The Structure of Evolutionary Theory

by Stephen Jay Gould

  Having disposed of this simple error in thought, Darwin must now face the real dilemma that intermediates rarely occur in the fossil record either, where they should exist in abundance: “Why then is not every geological formation and every stratum full of such intermediate links? Geology assuredly does not reveal any such finely graduated organic chain; and this, perhaps, is the most obvious and gravest objection which can be urged against my theory” (p. 280).

  Darwin's general answer stands out in the title to his chapter, and he in­vokes the crucial claim to solve each of the three issues that, in their increas­ing difficulty, define the flow and logic of his treatment. The very next sen­tence, following the quotation cited just above, states this comprehensive solution: “The explanation lies, as I believe, in the extreme imperfection of the geological record” (p. 280). This “argument from imperfection” became so indispensable to Darwin's dispersal of his major challenge from the fossil record that, later in the chapter, he ventured one of the most honest and re­vealing statements in all our scientific literature: a striking admission that the needs of theory had provoked an understanding of paleontological data that he otherwise might never even have considered. Such feedbacks always occur in science, but the empirical ethos of our profession leads us to underplay, or never to recognize in our own mental processing at all, this reverse flow from the expectations of theory to the perception and interpretation of factuality. Darwin's admission strikes me as wonderfully honest and self-scrutinizing, but also as potentially triggering a trap of circular reasoning, as the dictate of theory, mandating an expectation of imperfection, biases the reading of a fos­sil record that might actually be displaying more genuine signal than the “noise” of absence: “But I do not pretend that I should ever have suspected how poor a record of the mutations of life, the best preserved geological sec­tion presented, had not the difficulty of our not discovering innumerable transitional links between the species which appeared at the commencement and close of each formation, pressed so hardly on my theory” (p. 302).

  First, Darwin presents his easier defense of gross imperfection at global scales, as he argues convincingly that we lack an adequate sample of life's full richness because both natural limitations (non-deposition of strata during most intervals) and insufficient human effort in a very young science (poverty of existing collections relative to fossils that could be gathered, exploration of only a small and geographically restricted percentage of the earth's fossil bearing strata) preclude any adequate sampling of life's full richness.

  But Darwin must then admit that such general reasons do not resolve the second, and local, issue of why we do not find a “finely graduated organic chain” within single formations that do seem to preserve a continuous record of strata: “From the foregoing considerations it cannot be doubted that the geological record, viewed as a whole, is extremely imperfect; but if we confine our attention to any one formation, it becomes more difficult to understand, why we do not therein find closely graduated varieties between the allied spe­cies which lived at its commencement and at its close” (pp. 292-293).

  Again, for this second issue, Darwin stresses the record's imperfection, [Page 1298] now relying on discontinuity of deposition and environment in single regions through geological intervals. Strata may seem continuous, he correctly ar­gues, but most times contributed no sediment because accumulation only oc­curs during gentle subsidence of basins, and such conditions do not generally prevail. By the same token, the local environment occupied by a species in anagenetic transformation will not generally persist long enough in any single place, and organisms will track their moving habitats. For example, the shal­low marine habitat of so many invertebrate species can only continue (under conditions that also accumulate sediments with fossils) when rates of deposi­tion for strata evenly match rates of subsidence for substrates — and how of­ten, and for how long, can such a fine balance be maintained in any one place?

  Having ascribed, to his satisfaction, both global and local absences of transitional forms to the general imperfection of geological records, Darwin must finally, in the closing sections of Chapter 9 (and spilling over into substantial parts of Chapter 10), rebut a third geological challenge to evolution, and es­pecially to gradualistic explanations framed in terms of natural selection. To overcome this last obstacle, Darwin must tackle the harder problem of an ap­parently positive signal against his expectations, rather than (as in the first two cases) a negative result of failure to locate an anticipated confirmation. To complete his argument, Darwin must now explain away the evidence for global episodes of apparently sudden mass extinction or origin of entire faunas.

  We should first pause to ask why Darwin even considered this signal from the fossil record as such a problem, especially for episodes of mass extinction. Why did he view the prospect of simultaneous extirpation as an issue at all, either for evolution or for natural selection? Natural selection does not guar­antee the power of adaptation in all circumstances — and if environments change rapidly and profoundly enough, these alterations may exceed the power of adaptation by natural selection, with extinction of most forms as the expected result, even in the most strictly Darwinian of circumstances.

