The Structure of Evolutionary Theory Read online

  Epitomes for a Long Development

  LEVELS OF POTENTIAL ORIGINALITY

  Most of this book can be described as extensive narration of work already done, and ideas already expounded elsewhere. But no one should write at such length merely to organize the conventional material of a field, and without an original structure, or a set of unconventional ideas, to propose. I wrote The Structure of Evolutionary Theory because I felt that I had fol­lowed a sufficiently idiosyncratic procedure to devise a sufficiently novel the­oretical structure that then yielded a sufficient number of original insights on specific matters to qualify as a justification for spending so many years of a career, and daring to ask readers for such a non-trivial chunk of their at­tention.

  As implied by the foregoing sentence, I think that whatever originality this work possesses might best be conceptualized at three levels of basic structure, primary justifications for the major components of theory, and specific in­sights or discoveries then developed under the aegis of this structure and the­ory. At the first level of basic structure, I believe that three features of organi­zation set the novelty of presentation:

  1. Developing an exegesis of essential components in the logic of Darwin­ian theory, as expressed in the agency, efficacy, and scope of selection as an evolutionary mechanism (Chapter 2).

  2. Explicating the history of evolutionary thought as a complex and ex­tended debate about these essential components, developed negatively at first by early evolutionists who sought alternative formulations to Darwinism (Chapters 3–6), and then positively in our times by scientists who recognized the need for extensive revisions and expansions that would build an enlarged structure upon a Darwinian foundation, rather than uproot the theoretical core of selectionism (Chapters 7–12).

  3. Formulating an expanded theory that introduces substantial revisions on each branch of Darwinian central logic, but builds, in its ensemble, a coher­ently enlarged structure with a retained Darwinian base — moving from Dar­win's single level of agency to a hierarchical theory of selection on the first branch; balancing positive sources of internal constraint (for both structural and historical reasons) with the conventional externalism of natural selection on the second branch; and recognizing the disparate inputs of various tiers [Page 49] of time, rather than trying to explain all phylogenetic mechanics by uniformitarian extrapolation from microevolutionary processes, on the third branch.

  At the second level of validation for proposed revisions in the structure of evolutionary theory, I have tried to develop broad arguments and empirical justifications for major changes and expansions on each of the three branches of Darwinian central logic. On the first branch of agency, the theory of punc­tuated equilibrium itself, initially formulated by Niles Eldedge and me, estab­lishes the species as a true and potent Darwinian individual, and grants a minimal guarantee of descriptive independence to macroevolution by requir­ing a treatment of trends as the differential success of stable species rather than the adaptive anagenesis of lineages by accumulated and extrapolated organismal selection alone. Beyond punctuated equilibrium, the general ra­tionale for a hierarchical theory of selection, as presented here through the interactor approach based on emergent fitnesses at higher levels, may estab­lish a complete (and tolerably novel) framework not only for grasping the consistent logic of hierarchical selection, but also for viewing each level as po­tent in its own distinctive way, and for recognizing the totality of evolution­ary outcomes as a realized balance among these potencies, and not as the achieved optimality of a single causal locus — a substantial difference from Darwinian traditions for conceiving the dynamics of evolutionary change. In working through the differences among levels — see Chapter 8, pp. 714–744 — I was particularly struck by the surprising, but accurate and challeng­ing, analogies (Lamarckian inheritance at the organismal level with adaptive anagenesis at the species level, for example); and by the different modes of equally effective change implied by disparate structural reasons for the estab­lishment of individuality at various levels (particularly, the domination of se­lection over drift and drive at the organismal level vs. the potent balance among all three mechanisms at the species level).

  On the second branch of efficacy, I have tried to make the most comprehensive case yet advanced for internal constraint as a positive director and channeler of evolutionary change, and not only as a negative brake upon pure Darwinian functionalism. I proceed by explicating two conceptually different forms of constraint — structural constraints as consequences of physical prin­ciples, and historical constraints as channels from particular pasts. I argue that each category challenges a different central tenet of Darwinism — struc­tural constraint by establishing a substantial space for non-selectionist origin of important evolutionary features, and historical constraint for explaining the markedly inhomogeneous filling of morphospace as flow down ancient internal channels of deep homology, and not primarily as a mapping of adap­tive design upon current ecological landscapes. Beyond any novelty in this general formulation, I have attempted to develop a conceptual space, and to establish practical criteria, for the identification of non-adaptive sequelae (spandrels), the evolutionary importance of their later cooptation for utility (exaptation), and the importance of such reservoirs of potential (exaptive pools) in explicating the important concept of “evolvability” in structural rather than purely adaptational terms. [Page 50]

