But the truly curious aspect of both Dawkins's and Dennett's charge lies in their subsequent recognition, and fair discussion, of the important theoretical implication of punctuated equilibrium: the establishment of species as Darwinian individuals, and the consequent validation of species sorting and selection as a prominent process in a hierarchical theory of Darwinian evolution. In 1984, Dawkins acknowledged that this aspect of punctuated equilibrium “does, in a sense, move outside the neo-Darwinian synthesis, narrowly interpreted. This is about whether a form of natural selection operates at the level of entire lineages, as well as at the level of individual reproduction stressed by Darwin and neo-Darwinism.” In his 1986 book, Dawkins then devotes a substantial part of the chapter following his rejection of punctuated equilibrium to an evaluation of species selection. But he finishes his exploration by reimmersion in the same parochial trap of denying importance because the phenomenon doesn't explain his exclusive interest in adaptive organismal design: “To conclude the discussion of species selection, it could account for the pattern of species existing in the world at any particular time. It follows that it could also account for changing patterns of species as geological ages give way to later ages, that is, for changing patterns in the fossil record. But it is not a significant force in the evolution of the complex machinery of life ... As I have put it before, species selection may occur but it doesn't seem to do anything much!” (Dawkins, 1986, pp. 268-269). But doesn't “the pattern of species existing in the world at any particular time” and “changing patterns in the fossil record” represent something of evolutionary importance?
At the end of his long riff against punctuated equilibrium, Dennett also pauses for breath and catches a glimmer of the concept that seems important and theoretically intriguing to many students of macroevolution (Dennett, 1995, pp. 297-298): [Page 1021]
The right level at which to look for evolutionary trends, he [Gould] could then claim [indeed I do], is not the level of the gene, or the organism, but the whole species or clade. Instead of looking at the loss of particular genes from gene pools, or the differential death of particular genotypes within a population, look at the differential extinction rate of whole species and the differential “birth” rate of species — the rate at which a lineage can speciate into daughter species. This is an interesting idea ... It may be true that the best way of seeing the long-term macro-evolutionary pattern is to look for differences in “lineage fecundity” instead of looking at the transformations in the individual lineages. This is a powerful proposal worth taking seriously.
I am puzzled by the discordance and inconsistency, but gratified by the outcome. Dawkins and Dennett, smart men both, seem unable to look past the parochial boundaries of their personal interest in evolution, or their feelings of jealousy towards whatever effectiveness my public questioning of their sacred cow of Darwinian fundamentalism may have enjoyed (see Gould, 1997d) — so they must brand punctuated equilibrium as trivial. But they cannot deny the logic of Darwinian argument, and they do manage to work their way to the genuine theoretical interest of punctuated equilibrium's major implication, the source of our primary excitement about the idea from the start.
THE WISDOM OF AGASSIZ'S AND VON BAER'S THREEFOLD HISTORY OF SCIENTIFIC IDEAS. When I was writing Ontogeny and Phylogeny, I came across a wonderful, if playfully cynical, statement by the great embryologist Karl Ernst von Baer (1866, p. 63) about Louis Agassiz's view on the ontogeny of scientific theories (also quoted on p. 687): “Deswegen sagt Agassiz, dass wann eine neue Lehre vorgebracht wurde, sie drei Stadien durchzumachen habe; zuerst sage man, sie sei nicht wahr, dann, sie sei gegen die Religion, and im dritten Stadium, sie sei langst bekannt gewesen.” [Therefore, Agassiz says that when a new doctrine is presented, it must go through three stages. First, people say that it isn't true, then that it is against religion, and, in the third stage, that it has long been known.]