  As a general answer — and as the primary reason for treating this subject within a chapter on modern critiques of the third leg, or extrapolationist premise, of Darwinian central logic — Darwin's hostility to catastrophic mass extinction does not arise primarily from threats posed to the mechanism of natural selection itself, but more from the challenges raised by the prospect of sudden global change to the key uniformitarian, or extrapolationist, assump­tion that observable processes at work in modern populations can, given the amplitude of geological time, render the full panoply of macroevolutionary results by prolonged accretion and accumulation.

  The problem of mass extinction became acute for Darwin because geologi­cal paroxysm threatened something quite particular, vitally important, and therefore of much greater immediate pith and moment than his general meth­odological preference for locating all causality in the palpable observation of microevolution (see Chapter 2). Global catastrophe could undermine the ecological [Page 1299] argument that Darwin had so carefully devised (see Chapter 6, pp. 467–479) to validate something more particular but no less important: his culture's central belief in progress, especially when Darwin had so increased the difficulty of the problem by constructing a theory (natural selection itself) that could not render this consummately desired result through its bare-bones mechanics. (For this reason, I discuss mass extinction in this relatively short Chapter 12, conceived as a counterpart to the equally brief Chapter 6 of this book's historical half — for these chapters feature the surrogate geological de­fenses of extrapolation, rather than the arguments from biological theory in­herent in the first two legs of Darwin's logical tripod, and treated in extenso in Chapters 3–5 of the historical half, and 8–11 of this second half on contemporary debates.)

  To explain the general pattern of life's history, Darwin sought to extrapo­late the results of competition ordained by the immediacies of natural selec­tion in ecological moments. In particular (as discussed and documented in Chapter 6, pp. 467–479), he used his “metaphor of the wedge” to argue that most competition, in a world chock full of species, unfolds in the biotic mode of direct battle for limited resources, mano a mano so to speak, and not in the abiotic mode of struggle to survive in difficult physical conditions. If strug­gle by overt battle (which favors mental and biomechanical improvement) trumps struggle against inclement environment (which often favors coopera­tion rather than battle and usually leads, in any case, to specialized local ad­aptation rather than to general improvement), then a broad vector of prog­ress should pervade the history of life.

  These two geological chapters (9 and 10) include nearly all of Darwin's passages and notable arguments for linking general progress to the extrapola­tion of momentary biotic competition through geological time. “The th
eory of natural selection is grounded on the belief that each new variety, and ulti­mately each new species, is produced and maintained by having some ad­vantage over those with which it comes into competition; and the consequent extinction of less-favored forms almost inevitably follows” (p. 320). Or con­sider this passage, with multiple metaphors of victory and defeat: “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 ... I do not doubt that this process of improvement has affected in a marked and sensible manner the organisation of the more recent and victorious forms of life, in comparison with the ancient and beaten forms” (pp. 336-337). Most famously, Darwin writes in the summary of both chapters (p. 345): “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 sentiment, felt by many paleontologists, that organisation on the whole has progressed.”

  To bring the literal appearance of mass extinction back under the rubric of extrapolation, Darwin realizes that he does not have to deny episodes of [Page 1300] markedly increased extinction entirely. Rather, he need only “spread out” the appearance of true simultaneity into a period long enough to permit explana­tion by biotic competition, perhaps intensified by tough physical times that make organismal battles even more stringent than usual, while remaining within the ordinary range and mode. After all, and in the anachronism of modern slang, “when the going gets tough, the tough get going.”

  Thus, high rates of change in physical environments, so long as they stay within permissible uniformitarian limits, will enhance extinction by “turning up the gain” on the dial of input from the geological stage. But, to emphasize the key point of Darwin's efforts, a world of conceptual difference separates a false appearance of catastrophe that can, by invoking the imperfection of geological records, be spread over sufficient time to remain within the uni­formitarian range (leading to intensification of evolutionary rates by ordinary modes), and a true catastrophe that must impose its burden of extinction by direct environmental impress under rules different from those regulating the primarily biotic competition of normal times and ordinary ecology (and gen­erating a macroevolutionary vector of progress as a result). Darwin would therefore turn to his old standby of an imperfect geological record to disperse this third and greatest challenge to his extrapolationist vision.

  Darwin presents two basic arguments — the first more theoretical and bio­logical, and the second far more practical, crucial, operational and geologi­cal — to buttress his claim that a threatening appearance of simultaneity in mass extinction and origination should be “spread out” to occupy enough time for explanation on uniformitarian premises by the ordinary operation of natural selection. First, theory dictates that old species generally become ex­tinct by biotic competition with new and improved forms, not by direct ex­tirpation through marked changes in the physical environment. “The extinc­tion of old forms is the almost inevitable consequence of the production of new forms” (p. 343). Darwin even denies a possible “escape route” for the less fit by asserting that mean global diversity has remained fairly constant through time — so the poorly adapted must go to the wall in clearing limited space for improved forms, and cannot hang on at the peripheries of a gen­eral expansion that welcomes the new without necessarily destroying the old: “Thus the appearance of new forms and the disappearance of old forms, both natural and artificial, are bound together . . . We know that the num­ber of species has not gone on indefinitely increasing, at least during the later geological periods, so that looking to later times we may believe that the production of new forms has caused the extinction of about the same number of old forms” (p. 320). Moreover, to enhance the implausibility of truly catastrophic mass dying, Darwin holds that “the complete extinction of the species of a group is generally a slower process than their production” (p. 318).

