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Chance Variation and Evolutionary Contingency: Darwin, Simpson (The Simpsons) and Gould John Beatty Department of Philosophy University of British Columbia Vancouver, BC V6T 1Z1, Canada [email protected] 1. Introduction The unpredictability of evolution by natural selection was championed most famously by Stephen Gould in terms of the following thought experiment: I call this experiment “replaying life’s tape.” You press the rewind button and, making sure you thoroughly erase everything that actually happened, go back to any time and place in the past. . . . Then let the tape run again and see if the repetition looks at all like the original. (Gould 1989, p. 48) His expectation was that “any replay of the tape would lead evolution down a pathway radically different from the road actually taken” (p. 51). The basic idea—sans the hyperbole (and the high-tech metaphor)—goes back to Darwin. But in reasserting it, Gould was hardly stating the obvious. It had fallen out of favor with evolutionary biologists on more than one occasion. In this chapter, I will discuss the origins and fate of the idea, which is closely tied to considerations of the meaning and significance of chance variation. 2. Darwin on “The Details Left to Chance” We associate chance variation with Darwin. But the notion appears earlier, for example in William Paley’s Natural Theology (1809). Which is somewhat surprising, because throughout the book Paley argued that living things and most everything about them are the products of design, not chance. What does chance ever do for us? In the human body, for instance, chance, i.e. the operation of causes without design, may produce a wen, a wart, a mole, a pimple, but never an eye. [Never was] an organized body of any kind, answering a valuable purpose by a complicated mechanism, the effect of chance. In no assignable instance hath such a thing existed without intention somewhere. (Paley, 1809, pp. 62-63) Toward the end of the book, however, Paley acknowledged that God had left some important things to chance, and for (supposedly) good reason. Take for example the variety of occupational abilities that we see in every society, and that every society relies on: some people are cut out to be laborers, some are cut out to be land or factory owners, etc. Such differences are associated with differences in fortune, and therefore it is only fair that they be distributed by lot. So God designed the world with this sort of chance variation built-in (p. 520). Darwin, an admirer of Paley, also figured that variation was part of God’s plan. Toward the beginning of his “Transmutation of Species” notebooks, Darwin privately wondered why organisms reproduce and die rather than living forever. The clue to the puzzle, he decided, was variation. The “final cause” or purpose of reproduction and death, together with the “tendency to vary,” was to allow species to adapt to changing environmental circumstances (see, e.g., Darwin 2 1987 [1837, Notebook B, pp. 1-5, 64], pp. 170-171, 187; Kohn 1980; Ospovat 1981, pp. 39-86; Hodge and Kohn 1985). The natural theological roots of Darwin’s evolutionary theorizing, and his understanding of variation in particular, are important for reasons that I will address shortly. At first Darwin assumed that environmental changes cause adaptively appropriate variations to arise in the offspring of affected organisms. But he soon came to the conclusion that most variations arise by “accident” or “chance,” i.e., by causes that are 1) complex and unknown and 2) in no way related to what would be useful for surviving and reproducing. As Darwin expressed the first point, “No doubt each slight variation must have its efficient cause; but it is as hopeless an attempt to discover the cause of each, as to say why a chill or a poison affects one man differently from another” (1875, vol. 2, p. 282). The causes of variation were, to Darwin, so complex that even God—as omniscient as He is—might not foresee, or at least might not take note of the variations that would arise in a particular species in a particular environment (I will return to this point). Concerning the absence of any relationship between what variations arise and what variations would be useful, Darwin expressed himself by way of analogy: [Evolution by natural selection] absolutely depends on what we in our ignorance call spontaneous or accidental variation. Let an architect be compelled to build an edifice with uncut stones, fallen from a precipice. The shape of each fragment may be called accidental; yet the shape of each has been determined by the force of gravity, the nature of the rock, and the slope of the precipice,—events and circumstances, all of which depend on natural laws; but there is no relation between these laws and the purpose for which each fragment is used by the builder. In the same manner the variations of each creature are determined by fixed and immutable laws; but these bear no relation to the 3 living structure which is slowly built up through the power of selection, whether this be natural or artificial selection. (1875, vol. 2, p. 236) Many of Darwin’s readers could not see past the role he attributed to chance variation. Some accused him of explaining organic form in terms of chance alone. For example, Karl von Baer complained that the Origin reminded him of Gulliver’s Travels, especially Gulliver’s account of the Laputans and their amazing discovery machine. Laputan scholars had inscribed all the forms of all their words on the sides of dice, which were then placed in a mechanical grid. By cranking the handles of the device, they could spin each die independently, and generate different word combinations. They recorded the sensible word phrases and then cranked the handles