I won't vouch for the generality of this scenario, but Agassiz's rule certainly applies to the history of nonscientific debate about punctuated equilibrium, particularly to the aspect governed by jealousy of critics — as Eldredge and I recognized in a previous publication entitled: “Punctuated equilibrium at the third stage” (Gould and Eldredge, 1986). The first stage of empirical denial, extending roughly from our original publication in 1972 to the Chicago macroevolution meeting of 1980, featured studies of fossil sequences by paleontologists (notably Gingerich, 1974 and 1976), many of whom tried to deny that punctuated equilibrium occurred very frequently, if at all, by documenting cases of gradualism.
During the second phase, spanning the first half of the 1980's, the primary subject of this section, punctuated equilibrium, was vociferously dismissed as contrary to religion — that is, as apostate anti-Darwinian nonsense. Our theory, [Page 1022] falsely read as a saltationist doctrine proclaiming the overthrow of the Modern Synthesis, if not of Darwinism itself, received a hefty dose of anathematization in tones usually reserved for demonizing religious heterodoxies.
The third phase then began in the mid-1980's, as documented in Sections III-V of this chapter, and has continued ever since. The evidence became too great, and we withstood all ideological attacks without sustaining appreciable damage. Punctuated equilibrium now seemed both coherent in argument, and supported by a sufficient number of empirical studies to become a recognized evolutionary phenomenon — though at a relative frequency as yet undetermined. Such a situation must cause critics to remember the old cliche: if you can't beat 'em, join 'em (but don't grant 'em too much credit for innovation or originality). Move instead to phase three of Agassiz's continuum — “sure it's true, but we always knew this; punctuated equilibrium amounts to no more than a little wrinkle on the skin of neo-Darwinism.”
As an initiating episode of the third phase, Darwinian biologists began to construct models that rendered punctuational patterns (though not always cladogenetic events of true punctuated equilibrium) by standard formulae of population genetics under certain reasonable assumptions and conditions. We have always welcomed these formulations, for we never sought the radical content of punctuated equilibrium in novel microevolutionary processes, as I have emphasized throughout this chapter — and any demonstrated mechanism for punctuational patterns evokes both our interest and satisfaction. The first two studies in this genre appeared in 1985 — Newman, Cohen and Kipnis (1985) and Lande (1985). In 1986, Roger Lewin wrote a “news and views” commentary for Science entitled: “Punctuated equilibrium is now old hat.” He ended with a gratifying comment by Joel Cohen: “In terms of the tenor of the debate, which at times has been strident, the new results will bring the various parties closer together. Cohen readily concedes that population geneticists very probably would not have applied their mathematical tools to the issue in this way had there not been such a big fuss stirred up by the paleontologists' claims. 'They deserve credit for that,' he says.”
So I guess we won by Agassiz's scenario, even if personal motivations of an ungenerous nature lead our severest critics to belittle our achievement as true after all, but trivial from the outset. But why then did they ever make such a fuss?
A CODA ON THE KINDNESS AND GENEROSITY OF MOST COLLEAGUES. This section, devoted to unscientific critiques by professional colleagues, centers on unhappy themes of jealousy, pettiness and meanness of spirit. But I do not wish to leave the impression that these unpleasantnesses have dominated the totality of discussion about punctuated equilibrium. Quite to the contrary, in fact — and I have already discussed the numerous tough, spirited, helpful and scientific critiques of punctuated equilibrium in sections III-V of this chapter. Intense and useful debate has predominated throughout the history of punctuated equilibrium. Most of our colleagues have unstintingly followed the norms and ideals of scientific discussion, and we have primarily [Page 1023] wrestled with good argument and content, not primarily with deprecation and personal attack.
I have mentioned and cited se
veral of these generous reactions, these fair and accurate characterizations, throughout this section — the journalistic accounts of Tudge and Gleick (pp. 993, 994); the excellent textbook epitome of Curtis and Barnes (p. 998); the generous assessment of punctuated equilibrium's scientific importance by Rhodes (1983); and the acknowledgement of punctuated equilibrium's prod to further exploration of formulae in population genetics by Cohen (p. 1022). I also wish to emphasize that most professional colleagues have always given us generous credit, and have applauded both the debate and the interest generated by punctuated equilibrium.