  On the third branch of scope, my contribution cannot claim much novelty, if only because I have not worked professionally in this area of paleontological research. But I do explicate, perhaps more fully than before, both the historical and conceptual reasons for regarding catastrophic mass extinction, and catastrophic mechanics in general (within their limited scope of validity), not as anti-selectionist per se, but rather as fracturing the extrapolationist premise of Darwinian central logic, and requiring that substantial aspects of phyletic pattern be explained as interaction between temporal extensions of microevolution and different processes that only become visible and effective at higher tiers of time. I try to resolve “the paradox of the first tier” (the em­pirical failure of Darwin's logically airtight argument for a vector of progress) by arguing that punctuated equilibrium at the second tier of phyletic trends, and mass extinction at the third tier of faunal overturn, impose enough of their own, distinctive and different, patterning to forestall the domination or pure imprint of extrapolated microevolutionary results upon the general pag­eant of life's history.

  Finally, at the third level of those lovely details (where both God and the devil dwell, and where, ultimately, both the joy and power of science reside), I trust that any originality I have introduced at “higher” levels of theoretical structure gains primary expression and utility in the resolution of previously puzzling details, and in the identification of “little things” that had escaped previous notice or explicit examination.

  For example, most original analyses and discoveries in the historical first half of this book flow directly from my organizing theme of identifying essen­tial components in Darwinian logic, and then tracing both the early attempts to defeat, and our later efforts to modify and expand them through time. I was thus able to discover and identify Darwin's major encounter with higher level selection not in his recognized discussion of group selection for human altruism, but in his previously unexplicated admission of species selection to resolve the problem of diversity (see Chapter 3, pp. 246–250). In this case, I “lucked out” through an odd reason for previous ignorance of such an im­portant textual revision — for Darwin omitted this material in his compressed and hasty discussion of diversity in Chapter 4 of the Origin (on this subject, the only Darwinian source generally known to professional biologists, who would immediately highlight the importance of any acknowledgment of spe­cies selection). But Darwin agonized over levels of selection at explicit length in the unpublished “long version” that only saw the light of printed day in 1975 (Stauffer, 1975), and that virtua
lly no practicing biologist has ever read (whereas historians of science who do study this longer text usually lack suf­ficient knowledge of the technical debate about levels of selection to under­stand the meaning of Darwin's passages or to appreciate their import).

  The same context led me to appreciate the previously unanalyzed development of a full hierarchical model by Weismann in his later works (Chapter 3, pp. 223–224), a formulation that Weismann himself identified as the most important theoretical achievement of his later career. Previous historians had written about his much longer and earlier explications of lower level selection [Page 51] (germinal selection in his terms), if only in the context of modern reductionistic breakdowns of Darwinism to selection among “selfish genes.” But they had missed his later reversal and expansion to a full hierarchical model, despite Weismann's own emphasis. Similarly, de Vries's clear understanding of Darwinian logic had also been ignored because de Vries, as an opponent of the efficacy of Darwinian organismal selection (a painful decision for him, given his psychological fealty to Darwin, also explored herein), applied the logic to higher levels, and even devised the term “species selection” (Chapter 5, pp. 446–451) — a concept and coining previously entirely unremarked by historians (much to the embarrassment of scientists, including yours truly, who coined and explicated the same term much later in full expectation of pristine originality!).

  Similarly, my sense of the logic in conflicts between constraint and adapta­tion (or internal vs. external, or formal vs. functional approaches) on the second branch helped me to pinpoint, or to make sense of, several important historical events and arguments that have not been properly treated or under­stood. Historians of science had not previously discussed orthogenetic theo­ries in this fairest light, and had not distinguished the very different formula­tions of Hyatt, Eimer, and Whitman in terms of their increasingly greater willingness to accommodate Darwinian themes as well (see Chapter 5). The same framework allowed me to identify the crucial importance, and brilliant epitomization, of this issue in the final paragraphs of Chapter 6 (“Difficulties on Theory”) in Darwin's Origin, a significance that had not been highlighted before.

  I also traced the dichotomy of anglophonic preferences for functionalist accounts vs. continental leanings towards formalism back through the evo­lutionary reconstruction of the argument in the mid 19th century into the creationist formulations of Paley vs. Agassiz (Chapter 4), thus illustrating a pedigree for this fundamental issue in morphology that evolution may have recast in causal terms, but did not budge in basic commitments to the mean­ing of morphology. Among the little tidbits that emerge from such analy­ses, I even discovered that Darwin borrowed his clearest admission of co-opted utility from non-adaptive origins (unfused skull sutures in mammalian neonates, essential for passage through the birth canal, but also existing in birds and reptiles born from more capacious eggs) from the longer and more nuanced descriptions of Richard Owen, Britain's anomalous defender of for­malism.

