The Structure of Evolutionary Theory

by Stephen Jay Gould

  Note also how Darwin, in this passage, explicitly limits within tribal bound­aries the extent of such spread against organismal selection. If some form of group selection had to be acknowledged for a special case, Darwin sought to confine its operation to the smallest aggregation within the species — and then to let these small collectivities struggle with others in a minimal context of groupiness.

  Thus, in permitting a true exception to organismal selection, Darwin's pri­mary attitude exudes extreme reluctance — restriction to minimal groupiness, provision of other explanations in the ordinary organismal mode, limitation to a unique circumstance in a single species (human consciousness for the spread of an idea against the force of organismal selection), and placement within a more general argument for sexual selection, the strongest form of the orthodox mode.

  In my researches for this book, I made a discovery that strongly supports this view of Darwin's attitude towards supraorganismal selection. I found that the traditional sources (Ruse, Kottler and others) did not identify Dar­win's major, explicit struggle to contain an apparent need for higher-level se­lection, and to assert exclusivity for the organismal mode. He fought a far more important battle with himself on an issue well beyond particular prob­lems raised by single taxa (sterility of worker castes or human morality): the explanation of the principle that he ranked second only to natural selection itself as a component of evolutionary theory — the “principle of divergence.” (Evolutionists have not recognized this important component of Darwin's de­veloping ideas about selection because he excised this discussion as he ab­stracted his longer work to compose the Origin. But the full version exists in the uncompleted manuscript of his intended larger work — edited and pub­lished by Stauffer, 1975, but not widely read by practicing biologists.) More­over, in his long version, Darwin wrestles not with the lowest interdemic level of tribal selection, but with species selection itself. I will present a full exposi­tion in Chapter 3 (pp. 224–250), but should mention for now that Darwin's [Page 136] tactic closely follows his argument about human morality, and therefore emphasizes his extreme reluctance to embrace supraorganismal selection, and his almost desperate effort to confine explanation to the organismal mode. The recognition that Darwin, despite such strong reluctance, could not avoid some role for species selection, builds a strong historical argument for the ineluctability of a hierarchical theory of selection. (I shall show in Chapter 3 that none of the few 19th century scientists who truly grasped the full range and subtlety of selectionist theory could avoid important roles for levels other than the organismic.)

  As with the next topic of creativity for natural selection (pp. 137-159), the issue of levels in selection has resounded through the entire history of evolu­tionary theory, and continues to set a major part of the agenda for modern debate — as it must, for the subject lies (with only a few others) at the very heart of Darwinian logic. Wallace never comprehended the question of levels at all, as he searched for adaptation wherever he could find it, oblivious to any problems raised by the locus of its action; Kropotkin, in asserting mutual aid, never grasped the problem either; Weismann shared Darwin's insight about the problem's fundamental nature, but also came to understand, after a long and explicit intellectual struggle with his own strong reluctance, that ex­clusivity must yield to hierarchy (pp. 197-224).

  In our generation, Wynne-Edwards (1962) riled an entire profession by defending the classical form of group selection as a generality, while Williams (1966) penned a powerful rebuttal, urging us all to toe the Darwinian line (see Chapter 7 for a full account). The classical ethologists invoked various forms of group selection (often by default); the sociobiologists proclaimed a revolution by reaction and return to the pure Darwinism of individual advan­tage. Dawkins (1976) attempted an even stronger reduction to exclusivity for genic selection, but his false argument rests on a confusion of bookkeeping with causality, and his own later work (1982) negates his original claim, though Dawkins seems unaware of his own contradictions (see Chapter 8). Supporters of hierarchy theory — I am one, and this is a partisan book — are revising Darwinism into a multilevel theory of selection.

  This issue will not go away, and must excite both interest and passion. Nothing else lies so close to the raw nerve of Darwin's radicalism. The exclu­sivity of organismal selection, after all, provides the punch line that allowed the vision of Adam Smith to destroy the explicit beauty and harmony of Wil­liam Paley's world.

