The evolutionary synthesis [i.e., neo-Darwinian theory] came unraveled for me during the period since 1980. Historically, my examination of this period, after editing with Ernst Mayr “The Evolutionary Synthesis” (Mayer and Provine 1980), showed that it was not a synthesis, but rather a systematic diminution of the factors in evolution, and I now call it the “evolutionary constriction” (Provine 1989). The unity of evolutionary biology inherent in the “synthesis” has been replaced by a much more interesting and fascinating complex of different levels marching to different drummers….. In 1970 I could see the origins of theoretical population genetics as being an unalloyed good for evolutionary biology, and thus obviously a great subject for an historian. Now I see these same theoretical models of the early 1930s, still widely used today, as an impediment to understanding evolutionary biology, and their amazing persistence in textbooks and classrooms as a great topic for other historians.

— Provine, William B. The Origins of Theoretical Population Genetics. Chicago: Chicago University Press; 2001; c1971 pp. 203-204.

1895, Wilhelm Roux, in his manifesto for experimental embryology, postulated that there would be two types of developmental mechanics. The first—ontogenetic developmental mechanics—would uncover how development occurred. The second—phylogenetic developmental mechanics—would determine how changes in embryonic development caused evolutionary change. A century later, we are starting to make good on Roux’s prophecy. The homologies of process within morphogenetic fields provide some of the best evidence for evolution—just as skeletal and organ homologies did earlier. Thus, the evidence for evolution is better than ever. The role of natural selection in evolution, however, is seen to play less an important role. It is merely a filter for unsuccessful morphologies generated by development. Population genetics is destined to change if it is not to become as irrelevant to evolution as Newtonian mechanics is to contemporary physics. The population genetics of regulatory genes and their possible combinations within fields should become a major new research program. Developmental genetics would also change, reflecting an emphasis on the initiation and maintenance of genetic circuits within cells and epigenetic circuits within the field. One of its major research programs would be to find the target genes of these pathways which differ from field to field and from organism to organism, i.e., those genes that provide the diversity in evolution. (Gilbert, Opitz, and Raff 1996: 368)

Developmental biology is reclaiming its appropriate place in evolutionary theory. We conclude with a remarkable prophecy from one of those evolutionary-minded embryologists, Gavin de Beer (1951), who saw homology and fields as being crucial to the study of evolution:

But since phylogeny is but the result of modified ontogeny, there is the possibility of a causal analytic study of present evolution in an experimental study of the variability and genetics of ontogenetic processes. Finally, it may be possible that, freed from the trammels and fetters which have so long confined thought, the whole of the animal kingdom may appear in a new light, more homogeneous and compact than had been imagined, and with the gaps between its major groups less formidable and perhaps even bridgeable.

— Gilbert S. F., Opitz J. M. and Raff R. A. Resynthesizing evolutionary and developmental biology. Developmental Biology. In Developmental Biology, 173, 357-72.

~ ~ ~

Natural selection ranked as a standard item in biological discourse—but with a crucial difference from Darwin’s version: the usual interpretation invoked natural selection as part of a larger argument for created permanency. Natural selection, in this negative formulation, acted only to preserve the type, constant and inviolate, by eliminating extreme variants and unfit individuals who threatened to degrade the essence of created form. (Gould 2002: 137-139)

(….) Darwin’s theory … cannot be equated with the simple claim that natural selection operates. Nearly all his colleagues and predecessors accepted this postulate. Darwin, in his characteristic and radical way, grasped that this standard mechanism for preserving the type could be inverted, and then converted into the primary cause of evolutionary change. Natural selection obviously lies at the center of Darwin’s theory, but we must recognize, as Darwin’s second key postulate, that claim that natural selection acts as the creative force of evolutionary change. The essence of Darwinism cannot reside in the mere observation that natural selection operates—for everyone had long accepted a negative role for natural selection in eliminating the unfit and preserving the type. (Gould 2002: 139)

(….) We have lost this context and distinction today, and our current perspective often hampers an understanding of the late 19th century literature and its preoccupations. Anyone who has read deeply in this literature knows that no argument inspired more discussion, while no Darwinian claim seemed more vulnerable to critics, than the proposition that natural selection should be viewed as a positive force, and therefore as the primary cause of evolutionary change. The “creativity of natural selection”—the phrase generally used in Darwin’s time as a shorthand description of the problem—set the cardinal subject for debate about evolutionary mechanisms during Darwin’s lifetime and throughout the late 19th century. [It is poised to once again become the central question due to the scientific discoveries taking place in the fields of epigenetics and developmental biology (evo-devo).] (Gould 2002: 139)

Non-Darwinian evolutionists did not deny the reality, or the operationality, of natural selection as a genuine cause stated in the most basic or abstract manner. (….) They held, rather, that natural selection, as a headsman or executioner, could only eliminate the unfit, while some other cause must play the positive role of constructing the fit. (Gould 2002: 139)