again. Etc., etc. And this is how they planned to advance knowledge. It all sounded, to von Baer, vaguely Darwinian: chance begetting usefulness. For a long time the author of these reports was taken to be joking, because it is selfevident that nothing useful and significant could ever result from chance events. . . . [But] now we must acknowledge this philosopher as a deep thinker because he foresaw the present triumphs of science! (von Baer [1873] 1973, p. 419) Of course, in construing Darwinian evolution as a matter of chance variation alone, von Baer left out natural selection. In this regard, he did not make sufficient use of his own analogy. The Laputans relied on chance only to generate word combinations; then they carefully selected the phrases to be passed down to future generations. Similarly, in the case of Darwinian evolution, variations appear by chance, but it is not entirely a matter of chance which variations are perpetuated; it is a matter of natural selection. Nonetheless, even correcting for von Baer’s misconstrual, it is still the case that evolution by natural selection is a matter of chance. It may not be a matter of chance that the fitter 4 variations are selected and that the outcomes of evolution by natural selection are adaptive. But it is a matter of chance which variations arise, and in this sense also a matter of chance which variations will be selected and hence which adaptive outcomes will obtain. In principle, evolution by natural selection could result in very different outcomes, even starting with closely related and in all important respects identical species, inhabiting identical environments, depending on what variations happen to arise in each lineage, and in what order. Darwin set out to demonstrate the application of this principle in his book, On the Various Contrivances by which British and Foreign Orchids are Fertilised by Insects (1862). There he argued that, as diverse as orchid flowers are, they all serve basically the same function, namely to enlist flying insects in cross-pollination, and thus to avoid inbreeding. These otherwise very different “contrivances” for intercrossing had evolved, Darwin believed, under virtually the same environmental circumstances, e.g., the same range of available insects. Sometimes one part of the flower had been modified to entice insects in the vicinity, by mimicry or by scent; sometimes another part had been modified to do the same job. Once the insects had arrived, the pollen had to be attached. Some flowers were so constructed as to catapult pollen at the visiting insects; some catapult the insects against the pollen; some simply induce the visitors to travel past and brush-up against the pollen. Etc., etc. Thus cross-pollination is accomplished in very different ways, the different outcomes being due in large measure—Darwin argued—to natural selection acting on chance differences in variation among different lineages. Darwin might have explained the different outcomes mainly in terms of differences in environmental conditions—e.g., differences in the insects available for conscription as pollinators. Thus, he might have presumed that different lineages of orchids experience the same variations, in the same order, and then he might have argued that different variations are selected 5 in different lineages depending on which pollinators are in the vicinity. But he did not. His emphasis was on chance differences in variation among lineages. An example that particularly struck Darwin involved the position of the so-called “labellum” petal, which in most fully formed orchid flowers is the lowermost of the three petals. In that position, it often serves as a landing pad for pollinators. But interestingly, the labellum actually arrives at that position through a 180-degree twisting of the flower’s stem (usually including the ovarium) as the flower develops. Darwin reckoned that the position of the labellum in the ancestral orchid had been uppermost, presumably on the grounds that this is also the original position in development, and assuming more generally that the order of development reflects the order of ancestry. He understood the now-typical, lowermost position of the labellum to be an outcome of evolution by natural selection of the more twisted variations that had happened, by chance, to arise (1877, p. 284). But Darwin was especially intrigued by cases where the labellum had resumed its uppermost position, which in some cases had resulted from the selection of less and less twisted variations, and in other cases had come about as the result of selection for more and more twisted forms. Flowers of the latter sort twist a full 360 degrees to resume their starting position (see Fig. 1)! 6 Figure 1. The twisted stalk of Malaxis paludosa (from Darwin 1877, p. 130). See the text. As Darwin described the situation, . . . in many Orchids the ovarium (but sometimes the foot-stalk) becomes for a period twisted, causing the labellum to assume the position of a lower petal, so that insects can easily visit the flower; but . . . it might be advantageous to the plant that the labellum should resume its normal position on the upper side of the flower, as is actually the case with Malaxis paludosa, and some species of Catasetum, &c. This change, it is obvious, might be simply effected by the continued selection of varieties which had their ovaria less and less twisted, but if the plant [for whatever reason, by chance] only afforded variations with the ovarium more twisted, the same end could be attained by the selection of such variations, until the flower was turned completely on its axis. This seems to have 7 actually occurred with Malaxis paludosa, for the labellum has acquired its present upward position by the ovarium being twisted twice as much as is