The Structure of Evolutionary Theory

by Stephen Jay Gould

  Darwin's caveats, whenever he presents a prima facie case for abiotic con­trol, are even more revealing. Reports of biotic competition elicit only simple approbation, while putative examples of response to physical circumstances often provoke reminders that we may not be viewing the matter correctly, and that biotic competition may still be exerting a hidden sway.

  The structure of every organic being is related, in the most essential yet often hidden manner, to that of all other organic beings, with which it comes into competition for food or residence, or from which it has to escape, or on which it preys. This is obvious in the structure of the teeth and talons of the tiger; and in that of the legs and claws of the parasite, which clings to the hair on the tiger's body. But in the beautifully plumed [Page 473] seed of the dandelion, and in the flattened and fringed legs of the water beetle, the relation seems at first confined to the elements of air and wa­ter. Yet the advantage of plumed seeds no doubt stands in the closest re­lation to the land being already thickly clothed by other plants; so that the seeds may be widely distributed and fall on unoccupied ground. In the water-beetle, the structure of its legs, so well adapted for diving, al­lows it to compete with other aquatic insects, to hunt for its own prey, and to escape serving as prey to other animals (p. 77).

  And epidemic extirpations, not generally attributed to microorganisms in Darwin's day and therefore an apparently clear case of regulation by non-competitive forces, may be caused by “parasitic worms” — an organism large enough to engender thoughts about overt and visible competition between parasite and host: “When a species, owing to highly favorable circumstances, increases inordinately in numbers in a small tract, epidemics — at least, this seems generally to occur with our game animals — often ensue: and here we have a limiting check independent of the struggle for life. But even some of these so-called epidemics appear to be due to parasitic worms, which have from some cause, possibly in part through facility of diffusion amongst the crowded animals, been disproportionably favored: and here comes in a sort of struggle between the parasite and its prey” (p. 70).

  Wedging and the causes of extinction

  Darwin's most striking metaphor for biotic competition, invoked from his very first jotting about natural selection (after his Malthusian insight of Octo­ber 1838) to the Origin of Species, imagines a surface packed tightly with wedges, representing nature chock full to its carrying capacity. Such a maxi­mally crowded world provides only one path for entry — by forcing (“wedg­ing”) another creature out. Biotic competition rules with a vengeance: “The face of Nature may be compared to a yielding surface, with ten thousand sharp wedges packed close together and driven inwards by incessant blows, sometimes one wedge being struck, and then another with greater force” (1859, p. 67).

  The longer version of Natural Selection, Darwin's original manuscript, presents an even more revealing characterization, replete with almost frantic images of crowding, an explicit focus on species (in other passages, Darwin construes the wedges as individual organisms), and the relegation of physical limitation to an underlying layer, usually not penetrated, while the real work of nature proceeds by biotic struggle in the visible region above (Natural Se­lection, 1856-1858,1975 edition, edited by Stauffer, p. 208):

  Nature may be compared to a surface covered with ten-thousand sharp wedges, many of the same shape and many of different shapes represent­ing different species, all packed closely together and all driven in by in­cessant blows: the blows being far severer at one time than at another; sometimes a wedge of one form and sometimes another being struck; and one driven deeply in forcing out others; with the jar and shock often [Page 474] transmitted very far to other wedges in many lines of direction: beneath the surface we may suppose that there lies a hard layer, fluctuating in its level, and which may represent the minimum amount of food required by each living being, and which layer will be impenetrable by the sharp­est wedge.

  In this exigent world of intense and ubiquitous competition, severity of struggle will be directly proportional to degree of relationship — most intense among members of the same species, strong between individuals of closely re­lated species, and generally tapering with genealogical distance (and ecologi­cal dissimilarity). As a result, new species tend to eliminate their ancestors and closest relatives: “Each new variety or species, during the progress of its formation, will generally press hardest on its nearest kindred, and tend to ex­terminate them” (1859, p. 110).

  Extinction therefore becomes a consequence of failure in biotic struggle, for ecosystems generally stand chock full, and new wedges must be poised to make their move whenever a chink appears. All species become enmeshed in a perpetual upward spiral, running continuously just to keep pace with their fellows — the Red Queen hypothesis (Van Valen, 1973): “For as all organic beings are striving, it may be said, to seize on each place in the economy of Nature, if any one species does not become modified and improved in a corre­sponding degree with its competitors, it will soon be exterminated” (1859, p. 102).