I have particularly appreciated the fairness of severe critics who generally oppose punctuated equilibrium, but who freely acknowledge its legitimacy as a potentially important proposition with interesting implications, and as a testable notion that must be adjudicated in its own macroevolutionary realm. Ayala (1982) has been especially clear and gracious on this point:
If macroevolutionary theory were deducible from microevolutionary principles, it would be possible to decide between competing macroevolutionary models simply by examining the logical implications of microevolutionary theory. But the theory of population genetics is compatible with both punctualism and gradualism; and, hence, logically it entails neither. Whether the tempo and mode of evolution occur predominantly according to the model of punctuated equilibria or according to the model of phyletic gradualism is an issue to be decided by studying macroevolutionary patterns, not by inference from microevolutionary processes. In other words, macroevolutionary theories are not reducible (at least at the present state of knowledge) to microevolution. Hence, macroevolution and microevolution are decoupled in the sense (which is epistemologically most important) that macroevolution is an autonomous field of study that must develop and test its own theories.
Such statements stand in welcome contrast to the frequent grousing of strict Darwinians who often say something like: “but we know all this, and I said so right here in the footnote to page 582 of my 1967 paper; you have stated nothing new, nothing that can alter the practice of the field.” I will never forget the climactic moment of the Chicago macroevolution meeting, when John Maynard Smith rose to make such an ungenerous statement about punctuated equilibrium and macroevolutionary theory in general — and George Oster responded to him: “Yes, John, you may have had the bicycle, but you didn't ride it.” In the same vein, I appreciate the comment of Marjorie Grene (quoted in Stidd, 1985), a famous philosopher who has greatly aided the clarification of evolutionary theory:
Yet on both these counts — gradualism and neutralism — evolutionists other than paleontologists now appear unmoved. Just what we've said all along, they cry, even though, I swear, I've heard, and seen, them [Page 1024] emphatically assert just the opposite, time and time again. No sudden changes, no non-adaptive changes, they used to exclaim, while now they ask cheerfully: why not stasis, sudden change, and neutral mutations all over the place except for an adaptive innovation here and there, now and then? We always knew it was like that. Nothing really new, no revolution here.
Finally, I am heartened by the many top-ranking biologists who have found fruitful ideas and new wrinkles in the concept of punctuated equilibrium and its macroevolutionary implications — for utility in practice remains the ultimate criterion of judgment in science. I appreciate Dan Janzen's affirmation in his article “on ecological fitting” (Janzen, 1985, p. 308): “I suddenly realize that I have blundered through the front door of the turmoil over punctuated equilibria. We don't have to dig at the fossil record; punctuated equilibria are right here in front of us, represented by most of the species that you and I have anything to do with.”
I welcome the generous assessment of Kenneth Korey (1984) in the preface to his compendium of Darwin's best writing, The Essential Darwin:
Unquestionably no single challenge to the synthesis has provoked more attention than the theory of punctuated equilibrium advanced by Niles Eldredge and Stephen Gould. ... It is true that punctuated equilibrium was not a prediction of the synthesis; on the contrary, Simpson emphasized continuous, phyletic evolution as the most pervasive feature of evolution at this level . . . On the macroevolutionary front . . . punctuated equilibrium as an empirical proposition is not, perforce, in conflict with the synthesis, although if its wide province becomes established, then a more complete theoretical explanation for stasis will certainly be wanted. Species selection, in its present form, would seem to require the most profound reworking of evolutionary theory.
And I thank Paul Ehrlich (1986) for recognizing the genuine novelty and utility of punctuated equilibrium in his book, The Machinery of Nature:
The jury is not in on the punctuated-equilibria controversy. That the “snapshot” of differentiation we see today seems to reveal all stages of differentiation does not necessarily signal a win for the gradualists . . . And it is not fair to swallow the punctuationist view within the gradualist orthodoxy simply because the possibility of rapid speciation has always been part of that orthodoxy. The punctuationist view is about dominant patterns, not about what is possible — and it represents a genuine challenge to one widely held tenet within evolutionary theory.
