The Structure of Evolutionary Theory Page 4

After all, the notion of a general anatomical blueprint that contains all particular incarnations by acting as a fundamental building block (Goethe's leaf or Geoffroy's vertebra) moved long ago from conceptualization as a disem­bodied and nonmaterial archetype employed by a creator, to an actual struc­ture (or inherited developmental pathway) present in a flesh and blood ances­tor — a material basis for channeling, often in highly positive Ways, the future history of diversity within particular phyletic lineages. This switch from ar­chetype to ancestor permitted us to reformulate the idea of “essence” as broad and fruitful commonalities that unite a set of particulars into the most meaningful relationships of common causal structure and genesis. Our active use of this good word should not be hampered by a shyness and disquietude lacking any validity beyond the vestiges of suspicions originally set by battles won so long ago that no one can remember the original reasons for anathe­matization. Gracious (and confident) victors should always seek to revive the valid and important aspects of defeated but honorable systems. And the tran­scendental morphologists did understand the importance of designating a small but overarching set of defining architectural properties as legitimate es­sences of systems, both anatomical and conceptual.

  Hull correctly defines theories as historical entities, properly subject to all the principles of narrative explanation — and I shall so treat Darwinian logic and its substantial improvements and changes throughout this book. But the­ories of range and power also feature inherent “essences,” implicit in their logical structure, and operationally definable as minimal sets of proposi­tions so crucial to the basic function of a system that their falsification must undermine the entire structure, and also so necessary as an ensemble of mutual implication that all essential components must work in concert to set the theory's mechanism into smooth operation as a generator and explana­tion of nature's order. In staking out this middle Goldilockean ground be­tween (1) the “too little” of Hull's genealogical continuity without commit­ment to a shared content of intellectual morphology and (2) the “too much” of long lists of ideological fealty, superficially imbibed or memorized, and then invoked to define membership in ossified cults rather than thoughtful al­legiance to developing theories, I will argue that a Darwinian essence can be minimally (and properly) defined by three central principles constituting a tri­pod of necessary support, and specifying the fundamental meaning of a pow­erful system that Darwin famously described as the “grandeur in this view of life.”

  I shall then show that this formulation of Darwinian minimal commit­ments proves its mettle on the most vital ground of maximal utility. For not only do these three commitments build, in their ensemble, the full frame of a comprehensive evolutionary worldview, but they have also defined the chief objections and alternatives motivating all the most interesting debate within evolutionary theory during its initial codification in the 19th century. More­over, and continuing in our own time, these three themes continue to specify the major weaknesses, the places in need of expansion or shoring up, and the locus of unresolved issues that make evolutionary biology such a central and [Page 12] exciting subject within the ever changing and ever expanding world of mod­ern science.

  The Structure of Evolutionary Theory: Revising the Three Central Features of Darwinian Logic

  In the opening sentence of the Origin's final chapter (1859, p. 459), Darwin famously wrote that “this whole volume is one long argument.” The pres­ent book, on “the structure of evolutionary theory,” despite its extravagant length, is also a brief for an explicit interpretation that may be portrayed as a single extended argument. Although I feel that our best current formulation of evolutionary theory includes modes of reasoning and a set of mechanisms substantially at variance with strict Darwinian natural selection, the logical structure of the Darwinian foundation remains remarkably intact — a fasci­nating historical observation in itself, and a stunning tribute to the intel­lectual power of our profession's founder. Thus, and not only to indulge my personal propensities for historical analysis, I believe that the best way to ex­emplify our modern understanding lies in an extensive analysis of Darwin's basic logical commitments, the reasons for his choices, and the subsequent manner in which these aspects of “the structure of evolutionary theory” have established and motivated all our major debates and substantial changes since Darwin's original publication in 1859.1 regard such analysis not as an antiquarian indulgence, but as an optimal path to proper understanding of our current commitments, and the underlying reasons for our decisions about them.

  As a primary theme for this one long argument, I claim that an “essence” of Darwinian logic can be defined by the practical strategy defended in the first section of this chapter: by specifying a set of minimal commitments, or broad statements so essential to the central logic of the enterprise that disproof of any item will effectively destroy the theory, whereas a substantial change to any item will convert the theory into something still recognizable as within the Bauplan of descent from its forebear, but as something sufficiently differ­ent to identify, if I may use the obvious taxonomic metaphor, as a new subclade within the monophyletic group. Using this premise, the long argu­ment of this book then proceeds according to three sequential claims that set the structure and order of my subsequent chapters:

  1. Darwin himself formulated his central argument under these three basic premises. He understood their necessity within his system, and the difficulty that he would experience in convincing his contemporaries about such unfa­miliar and radical notions. He therefore presented careful and explicit de­fenses of all three propositions in the Origin. I devote the first substantive chapter (number 2) to an exegesis of the Origin of Species as an embodiment of Darwin's defense for this central logic.