  The geological extension of wedging

  If wedging rules the moment in a crowded world, then the extension of wedg­ing through time should build patterns of origination and extinction in the fossil record. Following the dictates of the wedge, Darwin presents extinction as gradual and natural — not as rapid elimination in the wake of environmen­tal catastrophe, but as slow diminution in the face of competition from “su­perior” forms, usually of close genealogical relationship (see Chapter 12, pp. 1296–1303, for a further development of this argument). Darwin chides us for ever regarding extinction as unusual, and draws an analogy to the inevita­bility of death, usually following a gradual course of prolonged weakening: “I may repeat what I published in 1845, namely, that to admit that species generally become rare before they become extinct — to feel no surprise at the rarity of a species, and yet to marvel greatly when it ceases to exist, is much the same as to admit that sickness in the individual is the forerunner of death — to feel no surprise at sickness, but when the sick man dies, to wonder and to suspect that he died by some unknown deed of violence” (p. 320).

  Darwin counterposes this view of extinction as gradual failure in biotic competition to the alternative that both he and Lyell so strongly rejected — catastrophic global paroxysm and resulting mass extirpation: “On the theory of natural selection the extinction of old forms and the production of new and improved forms are intimately connected together. The old notion of all [Page 475] the inhabitants of the earth having been swept away at successive periods by catastrophes, is very generally given up” (p. 317).

  Darwin centers his second geological chapter (“On the succession of or­ganic beings,” Chapter 10) upon an argument, framed in his usual mode of probing behind the literal appearance of an imperfect record, for the preva­lence of a pattern that would validate gradual, biotically-driven extinction as a norm for the history of clades. Darwin denies that much extinction occurs by simultaneous or coordinated removal of unrelated forms. On the contrary, he argues, groups wane slowly and individually as superior competitors wax, producing a distinctive pattern of “megawedging” through geological time. “The extinction of old forms is the almost inevitable consequence of the pro­duction of new forms” (p. 343). “As new species in the course of time are formed through natural selection, others will become rarer and rarer, and finally extinct. The forms which stand in closest competition with those un­dergoing modification and improvement, will naturally suffer most” (p. 110).

  The validation of progress

  For two reasons, Darwin could not find a rationale for progress in abiotic, physically-driven extinction and adaptation: first, a non-directional vector of environmental change can only elicit a set of meandering responses in the adaptive adjustments of organisms; second, the more serious challenge of ca­tastrophe and mass extinction raises the specter of randomness and death for reasons unrelated to the adaptive struggles of normal times — the wheel of fortun
e vs. the wedge of progress (Gould, 1989d).

  But victory over other creatures in an intense and unrelenting struggle for limited resources does permit an inference about progress. Now species triumph because, in some sense admittedly difficult to define, winners are “better” than the forms they vanquish. And the more uniformitarian the larger picture — the more that macroevolutionary pattern arises as a simple summation of immediate struggles — so do we gain increasing confidence that replacement and extinction must record the differential success of globally improved species. Thus, progress becomes an ecological concept for Dar­win — not a deduction from the inevitable mechanics of natural selection, but a mode of operation for natural selection in a particular kind of ecological world. If crowded habitats, where creatures must struggle to the death for limited resources, represent an ecological norm on earth, and if geological change usually proceeds at a sufficiently stately and unobtrusive pace to per­mit the fruits of biotic competition to accumulate into patterns of origination and extinction through time, then we may understand why “organization on the whole has progressed” (p. 345). Darwin links all his statements about progress firmly to his ecological theory of plenitude and to the prevalence of biotic competition.

  Consider Darwin's language and imagery (“inferior” forms “beaten” by “victorious” relatives) as he presents his key argument for linking the gradual geological decline of groups to the success of closely related competitors (a [Page 476] claim now strongly compromised by accumulating data on mass extinction — see Chapter 12): “The forms which are beaten and which yield their places to the new and victorious forms, will generally be allied in groups, from inherit­ing some inferiority in common; and therefore as new and improved groups spread throughout the world, old groups will disappear from the world; and the succession of forms in both ways will everywhere tend to correspond” (p. 327). Moreover, note Darwin's continual emphasis on advantage and competition in crowded ecosystems: “As natural selection acts solely by the preservation of profitable modifications, each new form will tend in a fully-stocked country to take the place of, and finally to exterminate, its own less improved parent or other less-favored forms with which it comes into compe­tition” (p. 172).

  The link of progress to biotic competition in a crowded world had perme­ated Darwin's thought from his first formulation of natural selection, as this passage from the E Notebook (January 18, 1839) indicates: “The enormous number of animals in the world depends on their varied structure and com­plexity. — Hence as the forms became complicated, they opened fresh means of adding to their complexity. — But yet there is no necessary tendency in the simple animals to become complicated although all perhaps will have done so from the new relations caused by the advancing complexity of others.”

