory of evolution,' as if no other could exist” (Olson, 1960, p. 524).

  Olson then identified three aspects of the logic and sociology of the syn­thetic theory that, in veering towards dogmatism, made him uncomfortable. First, the theory had become flexible enough to encompass all possible results almost a priori, thus setting itself no challenges for potential refutation:

  The feeling of a slight sense of frustration in the elasticity involved in developing a universal explanation is hard to avoid . . . There is little or nothing that cannot be explained under the selection theory, and, at pres­ent, this theory appears to be unique in this respect (1960, p. 530) . . . This possible danger is amply revealed in some studies of the last decade which seem more concerned with fitting results into the current theory than with evaluation of results in terms of a broader outlook. Further, of course, much research is conceived and carried out within the frame­work of the theory, and, no matter what its excellence, is not likely to break out of this framework (1960, p. 536).

  Second, the synthesists themselves often haughtily dismiss those who disagree as misguided, if not obtuse: “The statement is made, in effect, that those who do not agree with the synthetic theory do not understand evolution and are incapable of so doing, in most cases because they think typologically . . . Some avid proponents of the synthetic theory would appear to ... eliminate as competent students of evolution, because of their inability to understand the theory, those who may disagree” (1960, pp. 526-527; Olson's italics).

  Third, the success of consensus and consequent derision has silenced most doubters, but their numbers may be large and their questions cogent: “There exists, as well, a generally silent group of students engaged in biological pur­suits who tend to disagree with much of the current thought but say and write little ... It is, of course, difficult to judge the size and composition of this si­lent segment, but there is no doubt that the numbers are not inconsiderable. Wrong or right as such opinion may be, its existence is important and cannot be ignored or eliminated as a force in the study of evolution” (1960, pp. 523-524). As a paleontologist, Olson expressed most unhappiness with the “con­sistency argument” that awarded the synthetic theory hegemony over all scales of macroevolution — a misplaced confidence achieved by extrapolating, by fiat more than by evidence, a process that undoubtedly works in the ecological here and now to a sufficient explanation for all major changes oc­curring over hundreds of millions of years (1960, pp. 531 and 533).

  Yet, however cogent Olson's doubts, his attempt to inject more pluralism and skepticism into evolutionary theory ultimately failed — and for a valid reason from the orthodox point of view. A successful whistle blower must proceed beyond the exposure of faults in his boss's domain; he must also sug­gest a path towards greater accuracy and fuller explanation. And, on this [Page 576] constructive side, Olson could offer nothing. He ventured a few comments about cytoplasmic inheritance as a possible mechanism that might not follow all synthetic rules, but such a limited and inadequate speculation could not fuel such a comprehensive set of doubts! Revisionists would gain no hearing until they could propose an extensive and positive set of extensions or alter­natives — and I write this book because I believe that such an affirmative pro­gram has now been formulated. Olson's critique achieved no currency, and the hardened version descended from the empyrean academy into the vernac­ular world of textbooks, the ultimate test of establishment by social imposi­tion as well as by professional consensus.

  ALL QUIET ON THE TEXTBOOK FRONT

  Professional writing tends to be nuanced and judicious. Even the strongest partisan finesses his commitment and adds at least a footnote or tangential comment, so that any charge of oversimplification or dogmatism may be countered by stating: “but look on page 381 (in the small print); you see, I raised that caveat myself.”

  To learn the unvarnished commitments of an age, one must turn to the textbooks that provide “straight stuff” for introductory students. Yes, text­books truly oversimplify their subjects, but textbooks also present the central tenets of a field without subtlety or apology — and we can grasp thereby what each generation of neophytes first imbibes as the essence of a field. Moreover, many textbooks boast authorship by the same professionals who fill their technical writings with exceptions, caveats, and complexities.

  I have long felt that surveys of textbooks offer our best guide to the central convictions of any era. What single line could be more revealing, more at­tuned to the core commitment of a profession that bathed in the blessings of Victorian progressivism, and aspired to scientific status in Darwin's century, than the epigram that Alfred Marshall placed on the title page to innumerable editions of his canonical textbook, Principles of Economics: “natura non facit saltum.”

  The changing foci of 20th century textbooks provide direct insight into the history of evolutionary thought and the eventual triumph of Darwinism. In particular, if the Synthesis truly hardened, as I have argued, then texts fol­lowing the 1959 centennial celebrations — the apogee of strict selectionism — should describe evolution in unambiguously panadaptationist language, and should extol the sufficiency of natural selection to craft the entire range of evolutionary phenomena at all scales, ecological to geological.

  This section does not present a systematic survey of texts, though I have consulted everything I could find, including nearly all-major American books for introductory college biology (and several high school textbooks as well). A more complete search, extended back in time to cover the early days of the Synthesis, and the pre-synthetic period as well, would provide a fascinating topic for a dissertation in the history of science or education. This field of ver­nacular expression has been neglected by scholars, though the subject would [Page 577] yield great insight (for such material obviously represents the only formal contact that most students ever receive with any given discipline).

  I apologize for my almost anecdotal approach, but I think that I have identified a robust pattern supporting the hypothesis of hardening. I will focus on the two topics that authors of texts found most congenial in their efforts to explain synthetic evolutionism to introductory audiences: the centrality of adaptation, and the sufficiency of synthetic microevolution to explain events at all scales. (I consider here only the evolution chapters of comprehensive bi­ology texts for introductory courses, not entire textbooks on evolution. These short, unvarnished and straight-line accounts of adaptation and extrapola­tion appear in the context of such epitomes. Full texts on evolution, which cannot be called “introductory” or “elementary” (for such courses have al­ways been taught at intermediate or advanced levels in American universi­ties), do treat the subject more comprehensively, with a proper listing, often called “textbooky” in our jargon, of divergent views.)

  Adaptation and natural selection

  In this age of sound bites, even short chapters include final summaries to tell students the pith of what they must remember. Consider the following from Nelson, Robinson, and Boolootian (1967, p. 249), written to summarize a chapter entitled “Evolution, Evidences and Theories.” I cite the enti
re state­ment, not an excerpt:

  Principles

  1. Charles Darwin proposed a theory of evolution based on variation, competition, and consequent natural selection.

  2. The basic mechanism of evolution is now known to be changes in gene frequencies of populations through time, guided by natural selec­tion.

  On the subject of exclusivity, Darling and Darling (1961, p. 199) tell us “any organism is a bundle of interacting adaptations. Most all the fea­tures of all living things are adaptations.” Howells (1959, p. 24), a great evo­lutionary anthropologist publishing his popular text in Darwin's centennial year, discussed natural selection with his usual panache and good humor, but also in the all-encompassing celebratory mode: “So much for natural selec­tion, the external force, that finger beckoning to the otherwise unguided he­redity of an animal type. All other principles and facts of evolution may be satisfactorily related to it or explained by it, and the century following 1859 has seen Darwin triumphant.”

  Simpson, Pittendrigh and Tiffany (1957, p. 405), an excellent text that dominated the market for years (and featured a leading architect of the Syn­thesis as first author), also stated that any nonrandom evolution must be adaptive: “The evolutionary changes that result from nonrandom reproduc­tion are clearly adaptive: the changes are always, necessarily, of such a kind as to improve the average ability of the population to survive and reproduce in the environments that they inhabit.” [Page 578]