  2. As evolutionary theory experienced its growing pains and pursued its founding arguments in the late 19th and early 20th centuries (and also in [Page 13] its pre-Darwinian struggles with more inchoate formulations before 1859), these three principles of central logic defined the themes of deepest and most persistent debate — as, in a sense, they must because they constitute the most interesting intellectual questions that any theory for causes of descent with modification must address. The historical chapters of this book's first half then treat the history of evolutionary theory as responses to the three central issues of Darwinian logic (Chapters 3–7).

  3. As the strict Darwinism of the Modern Synthesis prevailed and “hard­ened,” culminating in the overconfidences of the centennial celebrations of 1959, a new wave of discoveries and theoretical reformulations began to challenge aspects of the three central principles anew — thus leading to an­other fascinating round of development in basic evolutionary theory, extend­ing throughout the last three decades of the 20th century and continuing to­day. But this second round has been pursued in an entirely different and more fruitful manner than the 19th century debates. The earlier questioning of Darwin's three central principles tried to disprove natural selection by offer­ing alternative theories based on confutations of the three items of central logic. The modern versions accept the validity of the central logic as a foun­dation, and introduce their critiques as helpful auxiliaries or additions that enrich, or substantially alter, the original Darwinian formulation, but that leave the kernel of natural selection intact. Thus, the modern reformula­tions are helpful rather than destructive. For this reason, I regard our modern understanding of evolutionary theory as closer to Falconer's metaphor, than to Darwin's, for the Duomo of Milan — a structure with a firm foundation and a fascinatingly different superstructure. (Chapters 8–12, the second half of this book on modern developments in evolutionary theory, treat this third theme.)

  Thus, one might say, this book cycles through the three central themes of Darwinian logic at three scales — by brief mention of a framework in this chapter, by full exegesis of Darwin's presentation in Chapter 2, and by lengthy analysis of the major differences and effects in historical (Part 1) and modern cri
tiques (Part 2) of these three themes in the rest of the volume.

  The basic formulation, or bare-bones mechanics, of natural selection is a disarmingly simple argument, based on three undeniable facts (overproduc­tion of offspring, variation, and heritability)1 and one syllogistic inference (natural selection, or the claim that organisms enjoying differential reproduc­tive success will, on average, be those variants that are fortuitously better adapted to changing local environments, and that these variants will then pass their favored traits to offspring by inheritance). As Huxley famously, and ruefully, remarked (in self-reproach for failing to devise the theory him­self), this argument must be deemed elementary (and had often been formulated [Page 14] before, but in negative contexts, and with no appreciation of its power — see p. 137), and can only specify the guts of the operating machine, not the three principles that established the range and power of Darwin's rev­olution in human thought. Rather, these three larger principles, in defining the Darwinian essence, take the guts of the machine, and declare its simple operation sufficient to generate the entire history of life in a philosophical manner that could not have been more contrary to all previous, and cher­ished, assumptions of Western life and science.

  The three principles that elevated natural selection from the guts of a working machine to a radical explanation of the mechanism of life's history can best be exemplified under the general categories of agency, efficacy, and scope. I treat them in this specific order because the logic of Darwin's own de­velopment so proceeds (as I shall illustrate in Chapter 2), for the most radical claim comes first, with assertions of complete power and full range of appli­cability then following.

  Agency. The abstract mechanism requires a locus of action in a hierar­chical world, and Darwin insisted that the apparently intentional “benevo­lence” of nature (as embodied in the good design of organisms and the har­mony of ecosystems) flowed entirely as side-consequences of this single causal locus, the most “reductionistic” account available to the biology of Darwin's time. Darwin insisted upon a virtually exceptionless, single-level theory, with organisms acting as the locus of selection, and all “higher” order emerging, by the analog of Adam Smith's invisible hand, from the (unconscious) “strug­gles” of organisms for their own personal advantages as expressed in dif­ferential reproductive success. One can hardly imagine a more radical refor­mulation of a domain that had unhesitatingly been viewed as the primary manifestation for action of higher power in nature — and Darwin's brave and single-minded insistence on the exclusivity of the organismic level, although rarely appreciated by his contemporaries, ranks as the most radical and most distinctive feature of his theory.

  Efficacy. Any reasonably honest and intelligent biologist could easily understand that Darwin had identified a vera causa (or true cause) in natural selection. Thus, the debate in his time (and, to some extent, in ours as well) never centered upon the existence of natural selection as a genuine causal force in nature. Virtually all anti-Darwinian biologists accepted the reality and action of natural selection, but branded Darwin's force as a minor and negative mechanism, capable only of the headsman's or executioner's role of removing the unfit, once the fit had arisen by some other route, as yet uniden­tified. This other route, they believed, would provide the centerpiece of a “real” evolutionary theory, capable of explaining the origin of novelties. Dar­win insisted that his admittedly weak and negative force of natural selection could, nonetheless, under certain assumptions (later proved valid) about the nature of variation, act as the positive mechanism of evolutionary novelty — that is, could “create the fit” as well as eliminate the unfit — by slowly accu­mulating the positive effects of favorable variations through innumerable generations. [Page 15]

  Scope. Even the most favorably minded of contemporaries often admit­ted that Darwin had developed a theory capable of building up small changes (of an admittedly and locally “positive” nature as adaptations to changing environments) within a “basic type” — the equivalent, for example, of making dogs from wolves or developing edible corn from teosinte. But these critics could not grasp how such a genuine microevolutionary process could be ex­tended to produce the full panoply of taxonomic diversity and apparent “progress” in complexification of morphology through geological time. Dar­win insisted on full sufficiency in extrapolation, arguing that his micro-evolutionary mechanism, extended through the immensity of geological time, would be fully capable of generating the entire pageant of life's history, both in anatomical complexity and taxonomic diversity — and that no further causal principles would be required.