  Viewed in this light, the Origin's very few statements about solace become particularly revealing. Darwin had just overturned a system that provided the philosophical basis of human comfort for millennia. What could he supply in return, as we continue to yearn for solace in this vale of tears? One might be tempted to read the few Darwinian statements about solace as peculiar, exceptional, even “soft” or illogical. But we should note another feature of these statements as well: they yield no ground whatever on the key issue of organismal struggle. Solace must be found in other guises; the linchpin of se­lection as struggle among organisms cannot be compromised. [Page 137]

  Darwin offers two sources for solace. First, the struggle, however fierce, usually brings no pain or distress to organisms (humans, with their intrusive consciousness, have introduced a tragic exception into nature). “When we re­flect on this struggle, we may console ourselves with the full belief, that the war of nature is not incessant, that no fear is felt, that death is generally prompt, and that the vigorous, the healthy, and the happy survive and multi­ply” (p. 79).

  Second, this struggle does lead to general improvement, if only as an epiphenomenon, and whatever the cost: “As natural selection works solely by and for the good of each being, all corporeal and mental endowments will tend to progress towards perfection” (p. 489). Darwin could never compro­mise his central logic; for even this “softest” of all his statements explicitly as­serts that selection can only work on organisms — “for the good of each be­ing.” And why not? The logic of organismal struggle includes both fierce beauty and empirical adequacy — whatever the psychic costs. And, since roses by other names smell just as sweet, then beauty, even as an epiphenomenon, becomes no less pleasing, and no less a balm for the soul.

  THE SECOND THEME: NATURAL SELECTION AS A

  CREATIVE FORCE

  The following kind of incident has occurred over and over again, ever since Darwin. An evolutionist, browsing through some pre-Darwinian tome in nat­ural history, comes upon a description of natural selection. Aha, he says; I have found something important, a proof that Darwin wasn't original. Per­haps I have even discovered a source of direct and nefarious pilfering by Dar­win! In the most notorious of these claims, the great anthropologist and writer Loren Eiseley thought that he had detected such an anticipation in the writ­ings of Edward Blyth. Eiseley laboriously worked through the evidence that Darwin had read (and used) Blyth's work and, making a crucial etymological mistake along the way (Gould, 1987c), finally charged that Darwin may have pinched the central idea for his theory from Blyth. He published his case in a long article (Eiseley, 1959), later expanded by his executors into a posthu­mous volume entitled “Darwin and the Mysterious Mr. X” (1979).

  Yes, Blyth had discussed natural selection, but Eiseley didn't realize — thus committing the usual and fateful error in this common line of argument — that all good biologists did so in the generations before Darwin. Natural selection ranked as a standard item in biological discourse — but with a crucial differ­ence from Darwin's version: the usual interpretation invoked natural selec­tion as part of a larger argument for created permanency.* Natural selection, [Page 139] in this negative formulation, acted only to preserve the type, constant and in­violate, by eliminating extreme variants and unfit individuals who threatened to degrade the essence of created form. Paley himself presents the following variant of this argument, doing so to refute (in later pages) a claim that mod­ern species preserve the good designs winnowed from a much broader range of initial c
reations after natural selection had eliminated the less viable forms: “The hypothesis teaches, that every possible variety of being hath, at one time or other, found its way into existence (by what cause or in what manner is not said), and that those which were badly formed, perished” (Paley, 1803, pp. 70-71).

  Darwin's theory therefore cannot be equated with the simple claim that natural selection operates. Nearly all his colleagues and predecessors ac­cepted this postulate. Darwin, in his characteristic and radical way, grasped that this standard mechanism for preserving the type could be inverted, and then converted into the primary cause of evolutionary change. Natural selec­tion obviously lies at the center of Darwin's theory, but we must recognize, as Darwin's second key postulate, the claim that natural selection acts as the cre­ative force of evolutionary change. The essence of Darwinism cannot reside in the mere observation that natural selection operates — for everyone had long accepted a negative role for natural selection in eliminating the unfit and preserving the type.

