As argued throughout Chapter 9, my greatest pride in punctuated equilibrium lies in the theoretical space it created for active study of subjects that could win neither definition nor existence under gradualistic presuppositions: particularly stasis (previously viewed as an embarrassing failure to detect evolution, and therefore as a non-subject), now generally seen as an important and surprising result at several levels in the history of life; and the punctuational explanation of trends by differential success of species treated as discrete Darwinian individuals (an alternative with explanatory options that simply didn't exist under older models of trends defined exclusively as anagenetic transformation). In several similar ways — I will cite just two here — the catastrophic impact hypothesis of mass extinction created an enlarged intellectual space that forced paleontologists to reevaluate data once viewed as comfortably consistent with gradualist assumptions, but clearly subject to extension and better definition as tests for gradualism vs. catastrophe. In this vital way, self-fulfilling claims for convention became sources for discrimination among rival hypotheses about some of the most important questions in the history of life.
For example, in a justly influential paper, Signor and Lipps (1982) recognized that the well-documented literal signal of taxa slowly “petering out” in the stratigraphic record before a mass extinction boundary might actually [Page 1310] be consistent with truly catastrophic removal — a “counterintuitive,” but actually rather obvious, option that paleontologists had never conceptualized because the literal signal matched their expectations, and they therefore never questioned the meaning. Signor and Lipps argued that if all taxa under consideration had truly died at once, their recorded disappearance in the stratigraphic record would still be sequential, as based on probability of fossilization. After all, a top, large-bodied carnivore like Tyrannosaurus, with a local geographic range and a relatively small N, might only yield a fossil once every several meters during the stratigraphic range of its actual existence; whereas a foraminifer that lived as billions of individuals at every moment of a continuous oceanic core, should provide abundant specimens in every mm of its stratigraphic existence. Thus, even if the dinosaur and the foram species died at the same instant in a worldwide catastrophe, the last dinosaur fossil might still appear several meters below the extinction boundary, while foram fossils should persist to the last stratum.
This artifactual pattern, now appropriately named the Signor-Lipps effect, can be distinguished from a genuine petering out that truly gradual extinction would produce — most obviously by testing for correlations between time of disappearance before the boundary and expectations of preservability in the fossil record (as measured by “waiting time” between fossils in normal strata between extinction events, not by working with abundances near the boundary itself, where circular reasoning may so easily intervene). Regardless of the outcome in any particular application — and some studies have yielded consistency with Signor-Lipps, whereas others seem to show genuine petering out — paleontologists had never formulated, or even conceptualized, this important methodology until the catastrophic impact scenario forced their attention to such questions.
As a different approach to the same basic situation, one might decide instead to take a group long interpreted as showing a clear literal signal of petering out — and then go to the most promising stratigraphic record in the world, pulling apart every single bedding plane to see if surpassingly rare specimens might occur nearer, if not right up to, the extinction boundary (for you only need one to disprove complete extinction). This “needle (or dinosaur) in the haystack” strategy represents the “flip-side” of the Signor-Lipps approach to the general problem of sampling in science — either use a global and statistical method to extract a clear signal from broad data, or sample with such intensity in a more limited area that you can effectively survey the available “universe,” and no longer even require the art of statistical inference. But why would one even think of sampling with such intensity, absent the prod of the impact hypothesis and its prediction of putative success. After all, if your world-view enjoins petering out, and your data (read literally) clearly display just such a pattern, why would you petition the National Science Foundation for cash, and then spend several summers sweating in a desert, pulling apart every bedding plane in a single place virtually guaranteed to yield nothing. Such behavior could only point to an unsound mind — [Page 1311] unless, of course, you had a really good reason to believe that a diamond lay hidden somewhere in that particular stratigraphic haystack.
This method of hyperintense sampling in optimal places has been applied, with great success in validating substantial, even fully maintained, abundance right up to the K-T boundary itself for two groups whose ostensible petering out had provided a mainstay of empirically based opposition to catastrophic mass extinction — ammonites as affirmed by Peter Ward's work in France and Spain (Ward, 1992), and dinosaurs as indicated by collections of Sheehan et al. (1991) in Montana and North Dakota. Seek and (perhaps) ye shall find, but in a real world of such limited time and opportunity in scientific careers, one does need a theoretical license, as well as a landowner's specific permission, to seek.
Despite my personal excitement at the theoretical and practical import of the impact hypothesis, I must confess my initial surprise at a statement that historian of science Bill Glen made to me in the early 1990's. For he asserted that the reforming power of the impact theory would surpass even that of Plate Tectonics in the history of geology. (And one cannot accuse Glen either of sour grapes or parochialism, for he not only wrote the book on “living” history of science for the impact debate (Glen, 1994), but had previously written an even more highly acclaimed history in progress for plate tectonics as well (Glen, 1982). Now I still don't fully agree with Glen, if only for two primary reasons:
First, despite all its successes in 20 years, the Alvarez scenario still applies, with proven power, only to the single event of the K-T mass extinction. None of the other four great mass dyings show clear iridium spikes or other evidence for triggering by impact (although some of the smaller extinction events have been more plausibly linked to possible impact). Thus, the Alvarez scenario remains a historical explanation, however elegantly affirmed, for a single event, and not a general theory of mass extinction. (And, as I came to know and understand Luis Alvarez late in his life, I rather suspect that this situation would have frustrated him intensely, for, as a theoretical physicist by trade, he remained committed to the view that science can only attain its true goal by establishing general explanations rooted in the spatio-temporal in-variance of natural law. To learn that he had become godfather to the contingent explanation of a great event, and not to the formulation of a general theory of mass extinction, would have left him unamused.)
