The Structure of Evolutionary Theory

by Stephen Jay Gould

  * This tripartite structure of the Origin is masked by our tendency to treat the two geo­logical chapters (9-10) as a unity. (Darwin even summarizes them together at the end of Chapter 10.) But Chapter 9, as the title proclaims (“On the imperfection of the geological record”), belongs to the discussion of difficulties in part 2 of the Origin — while Chapter 10 (“On the geological succession of organic beings”) initiates part three on documentation of evolution as a fact. (Even the consolidated summary of Chapter 10 makes a clear break be­tween these two disparate parts of Darwin's geological argument.)

  * Later Lord Avebury and an author of many fine evolutionary works himself. But Lubbock's greatest contribution to human thought was probably indirect, a result of neigh­borly fellowship — for he sold to Darwin a corner of property that became the famous “sandwalk” where Darwin, perambulating and kicking aside a flint cobble for each cir­cumnavigation, solved several riddles of life and human existence. Darwin graded the dif­ficulty of his problems by the number of circuits required for solution — two-flint problems, five-flint problems, etc. I suspect that macroevolutionary theory must present us with at least a fifty-flint problem!

  * The word adaptation did not enter biology with the advent of evolutionary theory. The Oxford English Dictionary traces this term to the early 17th century in a variety of mean­ings, all designating the design or suitability of an object for a particular function, the fit of one thing to another. The British school of natural theology used “adaptation” as a stan­dard word for illustrating God's wisdom by the exquisite fit of form to immediate function. Darwin, in borrowing this term, followed an established definition while radically revising die cause of the phenomenon.

  * “Hecatomb,” an unfamiliar word in English, should enter the vocabulary of all evolu­tionists as a wonderfully appropriate description for this key aspect of Darwinism. A heca­tomb is, literally, an offering of a hundred oxen in sacrifice. Yet, even in Homer, the word had come to designate any large number of deaths incurred as a sacrifice for some intended benefit — a good description of natural selection. And hecatomb trips so much more lightly off the tongue than “substitutional load.”

  * By the “syllogistic core” of natural selection (“the bare-bones argument”), I refer to the standard pedagogical presentation of the abstract mechanism of the theory as a set of three undeniable factual statements followed by the inference of natural selection (the fourth statement) as a logical entailment of the three facts, viz:

  1. Superfecundity: all organisms produce more offspring than can possibly survive.

  2. Variation: all organisms vary from other conspecifics, so that each individual bears distinguishing features.

  3. Heredity: at least some of this variation will be inherited by offspring (whatever the mechanism of hereditary transition — a mystery to Darwin, but the argument only requires that heredity exist, not that its mode of action be known).

  4. Natural selection: if we accept these foregoing three statements as factual (2 and 3 ranked as “folk wisdom” in Darwin's time and could scarcely be doubted; while Darwin took great pains to validate 1 in early chapters of the Origin, showing, for example, that even the most slowly reproducing of all animals, the African elephant, would soon fill the continent if all offspring survived and reproduced), then the principle of natural selection follows by syllogistic logic. If only some offspring can survive (statement 1), then, on aver­age (as a statistical phenomenon, not a guarantee for any particular organism), survivors will be those individuals that, by their fortuity of varying in directions most suited for ad­aptation to changing local environments, will leave more surviving offspring than other members of the population (statement 2). Since these offspring will inherit those favorable traits (statement 3), the average composition of the population will change in the direction of phenotypes favored in the altered local environment.

  As Darwin did himself in the Introduction to the Origin, nearly all textbooks and college courses present the “bare bones” of natural selection in this fashion (I have done so in more than 30 years of teaching). The device works well, but does not permit a teacher to go be­yond the simplest elucidation of selection as a genuine force that can produce adaptive change in a population. In other words, the syllogistic core only guarantees that selection can work. By itself, the core says nothing about the locus, the agency, the efficacy, or the range of selection in a domain — the sciences of natural history — where all assessments of meaning rest upon such claims about mode, strength, and relative frequency, once the prior judgment of mere existence has been validated. Thus, an elucidation of this “syllogistic core” can only rebut charges of hokum or incoherence at the foundation. An analysis of the three key issues of the Darwinian essence, the subject of the rest of this chapter, then en­gages the guts of natural history.