It has been a wonderful ride on Oster's bicycle, and we still have such a long road to travel.
[Page 1025]
CHAPTER TEN
The Integration and Adaptation
(Structure and Function) in
Ontogeny and Phylogeny:
Historical Constraints and the
Evolution of Development
Constraint As a Positive Concept
TWO KINDS OF POSITIVITY
An etymological introduction
After Job has endured, and countered with remarkable success in his straightened circumstance, three cycles of argument from each of his three supposed friends and comforters, a fourth participant in this moral and intellectual debate for the ages — the problem of theodicy, or why should the righteous suffer as much bodily torment and material deprivation as the unjust? — steps forward to make his pitch. Elihu, the son of Barachel the Buzite, offers an only slightly more comforting argument in admitting that Job's suffering may well be undeserved, but urging that Job view his distress as a salutary discipline leading to reconciliation with God (no physical relief, to be sure, but a damned sight more encouraging than the previous insistence of Zophar, Eliphaz and Bildad that Job must have sinned if God had so punished him).
Elihu states that he had hesitated to intervene previously because his youth demanded forbearance. (Modern Biblical scholars, on the other hand, regard Elihu's argument as so inconsistent with the rest of the book, in both style and content, that these late chapters probably represent a subsequent interpolation, thus explaining the curious fact that Elihu's name appears nowhere else in the entire book. Elihu probably “waited” his turn until the end because he didn't exist in the original story.) But he will now speak because he must. An internal force demands that he remain silent no longer: “I also will shew mine opinion. For I am full of matter; the spirit within me constraineth me. Behold my belly is as wine which hath no vent; it is ready to burst” (Job 32:17-19).
I choose this unconventional mode of beginning a scientific discussion with Biblical exegesis because this chapter (and a central theme in the logic of this entire book) rests upon a particular definition and construction of the concept of constraint — a meaning easily defended both terminologically and factually, [Page 1026] but often so buried in a confusing and contentious literature that the centrality of the argument becomes lost in collegial frustration.
In short, I emphasize two major premises at the outset: (1) The concept of constraint must be sharpened and restricted in meaning to a coherent set of causal factors that can promote evolutionary change from a structuralist perspective different from — in the helpful sense of “in addition to” or “in conjunction with, and yielding interesting nonlinear conclusions in the amalgamation,” rather th
an “in opposition to” — the functionalist logic of Darwinian natural selection. (2) The concept of constraint must include theoretically legitimate and factually important positive meanings — i.e., constraints as directing causes of particular evolutionary changes — rather than only the negative connotations of structural limitations that prevent natural selection from crafting an alteration that would otherwise be favored and achieved.
The passage from Job, needless to say, only provides an etymological justification for this crucial positivity of meaning. The case for actual existence, and important relative frequency, of these positive aspects then becomes the organizing theme of this chapter. But etymology provides a good beginning, because we must first establish the coherence of a case in language and logic before we can ask, with appropriate clarity, whether nature assents to such a reasonable and testable hypothesis.
The meanings and derivations of “constraint” are varied and complex. The Latin root stringere means both to compress or to draw tight (the negative connotations), but also to move, affect or touch (the positive aspects). The prefix con, meaning “with” or “together,” brings several items into the field of change or compression. Thus, constraints can surely be negative — as when we toss a group of miscreants into a jail cell in order to keep them close and restrict their movements. But constraints can also be positive, as when we force a group of items into closer conjunction so that their combined power and speed can grow and also become more focused in a particular direction towards a definite goal — as in the increased speed of fluids in narrowed pipes, according to Bernoulli's principle.
The Structure of Evolutionary Theory Page 163