Orthogenesis, the third and last of Kellogg's alternatives to Darwinism, expires with Fisher's initial argument on the virtues of particulate inheritance. The rates of mutation required by Darwinism become much lower under particulate models (and observation confirms that such a workably modest rate of mutation exists in nature). Since orthogenesis can only operate when mutation pressure becomes high enough to act as an agent of evolutionary change, empirical data on low mutation rates sound the death-knell of internalism. Fisher writes: “For mutations to dominate the trend of evolution it is thus necessary to postulate mutation rates immensely greater than those which are known to occur, and of an order of magnitude which, in general, would be incompatible with particulate inheritance” (1930, p. 20).
Fisher, with his mathematical training, reigned as the master of abstract
7-1. R. A. Fisher's classic illustration of the negative relationship between magnitude of change and probability of evolutionary utility. The vast majority of large mutations are deleterious; small mutations are both far more frequent and more likely to be useful. From Fisher, 1930.
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generality among the early synthesists. He favored infinitesimal changes in large panmictic populations, as illustrated by physical metaphors and analogies; and he maintained little interest in the historical quirks, vagaries and complex structuring of actual populations. When subject to such constant and unconstrained natural selection in large panmictic populations, living creatures become exquisitely fitted to their environments: “Organisms in general are, in fact, marvelously and intricately adapted, both in their internal mechanisms, and in their relations to external nature” (p. 41).
In Fisher's world of panadaptationism and pure Darwinian generality, neutralism can only maintain an insignificant relative frequency, and natural selection must reign. “Within such an unrelenting context of slow changes and large populations, Darwinian functionalism must triumph: “The very small range of selective intensity in which a factor may be regarded as effectively neutral suggests that such a condition must in general be extremely transient. The slow changes which must always be in progress, altering the genetic constitution and environmental conditions of each species, must also alter the selective advantage of each gene contrast” (1930, p. 95).
Many examples could be cited from Fisher's Genetical Theory, but two features strike me as particularly revealing in illustrating Fisher's maximally exclusive and general Darwinism.
The analogy of Fisher's “fundamental theorem” with the second law of thermodynamics. After explicating and justifying his “fundamental theorem” of natural selection — “the rate of increase in fitness of any organism at any time is equal to its genetic variance in fitness at that time” (p. 35, Fisher's italics) — Fisher, enamored as ever with physical analogies, compares this central principle of his own construction with the second law of thermodynamics:
It will be noticed that the fundamental theorem proved above bears some remarkable resemblances to the second law of thermodynamics. Both are properties of populations, or aggregates, true irrespective of the nature of the units which compose them; both are statistical laws; each requires the constant increase of a measurable quantity, in the one case the entropy of a physical system and in the other the fitness ... of a biological population ... Professor Eddington has recently remarked that “The law that entropy always increases — the second law of thermodynamics — holds, I think, the supreme position among the laws of nature.” It is not a little instructive that so similar a law should hold the supreme position among the biological sciences (1930, p. 36).
In a curious and even ironic sense, the most striking feature of this analogy lies in its imprecision and inaptness (especially as devised and presented by such an exacting thinker) — as Fisher himself admits directly after the statement quoted above, when he lists, as exceptions, several “profound differences” (p. 37). (The claim for such generality in physical terms must have struck Fisher as vitally important if he chose to make the comparison, declare its status as fundamental, and then immediately proceed to hedge or retract [Page 512] every important similarity!) Consider the first three exceptions listed by Fisher (1930, p. 37):
1. The systems considered in thermodynamics are permanent; species on the contrary are liable to extinction, although biological improvement must be expected to occur up to the end of their existence.
2. Fitness, although measured by a uniform method, is qualitatively different for every different organism, whereas entropy, like temperature, is taken to have the same meaning for all physical systems.
3. Fitness may be increased or decreased by changes in the environment, without reacting quantitatively upon that environment.
But what do these exceptions express in ordinary biological parlance? Contingency, individuality, and interaction, for the three points respectively. Could anyone have presented a better list of the peculiarly biological properties that make organisms and their history so intrinsically unlike simpler physical systems that operate by timeless and general laws? Do these differences between physical thermodynamics and Darwinian biology then rank as exceptions or invalidations?
The eugenical chapters. Fisher's Genetical Theory has generally been acknowledged, and properly so, as the keystone of 20th century evolutionary theory. Yet few contemporary biologists have actually read the book in extenso, and one feature of this common neglect seems especially puzzling. The last five chapters, nearly 40 percent of the entire volume, present a single coherent (if fatally flawed) argument in eugenics — a claim that modern industrial society (particularly the British version) has entered a potentially fatal decline as a result of “social promotion of the relatively infertile.” In essence, Fisher argues that people, who rise socially, by dint of moral or intellectual superiority, also tend to express ineluctable genetic propensities (not reversible, environmentally induced preferences) for infertility. This superior upper stratum will therefore be swamped by greater reproduction of less worthy social classes. Throughout human history, most great civilizations have declined for this reason. Society should fight this decay by rewarding gifted and highly fecund members of the lower classes, thereby helping them to rise and rejuvenate the reproduction of higher social strata.