(….) We can understand the trouble that Darwin’s contemporaries experienced in comprehending how selection could work as a creative force when we confront the central paradox of Darwin’s crucial argument: natural selection makes nothing; it can only choose among variants originating by other means. How then can selection possibly be conceived as a “progressive,” or “creative,” or “positive” force? (Gould 2002: 140)

The Requirements for Variation

In order to act as raw material only, variation must walk a tightrope between two unacceptable alternatives. First and foremost, variation must exist in sufficient amounts, for natural selection can make nothing, and must rely upon bounty thus provided; but variation must not be too florid or showy either, lest it become the creative agent of change itself. Variation, in short, must be copious, small in extent, and undirected. A full taxonomy of non-Darwinian evolutionary theories may be elaborated by their denials of one or more of these central assumptions. (Gould 2002: 141)

COPIOUS. Since natural selection makes nothing and can only work with raw material presented to its stringent review, variation must be generated in copious and dependable amounts…. Darwin’s scenario for selective modification always includes the postulate, usually stated explicitly, that all structures vary, and therefore evolve…. If these universally recognized distinctions arise as consequences of differences in the intrinsic capacity of species to vary, then Darwin’s key postulate of copiousness would be compromised—for failure of sufficient raw material would then be setting a primary limit upon the rate and style of evolutionary change, and selection would not occupy the driver’s seat. (Gould 2002: 141-142)

Darwin responds by denying this interpretation, and arguing that differing intensities of selection, rather than intrinsically distinct capacities for variation, generally cause the greater or lesser differentiation observed among domestic species. I regard this argument as among the most forced and uncomfortable in the Origin—a rare example of Darwinian special pleading. But Darwin realizes the centrality of copiousness to his argument for the creativity of natural selection, and he must therefore face the issue directly:

Although I do not doubt that some domestic animals vary less than others, yet the rarity or absence of distinct breeds for the cat, the donkey, peacock, goose, etc., may be attributed in main part to selection not having been brought into play: in cats, from the difficulty in pairing them; in donkeys, from only a few being kept by poor people and little attention paid to their breeding; in peacocks, from not being very easily reared and a large stock not kept; in geese, from being valuable only for two purposes, food and feathers, and more especially from no pleasure having been felt the display of distinct breeds (p. 42).

On the Origin of Species

Second, copiousness must also be asserted in the face of a powerful argument about limits to variation following modal departure from “type.” To use Fleeming Jenkin’s (1867) famous analogy: a species may be compared to a rigid sphere, with modal morphology of individuals at the center, and limits to variation defined by the surface. So long as individuals lie near the center, variation will be copious in all directions [isotropic; non-directional]. But if selection brings the mode to the surface, then further variation in the same direction will cease—and evolution will be stymied by an intrinsic limitation upon raw material, even when selection would favor further movement. Evolution, in other words, might consume its own fuel and bring itself to an eventual halt thereby. This potential refutation stood out as especially serious—not only for threatening the creativity of natural selection, but also for challenging the validity of uniformitarian extrapolation as a methodology of research. Darwin responded, as required by logical necessity, that such limits do not exist, and that new spheres of equal radius can be reconstructed around new modes: “No case is on record of a variable being ceasing to be variable under cultivation. Our oldest cultivated plants, such as wheat, still often yield new varieties: our oldest domesticated animals are still capable of rapid improvement or modification” (p. 8). (Gould 2002: 142)

(….) One of the most appealing features of Mendalism a strong reason for acceptance following its “rediscovery” in 1900 lay in the argument that mutation could restore variation “used up” by selection. (Gould 2002: 143) [See Klein & Tanaka, 2002, “Where Do We Come From: The Molecular Evidence for Human Descent,” p. 197-204, regarding atavism.]

SMALL IN EXTENT. If the variations that yielded evolutionary change were large—producing new major features, or even new taxa in a single step then natural selection would not disappear as an evolutionary force. Selection would still function in an auxiliary and negative role as headsman—to heap, up the hecatomb of the unfit, permit new saltation to spread among organisms in subsequent generations, and eventually to take over the population. But Darwinism, as a theory of evolutionary change, would perish—for selection would become both subsidiary and negative, and variation itself would emerge as the primary, and truly creative, force of evolution, the source of occasionally lucky saltation. For this reason, the quite properly, saltationist (or macromutational) theories have always been viewed as anti-Darwinian—despite the protestations of de Vries …, who tried to retrain the Darwinian label for his continued support of selection as a negative force. The unthinking, knee-jerk response of many orthodox Darwinians whenever they hear the word “rapid” or the name “Goldschmidt,” testifies to the conceptual power of saltation as a cardinal danger to an entire theoretical edifice. (Gould 2002: 143)

Darwin held firmly to the credo of small-scale variability as raw material because both poles of his great accomplishment required this proviso…. At the theoretical pole, natural selection can only operate in a creative manner if its cumulating force builds adaptation step by step from an isotropic pool of small-scale variability. If the primary source of evolutionary innovation must be sought in the occasional luck of fortuitous saltations, then internal forces of variation become the creative agents of change, and natural selection can only help to eliminate the unfit after the fit arise by some other process. (Gould 2002: 143-142)

(….)