  Because primates are visual animals, complex arguments are best portrayed or epitomized in pictorial form. The search for an optimal icon to play such a role is therefore no trivial matter (although scholars rarely grant this issue the serious attention so richly merited) — especially since the dangers of confu­sion, misplaced metaphor, and replacement of rigor with misleading “intu­ition” stand so high. I knew from the beginning of this work that I needed a suitable image for conveying the central logic of Darwinian theory. As one of my humanistic conceits, I hoped to find a historically important scientific im­age, drawn for a different reason, that might fortuitously capture the argu­ment in pictorial form. But I had no expectation of success, and assumed that I would need to commission an expressly designed figure drawn to a long list of specifications.

  The specific form of the image — its central metaphorical content, if you will — plays an important role in channeling or misdirecting our thoughts, and therefore also requires careful consideration. In the text of this book, I speak most often of a “tripod” since central Darwinian logic embodies three major propositions that I have always visualized as supports — perhaps be­cause I have never been utterly confident about this entire project, and I needed some pictorial encouragement to keep me going for twenty years. (And I much prefer tripods, which can hold up elegant objects, to buttresses, which may fly as they preserve great Gothic buildings, but which more often shore up crumbling edifices. Moreover, the image of a tripod suits my major claim particularly well — for I have argued, just above, that we should define the “essence” of a theory by an absolutely minimal set of truly necessary propositions. No structure, either of human building or of abstract form, captures this principle better than a tripod, based on its absolute minimum of three points for fully stable support in the dimensional world of our physical experience.)

  But organic images have always appealed more strongly, and I preferred a biological icon. If the minimal logic can be represented by a tripod pointing downward, then the same topology can be inverted into a structure growing upward. Darwin's own favorite image of the tree of life immediately sug­gested itself, and I long assumed that I would eventually settle on a botanical [Page 16] icon. But I also remembered Darwin's first choice for an organic metaphor or picture of branching to capture his developing views about descent with modification and the causes of life's diversity — the “coral of life” of his “B Note­book” on transmutation, kept during the 1830's as he became an evolutionist and struggled towards the theory of natural selection (see Barrett et al., 1987).

  As I began to write this summary chapter, I therefore aimlessly searched through images of Cnidaria from my collection of antiquarian books in paleontology. I claim no general significance whatsoever for my good fortune, but after a lifetime of failure in similar quirky quests, I was simply stunned to find a preexisting image — not altered one iota from its original form, I promise you, to suit my metaphorical purposes — that so stunningly embodied my needs, not only for a general form (an easy task), but down to the smallest de­tails of placement and potential excision of branches (the feature that I had no right or expectation to discover and then to exapt from so different an original intent).

  The following figure comes from the 1747 Latin version of one of the semi­nal works in the history of paleontology — the 1670 Italian treatise of the Sicilian savant
and painter Agostino Scilla, ha vana speculazione disingan-nata dal senso (“Vain speculation undeceived by the senses” — Scilla's de­fense, at the outset of “the scientific revolution” of Newton's generation, for empirical methods in the study of nature, and specifically, in this treatise, for a scientific paleontology and the need to recognize fossils as remains of an­cient organisms, not as independent products of the mineral kingdom). This work, famous not only for an incisive text, but also for its beautiful plates (see Fig. 1-3), engraved by an author known primarily as an artist of substan­tial eminence, includes this figure, labeled Coralium articulatum quod copio-sissimum in rupibus et collibus Messanae reperitur (“Articulated coral, found in great abundance in the cliffs and hills of Messina”).

  This model, and its organic features, work uncommonly well as a meta­phor for the Goldilockean position of definition by a barest minimum of truly fundamental postulates. For Scilla's coral, with its branching structure (see Fig. 1-4) — particularly as expressed in the lessening consequences of excising branches at ever higher levels nearer the top (the analogs of disconfirming theoretical features of ever more specialized and less fundamental import) — so beautifully captures the nature and operation of the intellectual structure that I defended above for specifying the essences of theories. The uncanny ap­propriateness of Scilla's coral lies in the fortuity that this particular specimen (accurately drawn from nature by Scilla, I assume, and not altered to as­sert any general point) just happens to include exactly the same number of branches (three) as my Darwinian essential structure. (They terminate at the same upper level, so I could even turn the specimen over into a tolerably unwobbly tripod!) Moreover, since this particular genus of corals grows in discrete segments, the joining points correspond ideally with my metaphor of chopping planes for excising parts of structures at various levels of impor­tance in an intellectual entity. But, most incredibly, the segmental junctions of