  In a long discussion on pages 325-327, Darwin collates all aspects of his biological argument that ordinary competition will explain the literal appear­ance of simultaneous global extinction and origination. The final paragraph summarizes his extrapolationist convictions (p. 327): [Page 1301]

  Thus, as it seems to me, the parallel, and, taken in a large sense, simul­taneous, succession of the same forms of life throughout the world, accords well with the principle of new species having been formed by dominant species spreading widely and varying; the new species thus produced being themselves dominant owing to inheritance, and to hav­ing already had some advantage over their parents or over other species; these again spreading, varying, and producing new species. 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.

  But Darwin's success hinges upon the second and more important geologi­cal argument — for his biological rationale only presents a theoretical defense, whereas he must overturn a strong signal from a literal reading of the fossil record: the appearance of true global simultaneity in mass extinction of entire groups and faunas, at a rate far too fast for any biological mechanism based on ordinary competition. At this crux, Darwin calls upon his standard argu­ment from imperfection to “spread out” this apparent moment into sufficient time for uniformitarian explanation.

  Darwin admits the literal signal (p. 322): “Scarcely any paleontological discovery is more striking than the fact, that the forms of life change almost simultaneously throughout the world.” But this impression must be an artifact produced by the markedly incomplete preservation of more gradual and con­tinuous change in a woefully imperfect geological record (pp. 317-318): “The old notion of all the inhabitants of the earth having been swept away at successive periods by catastrophes is very generally given up, even by those geologists . . . whose general views would naturally lead them to this conclu­sion. On the contrary, we have every reason to believe, from the study of the tertiary formations, that species and groups of species gradually disappear, one after the other, first from one spot, then from another, and finally from the world.”

  Among the many relevant aspects of imperfection, Darwin stresses two systematic factors that can compress a gradual transformation into a false appearance of simultaneity. First, sediments do not accumulate continuously, even in stratigraphic successions that look complete and uninterrupted. Strata pile up in continuity only when their basin of deposition slowly sub­sides, and this geological situation can occupy only a small percentage of to­tal time. Thus, most intervals will generate no sediments at all, and a group slowly petering out to extinction may seem to disappear all at once because sedimentation ceased when the group still included several declining species. If strata didn't begin to accumulate again until much later, all these species may have slowly dribbled out of existence during the intervening period of nondeposition: “We do not make due allowance for the enormous intervals of time, which have probably elapsed between our consecutive formations — [Page 1302] longer perhaps in some cases than the time required for the accumulation of each formation. These intervals will have given time for the multiplication of species from some one or some few parent-forms; and in the succeeding for­mation such species will appear as if suddenly created” (pp. 302-303).

  The second, and more sophisticated, argument follows from this principle of non-deposition during most intervals. We fall, Darwin argues, into circular reasoning in claiming that similar events in widely separated regions must have occurred simultaneously — for we make our judgment of temporal coin­cidence from the geological similarity alone, and not from any independent measure of time. For example, if we note the disap
pearance of several brachiopods in one stratum and the first appearance of several clams in the stratum just above, and we find the same pattern in a distant region on the other side of the earth, we might be tempted to proclaim a truly momentary wipeout followed by effectively simultaneous origin of functionally similar creatures. But this transition might actually occur very slowly in any single place, and leave no record of its true pace because a long interval of nondeposition fol­lowed the last preserved stratum of brachiopods. Moreover, this truly slow transition, prompted by ordinary biological competition of superior clams against inferior brachiopods, one species at a time, might have unfolded at quite different times in separated regions of the globe — for the process can only begin when the clam fauna migrates to a new area, and these migrations may span a considerable range of time (falsely compressed to simultaneity by our error in viewing the first stratum with clams as coeval throughout the world). Darwin summarizes this complex argument (pp. 327-329):

  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 ... If the several forma­tions in these regions have not been deposited during the same exact pe­riod, — a formation in one region often corresponding with a blank inter­val in the other ... in this case, the several formations in the two regions could be arranged in the same order, in accordance with the general succession of the form of life, and the order would appear to be strictly parallel.