UNDIRECTED. Textbooks of evolution still often refer to variation as “random.” We all recognize this designation is a misnomer, but continue to use the phrase by force of habit. Darwinians have never argued for “random” mutation in the restricted and technical sense of “equally likely in all directions,” as in tossing a die. [Rather it means statistical frequencies around a modal norm, like the bell curve for example, which does not imply that the underlying cause is totally random like tossing die.] But our sloppy use of “random” (see Eble, 1999) does capture, at least in a vernacular sense, the essence of the important claim that we do wish to convey—namely, that variation must be unrelated to the direction of evolutionary change; or, more strongly, that nothing about the process of creating raw material biases the pathway of subsequent change in adaptive directions. This fundamental postulate gives Darwinism its “two step” character, the “chance” and “necessity” of Monad’s famous formulation the separation of a source of raw material (mutation, recombination, etc.) from a force of change (natural selection). (Gould 2002: 144)

In a sense, the specter of directed variability threatens Darwinism even more seriously than any putative failure of the other two postulates. Insufficient variation stalls natural selection; saltation deprives selection of a creative role but still calls upon Darwin’s mechanism as a negative force. With directed variation, however, natural selection can be bypassed entirely. If adaptive pressures automatically trigger heritable variation in favored directions, then trends can proceed under regimes of random mortality; natural selection, acting as a negative force, can, at most, accelerate the change. (Gould 2002: 145)

(….) Darwin clearly understood the threat of directed variability to his cardinal postulate of creativity for natural selection. He explicitly restricted the sources of variation to auxiliary roles as providers of raw material, and granted all power over the direction of evolutionary change to natural selection…. He recognized biased tendencies to certain states of variation, particularly reversions toward ancestral features. But he viewed such tendencies as weak and easily overcome by selection. Thus, by the proper criterion of relative power and frequency, selection controls the direction of change: “When under nature the conditions of life do change, variations and reversions of character probably do occur; but natural selection, as will hereafter be explained, will determine how far the new characters thus arising shall be preserved” (p. 15) (Gould 2002: 145)

We may summarize Darwin’s third requirement for variation under the rubric of isotropy, a common term in mineralogy (and other sciences) for the concept of a structure or system that exhibits no preferred pathway as a consequence of construction with equal properties in all directions. Darwinian variation must be copious in amount, small in extent, and effectively isotropic. (….) Only under these stringent conditions can natural selection—a force that makes nothing directly, and must rely upon variation for all raw material—be legitimately regarded as creative. (Gould 2002: 145)

(….) Gradualism. Selection becomes creative only if it can impart direction to evolution by superintending the slow and steady accumulation of favored subsets from an isotropic pool of variation. If gradualism does not accompany this process of change, selection must relinquish this creative role and Darwinism then fails as a creative source of evolutionary novelty. If important new features, or entire new taxa, arise as large and discontinuous variations, then creativity lies in the production of the variation itself. Natural selection no longer causes evolution, and can only act as a headsman for the unfit, thus promoting changes that originated in other ways. Gradualism therefore becomes a logical consequence of the operation of natural selection in Darwin’s creative mode. (Gould 2002: 149)

(….) INSENSIBILITY OF INTERMEDIACY. We now come to the heart of what natural selection requires. This … “just right,” statement does not advance a claim about how much time a transition must take, or how variable a rate of change might be. (….) [And in this meaning of “gradualism,” it is simply asserted that] … in going from A to a substantially different B, evolution must pass through a long and insensible sequence of intermediary steps—in other words, that ancestor and descendant must be linked by a series of changes, each within the range of what natural selection might construct from ordinary variability. Without gradualism in this form, large variations of discontinuous morphological import—rather than natural selection—might provide the creative force of evolutionary change. But if the tiny increment of each step remains inconsequential in itself, then creativity must reside in the summation of these steps into something substantial natural selection, in Darwin’s theory, acts as the agent of accumulation. (Gould 2002: 150)

(….) If the altered morphology of new species often arose in single steps by fortuitous macromutation, then selection would lose its creative role and could act only as a secondary and auxiliary force to spread the sudden blessing through a population. But can we justify Darwin’s application of the same claim to single organs? (….) Would natural selection perish if change in this mode were common? I don’t think so. Darwinian theory would require some adjustments and compromises particularly a toning down of assertions about isotropy of variation, and a more vigorous study of internal constraint in genetics and development … —but natural selection would still enjoy a status far higher than that of a mere executioner. A new organ does not make a new species; and a new morphology must be brought into functional integration—a process that requires secondary adaptation and fine tuning, presumably by natural selection, whatever the extent of the initial step. (Gould 2002: 150)