  We have lost this context and distinction today, and our current perspective often hampers an understanding of the late 19th century literature and its preoccupations. Anyone who has read deeply in this literature knows that no argument inspired more discussion, while no Darwinian claim seemed more vulnerable to critics, than the proposition that natural selection should be viewed as a positive force, and therefore as the primary cause of evolutionary change. The “creativity of natural selection” — the phrase generally used in Darwin's time as a shorthand description of the problem — set the cardinal subject for debate about evolutionary mechanisms during Darwin's lifetime and throughout the late 19th century.

  Non-Darwinian evolutionists did not deny the reality, or the operationality, of natural selection as a genuine cause stated in the most basic or abstract manner — in the form that I called the “syllogistic core” on page 125 (still used as the standard pedagogical device for teaching the “bare bones” logic of Darwinism in general and introductory college courses). They held, rather, that natural selection, as a headsman or executioner, could only eliminate the unfit, while some other cause must play the positive role of constructing the fit.

  For example, Charles Lyell — whom Darwin convinced about the factuality of evolution but who never (much to Darwin's sadness and frustration) accepted the mechanism of natural selection — admitted that he had be­come stymied on the issue of creativity. He could understand, he wrote in his fifth journal on the “species question” in March 1860, how natural se­lection might act like two members of the “Hindoo Triad” — like Vishnu the preserver and Siva the destroyer, but he simply could not grasp how [Page 140] such a force could also work like Brahma, the creator (in Wilson, 1970, p. 369).

  E. D. Cope, chief American critic and exponent of neo-Lamarckism, chose a sardonic title to highlight Darwin's supposedly fatal weakness in claiming a creative role for natural selection. He called his book The Origin of the Fittest (1887) — a parody on Darwin's “survival of the fittest,” and a motto for what natural selection could not accomplish. Cope wrote: “The doctrines of 'selec­tion' and 'survival' plainly do not reach the kernel of evolution, which is, as I have long since pointed out, the question of 'the origin of the fittest.' This omission of this problem from the discussion of evolution is to leave Hamlet out of the play to which he has given the name. The law by which structures originate is one thing; those by which they are restricted, directed, or de­stroyed, is another thing” (1887, p. 226).

  We can understand the trouble that Darwin's contemporaries experienced in comprehending how selection could work as a creative force when we confront the central paradox of Darwin's crucial argument: natural selec­tion makes nothing; it can only choose among variants originating by other means. How then can selection possibly be conceived as a “progressive,” or “creative,” or “positive” force?

  In resolving this paradox, Darwin recognized his logical need, within the basic structure of his argument, to explicate the three main requirements and implications of an argument for selection's creativity: (1) the nature of varia­tion; (2) the rate and continuity of change; (3) the meaning of adaptation. This interrelated set of assertions promotes natural selection from mere exis­tence as a genuine, but secondary and negative, mechanism to domination as the primary cause of evolutionary change and pattern. This set of defenses for selection's creativity therefore ranks as the second of three essential postu­lates, or “minimal commitments” of Darwinian logic.

  As the epitome of his own solution, Darwin admitted that his favored mechanism “made” nothing, but held that natural selection must be deemed “creative” (in any acceptable vernacular sense of the term) if its focal action of differential preservation and death could be construed as the primary cause for imparting direction to the process of evolutionary change. Darwin reasoned that natural selection can only play such a role if evolution obeys two crucial conditions: (1) if nothing about the provision of raw materials — that is, the sources of variation — imparts direction to evolutionary change; and (2) if change occurs by a long and insensible series of intermediary steps, each superintended by natural selection — so that “creativity” or “direction” can arise by the summation of increments.