Second, even though we now have confidence in the factuality of impact as a trigger for the K-T extinction, we still cannot specify a satisfactory “killing scenario” to explain the timing and differential susceptibility to dying among life's various taxa — a scarcely surprising circumstance, given the complexity of the event and the potential number of dire consequences that impact might unleash, but still a damper upon any feeling of full satisfaction. Indeed, I remain amused by how the competing (and, to be sure, partly complementary) ideas follow the canonical scenarios for disaster in Western culture — the ten plagues of Moses. And whenever our scientific preferences so clearly match [Page 1312] our culturally inherited stories, we should begin to question the complex intellectual and psychological bases of the hypotheses we have chosen to test.
Moses tried them all (and eventually triumphed, of course). For the most popular theme of “nuclear winter” and the cutting off of light (and photosynthesis) by a persisting worldwide dust cloud raised by the impact, “he sent a thick darkness over all the land, even darkness which might be felt.” For acid rain and worldwide fires, “he gave them hailstones for rain; fire mingled with the ha
il ran along upon the ground.” For poisoning of the oceans, “he turned their water into blood.” For killing plagues induced by the few successful survivors, “their land brought forth frogs, yea even unto their King's chambers.” And for selective diseases unleashed in this novel environment, we have the analog of Moses' ultimate weapon: “He smote all the firstborn of Egypt, the chief of all their strength.”
Nonetheless, I do appreciate the undeniably valid part of Glen's contention. For many scientists and historians have noted that, whereas plate tectonics changed our view of the earth's structure more profoundly than any previous theory had ever done — and may even be described as supplying our first adequate account for the physics of earth-sized bodies in general — plate tectonics also, albeit ironically, promulgated a conservative reform in not challenging in the slightest way, but rather supplying a mechanism to validate, the deepest of all geological presuppositions: uniformitarianism itself. For if the earth's largest mountains can rise as a result of two plates crunching together at their characteristic moving rate of millimeters per year, and if the great faultlines, earthquakes and volcanic provinces also mark the edges of such slowly drifting plates, then the central Lyellian dictum of explaining all grand, and all apparently quick, events by the accumulation of slow movements, utterly imperceptible at the scale of our daily lives, gains a kind of planetary and mathematical validity that can only be deemed awesome in its phenomenological range and intellectual reach.
On the other hand, Glen continued, the impact theory — even if we never succeed in establishing this mechanism as a general theory, and even if such catastrophes remain confined to explanations of particular events — directly fractured Lyellian uniformity, therefore penetrating far deeper in its iconoclasm than the admittedly more comprehensive theory of plate tectonics could ever bore. In any case, and in the terms and concerns of this book, the validation of a truly catastrophic triggering mechanism for at least some events of mass extinction dramatically fractured the support that Darwin needed from the kind of geological stage necessarily set for playing out his preferred game of life. The vital extrapolationist premise of the third leg on the tripod of essential Darwinian logic must fail if global paroxysm can undo, redirect, or even substantially impact a pattern of life's history that, in a fully Darwinian scheme of explanation, must scale up in full continuity from the microevolutionary realities of competition in observable ecological time.
I have, in my own writings, tried to summarize the theoretical importance of readmitting truly catastrophic scenarios of mass extinction back into scientific [Page 1313] respectability (after 150 years of successful Lyellian anathematization) by stating an emerging consensus about four crucial and general features of such events, each strongly negative (and, in their ensemble, probably fatal) for the key extrapolationist premise needed to maintain a claim of exclusivity for a strictly Darwinian theory of evolutionary process (with descriptive aspects of life's pageant still left for paleontological documentation, because a general theory must underpredict an actual outcome in a historically contingent world): mass extinctions are more frequent, more rapid, more intense, and more different in their effects than paleontologists had suspected, and that Lyellian geology and Darwinian biology could permit.
In terms of the broad categories of pattern in life's history that will now require at least partial explanation from catastrophic theories of mass extinction, and will probably not be rendered on Darwinian assumptions of extrapolation from microevolutionary theory,* I would summarize the most important [Page 1314] changes under two rubrics: the “random” and the “different rules” models for alternating regimes of faunal turnover in mass extinction vs. ordinary, sequential (and, in Darwin's own preferences, overtly competitive) replacement in “normal” times between these infrequent but intense episodes.