  * Only two exceptions have been noted to this generality — both in the domain of anoma­lies that prove the rule. The Scottish fruit grower Patrick Matthew (in 1831) and the Scottish-American physician William Charles Wells (in 1813, published in 1818) spoke of natural selection as a positive force for evolutionary change, but neither recognized the sig­nificance of his speculation. Matthew buried his views in the appendix to a work entitled “Naval Timber and Arboriculture”; Wells published his conjecture in a concluding section, treating the origin of human races, to a paper on the medical case of a piebald woman. He presented this paper to the Royal Society in 1813, but only published it as he lay dying in 1818 — as a subsidiary to his two famous essays on the origin of dew, and on why we see but one image with two eyes.

  Matthew, still alive and vigorously kicking when Darwin published the Origin, wrote to express his frustration at Darwin's non-citation. Darwin offered some diplomatic palliation in the historical introduction added to later editions of the Origin, while professing, with ample justice, that he had meant no malice, but had simply never encountered Matthew's totally forgotten and inauspiciously located speculation. He responded to Matthew's ire in the Gardener's Chronicle for April 21, 1860: “I freely acknowledge that Mr. Matthew has anticipated by many years the explanation which I have offered of the origin of species, un­der the name of natural selection. I think that no one will feel surprised that neither I, nor apparently any other naturalist, has heard of Mr. Matthew's views, considering how briefly they are given, and that they appeared in the Appendix to a work on Naval Timber and Ar­boriculture.”

  Wells' article is particularly intriguing, if only for an antiquarian footnote, in the context of this book's focus on supraorganismal levels of selection. Although Wells has often been cited as a precursor, very few citationists have read his paper, and have therefore simply as­sumed that he spoke of natural selection by Darwin's route of advantages to individuals within populations. In fact, as I discovered (Gould, 1983a), Wells attributes racial differen­tiation in skin color to group selection among populations.

  I do not wish to make overly much of this point, as “precursoritis” is the bane of histori­ography; yet I am tickled by the ironic tidbit, in the light of later orthodoxy, that the first formulation of natural selection went forward in the supraorganismic mode. The point should not be overstressed, if only because Wells reached this alternative by the fallacious argument that favorable variants could not spread within populations. Echoing Jenkins' later criticism of Darwin, Wells held that blending inheritance prevents the transformation of populations from within because advantageous variants “quickly disappear from the in­termarriages of different families. Thus, if a very tall man be produced, he very commonly marries a woman much less than himself, and their progeny scarcely differs in size from their countrymen” (1818, pp. 434-135).

  Populations must therefore be transformed by fortuitous spread and propagation within small and isolated groups: “In districts, however, of very small extent, and having little in­tercourse with other countries, an accidental difference in the appearance of the inhabitants will often descend to their
late posterity” (p. 435). Change may then occur within an entire species by group selection among these differentiated populations:

  Of the accidental varieties of man, which would occur among the first few and scattered inhabitants of the middle regions of Africa, some would be better fitted than the others to bear the diseases of the country. This race would consequently multiply, while the others would decrease, not only from their inability to sustain the attacks of disease, but from their incapacity of contending with their more vigorous neighbors. The color of this vigorous race I take for granted ... would be dark. But the same disposition to form varieties still existing, a darker and a darker race would in the course of time occur, and as the darkest would be the best fitted for the climate, this would at length become the most prevalent, if not the only race, in the particular country in which it had originated (pp. 435-436).