  Under these provisos, variation becomes raw material only — an isotropic sphere of potential about the modal form of a species. Natural selection, by superintending the differential preservation of a biassed region from this sphere in each generation, and by summing up (over countless repetitions) the tiny changes thus produced in each episode, can manufacture substantial, directional change. What else but natural selection could be called “creative,” or direction-giving, in such a process? As long as variation only supplies raw [Page 141] material; as long as change accretes in an insensibly gradual manner; and as long as the reproductive advantages of certain individuals provide the statisti­cal source of change; then natural selection must be construed as the direc­tional cause of evolutionary modification.

  These conditions are stringent; and they cannot be construed as vague, unconstraining, or too far in the distance to matter. In fact, I would argue that the single most brilliant (and daring) stroke in Darwin's entire theory lay in his willingness to assert a set of precise and stringent requirements for varia­tion — all in complete ignorance of the actual mechanics of heredity. Darwin understood that if any of these claims failed, natural selection could not be a creative force, and the theory of natural selection would collapse. We pay our highest tribute to the power of natural selection in recognizing how Darwin used the theory to deduce a set of necessary properties for variation, well be­fore science understood the mechanism of heredity — and in noting that these properties then turned out to be both basically correct and also entailed by the causes later discovered!

  The requirements for variation

  In order to act as raw material only, variation must walk a tightrope between two unacceptable alternatives. First and foremost, variation must exist in suf­ficient amounts, for natural selection can make nothing, and must rely upon the bounty thus provided; but variation must not be too florid or showy ei­ther, lest it become the creative agent of change all by itself. Variation, in short, must be copious, small in extent, and undirected. A full taxonomy of non-Darwinian evolutionary theories may be elaborated by their denials of one or more of these central assumptions.

  COPIOUS. Since natural selection makes nothing and can only work with raw material presented to its stringent review, variation must be generated in copious and dependable amounts (especially given the hecatomb of selec­tive deaths accompanying the establishment of each favorable feature). Dar­win's scenario for selective modification always includes the postulate, usu­ally stated explicitly, that all structures vary, and can therefore evolve. He argues, for example, that if a short beak were favored on a full-grown pigeon “for the bird's own advantage” (p. 87), then selection would also work within the egg for sufficient beak strength to break the shel
l despite diminu­tion in overall size of the beak — unless evolution followed an alternate route of selection for thinner shells, “the thickness of the shell being known to vary like any other structure” (p. 87).

  Darwin's faith in the copiousness of variation can be gauged most clearly by his response to the two most serious potential challenges of his time. First, he acknowledges the folk wisdom that some domestic species (dogs, for ex­ample) have developed great variety, while others (cats, for example) differ far less among populations. If these universally recognized distinctions arise as consequences of differences in the intrinsic capacity of species to vary, then Darwin's key postulate of copiousness would be compromised — for failure of [Page 142] sufficient raw material would then be setting a primary limit upon the rate and style of evolutionary change, and selection would not occupy the driver's seat.

  Darwin responds by denying this interpretation, and arguing that differing intensities of selection, rather than intrinsically distinct capacities for varia­tion, generally cause the greater or lesser differentiation observed among do­mestic species. I regard this argument as among the most forced and uncom­fortable in the Origin — a rare example of Darwinian special pleading. But Darwin realizes the centrality of copiousness to his argument for the creativ­ity of natural selection, and he must therefore face the issue directly:

  Although I do not doubt that some domestic animals vary less than oth­ers, yet the rarity or absence of distinct breeds of the cat, the donkey, peacock, goose, etc., may be attributed in main part to selection not hav­ing been brought into play: in cats, from the difficulty in pairing them; in donkeys, from only a few being kept by poor people and little attention paid to their breeding; in peacocks, from not being very easily reared and a large stock not kept; in geese, from being valuable only for two pur­poses, food and feathers, and more especially from no pleasure having been felt in the display of distinct breeds (p. 42).