A tradition of discreet silence has enveloped these chapters. Provine (1971), for example, relegates this material to a single sentence in his important book (pp. 153-154): “In the concluding five chapters he extended his genetical ideas to human populations.” This discretion, I suppose, reflects our embarrassment that such a paragon of our profession should have ended his canonical book with such a long argument for a politically discredited movement (see Gould, 1991c, for an analysis and critique of Fisher's eugenical arguments). This professional silence surely cannot reflect a belief that these chapters bear no connection to the rest of the book, and that Fisher merely appended this material to grind his political axe — for Fisher states that he could have dispersed this 40 percent among the other chapters, and then adds [Page 513] (p. x): “The deductions respecting Man are strictly inseparable from the more general chapters.”
I regard the conspiracy of silence about these chapters as both unscholarly and overly fastidious. First of all, how can we justify silence about integral parts of an important thinker's work because we now recoil at his beliefs? (Wagner's antisemitism retains intimate linkage with his musical productions, but we cannot ban such glorious operas.) Second, even if we wish to defend such posthumous cleansing, Fisher's eugenics can only be judged as “garden variety” material for his time, and not as especially benighted or vengeful. His visions of proper social stratification may surely be judged elitist (scarcely a rare attitude for an Oxbridge don in class-conscious Britain), but anachronistic exponents of modern political correctness will appreciate other facets of his argument. (Fisher, for example, cautiously advocates racial mixing for its role in increasing genetic variance, the
reby supplying more material at the right tail of the human distribution, even though admixture with a “lower race” — Fisher did not espouse egalitarian beliefs! — might depress the mean.)
But the central relevance of these final chapters lies in the consonance of Fisher's eugenical argument with his commitment to a general and statistical Darwinism. Fisher's eugenics provides our most interesting and incisive affirmation of his evolutionary philosophy. Darwinian triumph must be measured as differential reproductive success, statistically defined in large populations — not as particularistic victory for nifty bits of morphology (or mentality) in Tennyson's world of “nature red in tooth and claw.” Moreover, Fisher maintains that our current pattern of degeneration arises from differentials in birth rate, not from selective superiority in resisting death — so Darwinian “success” can only be viewed as statistical leverage in components of reproductive advantage, not as improvement in any social or vernacular sense: “Even the highest death-rate in this period, that in the first year of life, must be quite unimportant compared with slight differences in reproduction; for the infantile-death rate has been reduced in our country to about seven percent of the births, and even a doubling of this rate would make only about a third as much difference to survival as an increase in the family from three children to four” (1930, p. 194).
Finally, this eugenical example illuminates the central Darwinian claim for the power of slight statistical advantage. A truly effective, and truly Darwinian, eugenics, Fisher argues, will focus on apparently tiny reproductive differentials, and not on the elimination of rare and overt “saltations” — sterilization of the genetically diseased or mentally defective, as in the programs favored by most eugenicists who did not grasp the Darwinian imperative. We might regard small differences in birth rates as trifling, and unlikely to exert much effect upon the rapid time scales of human history. But anything that can be measured at all over the minimal span of a generation or two translates to an enormous effect in evolutionary time. Thus, the social promotion of relative infertility, however “invisible” in comparison to the devastation of war or the progress of technology, will yield an evolutionary degeneration far [Page 514] in excess of almost any other Darwinian change in nature. In evolutionary time, Fisher laments, our social structures disintegrate rapidly; we had better pay heed: “Civilized man, in fact, judging by the fertility statistics of our own time, is apparently subjected to a selective process of an intensity approaching a hundredfold the intensities we can expect to find among wild animals, with the possible exception of groups which have suffered a recent and profound change in their environment” (1930, p. 199).
J. B. S. HALDANE AND THE INITIAL PLURALISM
OF THE SYNTHESIS
Haldane purposely included a plural in the title of his book — The Causes of Evolution (1932) — for he believed that nothing so encompassing could be so unifactorial. But Haldane wrote his book in the tradition of restriction, primarily to debunk Kellogg's triad of alternatives by showing the power of natural selection. He states (p. v) that his book began as a series of lectures entitled “A Reexamination of Darwinism,” and he then announces his primary aim in the preface (p. vi): “To prove that mutation, Lamarckian transformation, and so on, cannot prevail against natural selection of even moderate intensity.” (Haldane treats the same subject more formally in the book's lengthy mathematical appendix, thus uniting both the front and back matter for a single purpose.)
Haldane presents a conventional account of the revivification of Darwinism and the rejection of alternatives. Darwinism had fallen on bad times before the synthesis: “Criticism of Darwinism has been so thoroughgoing that a few biologists and many laymen regard it as more or less exploded” (p. 32). The Darwinian resurrection followed from the recognition that continuous, small-scale variation could also claim a Mendelian basis (p. 71) and, especially, that tiny selection pressures, working in a cumulative manner on such minor variations, could effectively explain all evolution: “But however small may be the selective advantage the new character will spread, provided it is present in enough individuals of the population to prevent its disappearance by mere random extinction.... An average advantage of one in a million will be quite effective in most species” (1932, p. 100).