  Note Wells' unquestioned assumption that our original color must have been white, and that dark skin could only arise as a modification of the type. As a final interesting footnote, Wells denied (probably wrongly) that dark skin could be adaptive in itself, and argued for its establishment in Africa as a result of noncausal correlation with unknown physiological mechanisms for protection against tropical disease. Thus, Wells presents an “internalist” explanation based on what Darwin would later call “correlation of growth.” With this ar­gument about channels, and his basic claim for group selection, Wells' departure from Dar­win's later preferences lie very much in the spirit of modern critiques, though for reasons that we would now reject (as if our anachronistic judgment mattered).

  * Charles Darwin surely ranks as the most genial of history's geniuses — possessing none of those bristling quirks and arrogances that usually mark the type. Yet, one subject invari­ably aroused his closest approach to fury — the straw-man claim, so often advanced by his adversaries, that he regarded natural selection as an exclusive mode of change in evolution. Darwin, who understood so well that natural history works by relative frequency, explicitly denied exclusivity and argued only for dominance. So frustrated did he become at the al­most willful misunderstanding of a point so clearly made, that he added this rueful line to the 6th edition of the Origin (1872b, p. 395): “As my conclusions have lately been much misrepresented, and it has been stated that I attribute the modification of species exclu­sively to natural selection, I may be permitted to remark that in the first edition of this work, and subsequently, I placed in a most conspicuous position — namely at the close of the Introduction — the following words: 'I am convinced that natural selection has been the main, but not the exclusive means of modification.' This has been of no avail. Great is the power of steady misinterpretation.”

  Darwin's good friend G. J. Romanes, author of a famous essay on Darwin's pluralism vs. the panselectionism of Wallace and Weismann, wrote of this statement (1900, p. 5): “In the whole range of Darwin's writings there cannot be found a passage so strongly worded as this: it presents the only note of bitterness in all the thousands of pages which he has pub­lished.” But Darwin wrote other bristling statements on the same sensitive subject. In 1880, for example, he castigated Sir Wyville Thomson for caricaturing him as a panselectionist: “This is a standard of criticism not uncommonly reached by theologians and metaphysi­cians when they write on scientific subjects, but is something new as coming from a natu­ralist ... Can Sir Wyville Thomson name any one who has said that the evolution of species depends only on natural selection?” (1880b, p. 32).

  * Some modern evolutionists have made the error of assuming that contemporary de­bates about gradualism engage this now obvious and entirely uncontroversial meaning. Thus Gingerich (1984a), abandoning his earlier and properly empirical approach to gradu­alism (sense iii of p. 152) vs. punctuation (1976), argues that gradualism must be true a pri­ori, as equivalent to “empiricism” in paleontology. He then provides a curious definition of stasis as “gradualism at zero rate” — an oxymoron with respect to the definition of gradual­ism that punctuated equilibrium opposes with a prediction of stasis. I was, at first, deeply puzzled by Gingerich's definition until I realized the source of his confusion. He had switched definitions from the empirical issue of rates (meaning iii of this discussion) — a lively and testable argument opposing stasis to gradualism defined as a rate of change — to the completely settled question of historical continuity. Does anyone seriously think that supporters of punctuated equilibrium, or any scientist for that matter, would deny histori­cal continuity? His argument therefore dissolves into the empty linguistic effort of trying to win a debate by shifting a definition. The question of punctuated equilibrium will be re­solved by empirical testing under the third definition of gradualism. (See Chapter 9 for a full discussion of this issue.)

  * This reversed order does not constitute a general claim for evolution against previous creationist models. This reversal represents, rather, one major style of evolutionary argu­ment — the functionalist response. The alternative, structuralist stance — the evolutionary version of function following form — sets a major theme of this book, both in the historical precedents of Goethe, Geoffroy, Bateson and others, and in modern notions of constraint and exaptation.

  * Lamarck's commitment to gradualism as a general philosophy matched Darwin's in centrality and strength, thus forging a connection deeper than a shared attitude toward en­vironment alone. Lamarck wrote (1809, p. 46): “Consider . . . that in all nature's works nothing is done abruptly, but that she acts everywhere slowly and by successive stages.”