  I also included some historical analyses in the book's second half on mod­ern advances because I thought they could make an original contribution to arguments usually developed only in contemporary terms and findings. I have already mentioned my analysis of how the initial pluralism of the Modern Synthesis (embracing any mode of change consistent with known genetic mechanisms) hardened through subsequent editions of the founding volumes into pronounced preferences for adaptationist accounts framed only in terms of natural selection (Chapter 7). In addition, I think that my reexhumation of the debate between Falconer and Darwin on fossil elephants provides a [Page 52] good introduction to punctuated equilibrium (Chapter 9, pp. 745–749). The largely unknown paradox of Lankester's original definition of homoplasy as a category of homology, rather than in the opposite status held by the term to­day, provides the best entry I could devise for understanding the vital, but lit­tle appreciated and rarely acknowledged, theoretical differences between par­allelism and convergence. In the absence of this context and distinction, the key importance of evo-devo and the discovery of deep homology among dis­tant phyla cannot properly be grasped as a challenge and expansion of Dar­winian expectations (Chapter 10).

  I hope that my sympathetic portrayal of D'Arcy Thompson's theory of form (Chapter 11), despite my general disagreement with his argument, will help colleagues to understand the thrust and potential power of this unusual formulation of structuralist constraint on external grounds of universal phys­ics. Although I am chagrined that I discovered Nietzsche's account of the dis­tinction between current utility and historical origin so late in my work, I know no better introduction — from one of history's greatest philosophers to boot, and in his analysis of morality, not of any scientific subject — to the the­oretical importance of spandrels and exaptation in the rebalancing of con­straint and adaptation within evolutionary theory (Chapter 11, pp. 1214–1218). In a final historical analysis of the second part, I think that Dar­win's own rationale for progress (Chapter 12, pp. 1296–1303), rooted not in the mechanics of natural selection itself, but in an ecological argument for extrapolation of biotic competition through time in a perpetually crowded world — an aspect of Darwin's thought that has very rarely been appreciated, formulated or discussed by historians — provided the best context I could de­vise for understanding why catastrophic mass extinction in particular, and non-extrapolation through tiers of time in general, play such havoc with Dar­win's need for uniformity on the third branch of his essential logic.

  The original claims in the book's second half on modern reformulations of evolutionary theory rest, necessarily and primarily, on theoretical insights and unusual conceptual parsings, rather than on novel data — if only be­cause custom dictates that my extensive empirical documentation be pre­sented in “review” format by collating published studies in support or refuta­tion of general themes under discussion. But I have sometimes presented existing data in novel contexts — as in my analysis of the proper category for understanding the exaptive value of genes lost by founder drift in establish­ing the social cohesion (albeit transient) that has made the Argentine ant Linepithema humile such a successful invader of non-native Californian habi­tats (Chapter 11, pp. 1282–1284). I have also cited my own empirical studies, previously published but original in the more conventional sense, to support important pieces of more general arguments, including validation of punctu­ated equilibrium by dissection of a single bedding plane to reveal transition by absolute age dating of individual shells (Goodfriend and Gould, 1996), the “employment” of constraint by selection to yield several adaptive features by one heterochronic change in a case of neoteny in Gryphaea (Jones and Gould, 1999), and the explanation of most ordered geographic variation within [Page 53] a major subregion of Cerion as consequences of allometric correlations in growth (Gould, 1984b).

  I tried (and utterly failed) to compose a selective listing, as provided above for the book's historical half, for original ideas about theoretical details devel­oped in revising the three branches of Darwinian central logic in the book's second half on modern reformulations of evolutionary theory. I ripped up several attempts that read like the hodge-podge of a random laundry list rather than the ordered “sweet places” on a logical continuum. These high­lights, I finally recognized, have little meaning outside the broader context of a linearly developing argument for each branch, and I will therefore make a second attempt, within the more detailed epitome of the next and final sec­tion of this chapter, to designate the points that struck me with the force of “aha,” or that conveyed a hint of deeper, surprising, or more radical implica­tions for reasons that I couldn't quite fathom directly, but that tickled my in­tuition at the edge of that wonderful, if elongate, German word: Finger — spitzengefuhl, or feeling at the tip of one's finger. Most inchoate excitements of this sort lead to nowhere but foolishness and waste of time, but every once in a while, the following of one's nose catches a whiff of novelty. At least we must trust ourselves enough to try — a
nd not take ourselves so seriously that we forget to laugh at our more frequent and inevitable stumbles.