The development of mathematical population genetics establishes the centerpiece of Darwinian revival. Haldane even begins the tradition of a founding trinity in stating, however immodestly (p. 33): “I can write of natural selection with authority because I am one of the three people who know most about its mathematical theory.”
However, in contrast to Fisher's quest for pervasive and abstract generality, Haldane felt compelled to bring the smaller and more particular puzzles of natural history under his theoretical umbrella. Here he allows a substantial range of exceptions to Darwinism, albeit at subsidiary frequency — thus illustrating the predominant pluralism of the early synthesis. Haldane rejects Lamarckism outright, as contrary in principle to the known workings of inheritance. But, in a remarkable passage, he finds some space, in chinks and [Page 515] corners of the new world of fusion between Darwin and Mendel, for the two internalist theories in Kellogg's triad of alternatives — saltation and orthogenesis. (In fact, Haldane even repeats the “standard” anti-Darwinian claim for selection's merely subsidiary and negative role in enhancing and stabilizing a saltational change arising by other means — even though Haldane regards this alternative mechanism as rare in nature.) Galton's polyhedron cannot be fully rounded by the emerging Darwinian consensus:
But if we come to the conclusion that natural selection is probably the main cause of change in a population, we certainly need not go back completely to Darwin's point of view. In the first place, we have every reason to believe that new species may arise quite suddenly, sometimes by hybridization, sometimes perhaps by other means. Such species do not arise, as Darwin thought, by natural selection. When they have arisen they must justify their existence before the tribunal of natural selection, but that is a different matter... Secondly, natural selection can only act on the variations available, and these are not, as Darwin thought, in every direction. In the first place, most mutations lead to a loss of complexity (e.g. substitution of leaves for tendrils in the pea and sweet pea) or reduction in size of some organ (e.g. wings in Drosophila)... Mutations only seem to occur along certain lines (1932, pp. 138—139).
Two modes of non-Darwinian change especially intrigued Haldane. First, though he tried to reinterpret as many cases as possible in a Darwinian manner, Haldane accepted some paleontological claims for supposedly orthogenetic trends, and he admitted that the developing Darwinian synthesis could find no place for such phenomena: “Many such cases — for example the development of large size or large horns — can, I think, be put down to the ill effects of competition between members of the same species. Others, such as the exaggerated coiling of Gryphaea cannot at present be explained with any strong degree of likelihood” (1932, p. 141). (This example seems especially ironic in retrospect, because Gryphaea's supposed overcoiling to necessary extinction never occurred, and the claim rested upon misreported and misinterpreted data — see Chapter 10, pp. 1040–1045 and Gould, 1972.)
As his favorite general argument for awarding a small space to orthogenesis, Haldane cited the putatively higher frequency of degenerational over progressive evolution, arguing that such a tendency probably required an internalist explanation rooted in a bias for deletional mutations: “Degeneration is a far commoner phenomenon than progress. It is less striking because a progressive type, such as the first bird, has left many different species as progeny, while degeneration often leads to extinction, and rarely to a widespread production of new forms ... But if we consider any given evolutionary level we generally find one or two lines leading up to it, and dozens leading down” (1932, pp. 152-153).
Second, Haldane accepted the common wisdom of taxonomists in his generation that most diffe
rentia of species expressed no adaptive significance. He [Page 516] also acknowledged this factual substrate as a primary source of legitimate doubts, then common among taxonomists, about Darwinism: “But when we have pushed our analysis as far as possible, there is no doubt that innumerable characters show no sign of possessing selective value, and, moreover, these are exactly the characters which enable a taxonomist to distinguish one species from another. This had led many able zoologists and botanists to give up Darwinism” (1932, pp. 113-114).
Haldane even presents the interesting argument that we have been fooled into accepting a dominant frequency for adaptation by a pronounced bias in the fossil record — the differential preservation of species with persistently large populations subject to control by small Fisherian differentials in natural selection. Perhaps most species exist as much smaller populations, and therefore become subject to Wrightian dynamics of genetic drift — even if such species rarely enter the fossil record and therefore fail to leave evidence for their dominant relative frequency. Haldane even cites the highest of all authorities to buttress this idea:
But Wright's theory certainly supports the view taken in this book that the evolution in large random-mating populations, which is recorded by paleontology, is not representative of evolution in general, and perhaps gives a false impression of the events occurring in less numerous species. It is a striking fact that none of the extinct species, which, from the abundance of their fossil remains, are well known to us, appear to have been in our own ancestral line. Our ancestors were mostly rather rare creatures. “Blessed are the meek: for they shall inherit the earth” (1932, pp. 213-214).
The Structure of Evolutionary Theory Page 82