  * Darwin, in distancing himself from precursors as he formulated and refined his own theory in the years before 1859, usually drew a primary contrast with Lamarck. But he sometimes added the anonymous author of the Vestiges of the Natural History of Creation (written by the Scottish publisher Robert Chambers in 1844, though his authorship did not become officially known until the last edition of 1884, two years after Darwin's death), pairing “Mr. Vestiges” with Lamarck as the entirety of a background to be rejected with vigor on the old principle that the enemy within can be more distressing than the enemy without. Darwin wrote to Hooker in an undated letter between 1849 and 1853: “Lamarck ... in his absurd though clever work has done the subject harm, as has Mr. Vestiges.” Dar­win then adds, with his endearing capacity for self-deprecation: “... and, as (some future naturalist attempting the same speculations will perhaps say) has Mr. D . . .” (in F. Darwin, 1887, vol. 2, p. 39).

  The publication of the Vestiges in 1844 unleashed a firestorm of criticism from all sides. “From the bottom of my soul,” wrote the dour creationist (and Darwin's teacher in geol­ogy) Adam Sedgwick, “I loathe and detest the Vestiges.” Following the prejudices of his age, Sedgwick conjectured that a woman must have written anything so stupid. But serious evolutionists also took offense at Chambers's rank amateur ignorance of natural history, and at the purely speculative character of his assertions (including the claim that birds evolved to mammals in two steps via the intermediary of a duck-billed platypus). Nonetheless, Vestiges became a succes de scandale, going through 12 editions in 40 years, and surely doing some service in making the subject of evolution discussible. The pairing of Lamarck with Chambers does, therefore, represent the totality of highly noticed, fully evo­lutionary systems available in England as Darwin formulated his theory.

  This pairing becomes an important footnote to my argument because Chambers's theory is also hierarchical in the same general sense as Lamarck's — that is, in advocating separate realms of causes for progress and deviation, with progress at a higher level and difficult to discern, and deviation as immediate and palpable, but incapable of generating the full taxonomic order of life.

  Chambers's particular theory, however, differed radically from Lamarck's, and “Mr. Ves­tiges” attacked his French predecessor and inspiration (via Lyell) as a man “whose notion is obviously so inadequate to account for the rise of the organic kingdoms, that we only can place it with pity among the follies of the wise” (1
844, p. 231). But Chambers thoroughly misunderstood Lamarck, accepting the usual caricature of mystical will leading to adapta­tion as Lamarck's complete system. Somehow Chambers missed the hierarchical character of Lamarck's theory; he understood that the force of adaptation could not yield progress, but, unaware that Lamarck had ascribed life's ladder to different causes acting at another level, he concluded that Lamarck had erred in trying to explain all evolution by adaptation alone.

  Chambers remedied this misperception by devising a theory every bit as hierarchical as Lamarck's actual proposal, but radically different in mechanism. Chambers's system works by extended analogy to von Baer's laws of embryonic differentiation (Gould, 1977b, pp. 109-112). Linear progress follows the embryonic sequence of the highest organism. The higher-level cause of progress pushes creatures up the sequence. Meanwhile, the lower-level force of deviation pulls organisms into adaptive configurations at various plateaus of de­sign. A small impetus from the cause of progress may bring a developing animal to the pla­teau of fishes; a greater push will lead to reptilian, mammalian, or even human grade. Prog­ress depends upon an ability “to protract the straightforward part of the gestation over a small space” (1844, p. 213), thereby resisting the lateral force of adaptation at lower pla­teaus. Chambers even introduces the cogent idea (still arousing modern debate) that higher-level causes may be hard to ascertain because they operate so rarely. They may work quite regularly and in an absolutely law like way, but only once every million years or so — and our chance of observing them then becomes vanishingly small. (This attack on a dogmatic uniformity, based only on observed modern causes, emerges in such recent ideas as the bo­lide impact theory of mass extinction.) Thus, for Chambers, embryological transcendence to the next stage of progress may occur regularly and rapidly, but very rarely — while the lateral forces of adaptation operate around us all the time.