In the purely random model, which I do not consider of great importance in life's history, a group might die in a mass extinction for no reason of particular sensitivity towards the catastrophic agents of extermination, but for reasons little beyond the luck of the draw — the “bad luck” vs. the traditional “bad genes” in Raup's amusing characterization (Raup, 1991a). (I consider this random model as less important than deterministic reactions to the “different rules” of these parlous times because, even in these extreme episodes, the species number of major taxa generally remains large enough to preclude full removal by random inclusion of all members of one kind within a percentage of totality. All ten red beans in a bag of 100 may disappear often enough in a random destruction of 75 beans. But we would not expect all 10,000-dinosaur species in a fauna of 100,000 land taxa to die in a truly random reduction to 25,000 taxa.)
Nonetheless, a fact of the fossil record, not widely known or appreciated outside the community of professional paleontologists, may grant the random model an important role in a few crucial circumstances during life's history (and such moments make all the difference in contingent sequences, as Jimmy Stewart discovered when his guardian angel showed him the alternate history of his town, absent his existence, in It's A Wonderful Life): some groups, formerly dominant in their habitats and therefore viewed as persistently “major” in our conceptions, fluctuated enormously in diversity throughout geological time, and just happened to be surviving at very low N (a situation from which they had always rebounded before, and in normal times) when a mass extinction intervened. And when an event of latest Permian magnitude occurs — the largest of all mass dyings, with estimates of species loss ranging up to 96 percent (see Raup, 1992) — and your group contributes only a lineage or two to the global fauna, you can easily disappear, entirely and forever, for little reason beyond bad luck (those two red beans, both included among the 96 percent of dead benthic taxa, but entirely equivalent in ordinary Darwinian prowess to the 4 percent that survived).
Among several plausible cases in this mode, the two specifically cited by [Page 1315] Darwin (see p. 1302) to express his confidence in gradualistic and uniformitarian models — trilobites and ammonites as the “signatures” of the Permian and Cretaceous extinctions respectively — may, ironically, fit this potential scenario best. Trilobites abounded in most Paleozoic faunas (at least in the affections of fossil collectors), but they had been reduced to only two lineages of low diversity by latest Permian times. The death of these two, and the consequent termination of one among only four great arthropod classes, may reveal no insufficiency of trilobite anatomy, ecology or development, and may only record the failure of any among a very few coins to come up heads at a weird moment that diverted 96 percent of all flips into a tailspin towards oblivion.
The case of ammonites is both more complex and more instructive. They suffered greatly during the last three of five great Phanerozoic dyings — latest Permian, latest Triassic, and latest Cretaceous. They barely survived the first two, with only one or two lineages persisting in each case, and then died entirely in the K-T event (although their less speciose relatives, the chambered nautiloids, survived to this day to become favored items in aquariums and shell collections, and to become, as the “ship of pearl,” Oliver Wendell Holmes's celebrated metaphor of eternity: “Build thee more stately mansions, O my soul...”).
After the first two restrictions, the ammonites reradiated mightily and became major components of Triassic, and then of Jurassic-Cretaceous, faunas. Perhaps we do need an “ammonite-specific” reason for the final Cretaceous death, as Ward has shown (1992) that their latest Maastrichtian diversity remained respectable. But I raise a different point here: Can we specify any “real” mathematical or biological difference among reduction to 1, 2 or 0 lineages? That is, can we say that the Cretaceous extinction was causally worse for ammonites than the preceding Permian or Triassic events? Surely not from the numbers themselves (and we have no other basis for such an assertion, at least at this moment of research and understanding). We can state no causal or statistical difference among 0, 1 or 2 survivor
s. These results are effectively equivalent. We might as well put slips with the three numbers in a bag and let nature choose one at random for each of the last three mass extinctions. And yet, in a real world of genealogical history, zero vs. anything else makes all the difference in the universe: absolute termination absolutely forever, vs. the possibility of redemption and reassertion. In this sense as well, and again in a sequence so influenced by contingency, effectively random removal based on small numbers in the face of catastrophe can impact the history of life in truly fateful and permanent ways. Principles regulate ranges of events, but particular and unpredictable events make history.
Nonetheless, I am confident, and I think nearly all paleontologists would agree, that the “different rules” model — a more conventionally causal view of history — plays a far more important role in expressing the power of catastrophic mass extinction to fracture the crucial extrapolationist premise of Darwinian central logic. I devoted the first part of this section (pp. 1296–1303) [Page 1316] to Darwin's own emphasis on the importance of “spreading out” the timing of apparent “mass extinction” into sufficient spans for explanation by ordinary Darwinian competition, with sequential and individual deaths of species occurring in conventional realms of natural selection, at most against a background of unusual environmental perturbation that may “turn up the gain” of intensity for standard causes, but will not change the usual rules or reasons. I also showed, in particular, how Darwin needed this extrapolationist argument to validate a concept of progress through life's history that his cultural context demanded (and to which he personally assented), but that he recognized (and, in his philosophical radicalism, greatly appreciated) as underivable from the “pure” operation of natural selection, and therefore recoverable only by an additional ecological postulate about the predominance of biotic competition in a perpetually crowded biota (the “metaphor of the wedge”).
The Structure of Evolutionary Theory Page 209