  In the Origin of Species, all explicit statements about progress invoke a rationale of biotic competition, and employ a metaphor of battle. I find Dar­win's conviction especially revealing in the light of his frank admission that he can neither formulate a way to test his proposal, nor specify a criterion by which progress might be measured:

  But in one particular sense the more recent forms must, on my theory, be higher than the more ancient; for each new species is formed by having had some advantage in the struggle for life over other and preceding forms. If under a nearly similar climate, the eocene inhabitants of one quarter of the world were put into competition with the existing inhabit­ants of the same or some other quarter, the eocene fauna or flora would certainly be beaten and exterminated; as would a secondary fauna by an eocene, and a paleozoic fauna by a secondary fauna. I do not doubt that this process of improvement has affected in a marked and sensible man­ner the organization of the more recent and victorious forms of life, in comparison with the ancient and beaten forms; but I can see no way of testing this sort of progress (pp. 336-337).

  Darwin's most widely quoted statement about progress appears in the summary to his two geological chapters. This famous passage also includes an odd mixture of firm conviction based on metaphors of competition (“the race for life” in this case), combined with some discomfort about the absence of a crisp definition: “The inhabitants of each successive period in the world's history have beaten their predecessors in the race for life, and are, in so far, higher in the scale of nature; and this may account for that vague yet ill-defined [Page 477] sentiment, felt by many paleontologists, that organization on the whole has progressed” (p. 345).

  Sequelae

  The central importance to Darwin of a link between progress and biotic competition seems especially clear in the various ramifications that branch so richly from his basic proposition. All these sequelae point to certain “smoothness,” a form of predictability, an accumulation through time of the reasonable and little into the sensible and big. Nature is not capricious; supe­rior forms prevail for cause; their triumph breeds further success and wider expansion; change proceeds in an orderly fashion — not in a clocklike manner to be sure, but at least decorously.

  Widespread and speciose genera usually include the ancestral stocks of later successes, for extended geographic ranges and large populations indi­cate triumph in competition, and good mettle for future progress: “The great and flourishing genera both of plants and animals, which now play so impor­tant a part in nature, thus viewed become doubly interesting, for they include the ancestors of future conquering races. In the great scheme of nature, to that which has much, much will be given” (Natural Selection, 1856-1858, 1975 edition, edited by Stauffer, p. 248).

  If brought into competition after previous isolation, big clades from large regions will prevail over less speciose groups from smaller areas because their members have been tested in hotter fires of competition: “For in the larger country there will have existed more individuals, and more diversified forms, and the competition will have been severer, and thus the standard of perfec­tion will have been rendered higher” (p. 206). Thus the success of North American mammals in South America following the rise of the Isthmus of Panama “is due to the greater extent of land in the north, and to the northern forms having existed in their own homes in greater numbers, and having consequently been advanced through natural selection and competition to a higher stage of perfection or dominating power, than the southern forms” (p. 379). In a revealing metaphor, Darwin then praises “the larger areas and more efficient workshops of the north” (p. 380).

  Looking at the complementary theme of failure, aberrant genera include few species because such creatures have been beaten by superior forms in competition (and not for a variety of other potential reasons including limited speciation, or specialization to rare and unusual environments): “Such rich­ness in species, as I find after some investigation, does not commonly fall to the lot of aberrant genera. We can, I think, account for this fact only by look­ing at aberrant forms as failing groups conquered by more successful compet­itors, with a few members preserved by some unusual coincidence of favor­able circumstances” (p. 429).

  Since competition will be ubiquitous, efficient, and unrelenting in a crowded world, steady change should represent a norm, while stasis must re­cord the unusual circumstance of reduced competition — as in the “living fos­sils” explicitly dubbed “anomalous” by Darwin: “These anomalous forms [Page 478] may almost be called living fossils; they have endured to the present day from having inhabited a confined area, and from having thus been exposed to less severe competition” (p. 107). In explaining why so few pairs of living species consist of one highly modified descendant and one unchanged surviving an­cestor, Darwin invokes the high probability of substantial change, due to bi­otic competition, in both lineages stemming from a common root:

  It is just possible by my theory that one of two living forms might have descended from the other; for instance, a horse from a tapir; and in this case direct intermediate links will have existed between them. But such a case would imply that one form had remained for a very long period un­altered, whils
t its descendants had undergone a vast amount of change; and the principle of competition between organism and organism, be­tween child and parent, will render this a very rare event; for in all cases the new and improved forms of life will tend to supplant the old and un­improved forms (p. 281).

  Darwin's thought lies best revealed in a remarkable paragraph from the Origin's final summary. All themes of this section now flow together — the de­nial of mass extinction (as Darwin borrows Lyell's favorite rhetorical trick of conflating this concept with nonscientific views of creation), the linkage of improvement in some groups to the extermination of competitors, and the strongest statement in the entire Origin about the predominant relative fre­quency of biotic competition vs. response to altered physical conditions. For Darwin now makes the boldest possible claim of all — an assertion that the ubiquity, continuity, and gradualism of biotic competition might actually per­mit us to use morphological change as a rough measure of elapsed time!*