usual. (pp. 284-285) So it had apparently become advantageous for Malaxis and some species of Catasetum to have their labellae uppermost. But due to differences in the variations that chanced to occur in the two lineages, evolution by natural selection had resulted in very different means of serving this end: a 360-degree twist in the first case, and no twist in the second (see also Lennox 1993). Darwin invoked basically the same process—evolution by natural selection of chance differences in variation—to account for the endless diversity of floral morphologies among orchids. To quote him, summarizing the process leading to the diversity of orchid flowers: In my examination of Orchids, hardly any fact has struck me so much as the endless diversities of structure,—the prodigality of resources,—for gaining the very same end, namely, the fertilization of one flower by pollen from another plant. This fact is to a large extent intelligible on the principle of natural selection. As all the parts of a flower are co-ordinated, if slight variations in any one part were preserved from being beneficial to the plant, then the other parts would generally have to be modified in some corresponding manner. But these latter parts might not vary at all [who knows, it is a matter of chance], or they might not vary in a fitting manner [again, who knows], and these other variations, whatever their nature might be [ditto], which tended to bring all the parts into more harmonious action with one another, would be preserved by natural selection. (p. 284; my emphasis) Darwin came to believe that much of the diversity of life could be understood in this way: as different means to the same ends, reached by natural selection of whatever variations happen to arise. He hammered the point home in later editions of the Origin: 8 [I]t is a common rule throughout nature that the same end should be gained, even sometimes in the case of closely-related beings, by the most diversified means. (1872, p. 153) [T]his subject of the same end being gained by the most diversified means well deserves attention. (p. 154) How, it may be asked . . . can we understand the graduated scale of complexity and the multifarious means for gaining the same end? (p. 156) [T]he common rule throughout nature is infinite diversity of structure for gaining the same end. (p. 165) The sometimes amusing (e.g., in the case of Malaxis paludosa), and at other times disturbing contingency of evolutionary outcomes had, for Darwin, profound theological implications that he first discussed privately in correspondence with Asa Gray, and then publicly as well, all in response to Gray’s review of the Origin. Gray had intended to defend Darin against charges that the theory of evolution by natural selection was atheistic. Just because Darwin had remained silent concerning God’s role in evolution, Gray argued, that is no reason to think that Darwin denied God a role. As for what that role might be, Gray suggested that it would surely be guiding variation “along certain beneficial lines,” like a stream “along definite and useful lines of irrigation” (1860, pp. 413-414). In other words, God either directly causes particular variations to appear at particular times in order to be perpetuated by natural selection, or else indirectly provides for their occurrence. Indeed, Darwin had not intended to write God out of creation. God had, he initially believed, designed the sorts of laws—including laws of variation, heredity, population growth, etc.—that would lead to the production of well adapted species. But Darwin balked at Gray’s 9 suggestion that God also arranged the sequence of variations that were subsequently selected. To what end would God have done so? Presumably to guarantee the existence of humans (“in His own image,” Genesis 1:27). But then did He also directly or indirectly cause just the right variations to occur at just the right times to ensure the evolution of every other species that has ever existed? Did He, for example, arrange the sequence of variations that led to the existence of the ichneumonids, parasitic wasps that lay their eggs in living caterpillars so that the larvae can devour their hosts from the inside out? Given the complexity of variation, God would have had to go to an awful lot of trouble to produce such mean beasts. Better to think that He did not attend to that! Thus, Darwin counter-suggested that the living world is the result of “designed laws, with the details, whether good or bad, left to the working out of what we may call chance.” But if God did not set things up in such a way as to get ichneumonids, then why suppose that he arranged for humans? Getting out of one theological jam got Darwin into another, but he seems to have preferred the latter conundrum to the former. As he wrote to Gray, With respect to the theological view of the question; this is always painful to me.—I am bewildered.—I had no intention to write atheistically. But I own that I cannot see, as plainly as others do, & as I shd wish to do, evidence of design & beneficence on all sides of us. There seems to me too much misery in the world. I cannot persuade myself that a beneficent & omnipotent God would have designedly created the Ichneumonidae with the express intention of their feeding within the living bodies of caterpillars, or that a cat should play with mice. Not believing this, I see no necessity in the belief that the eye was expressly designed. On the other hand I cannot anyhow be contented to view this wonderful universe & especially the nature of man, & to conclude that everything is the result of brute force. I am inclined to look at everything as resulting 10 from designed laws, with the details, whether good or bad, left to the working out of what we may call chance. Not that this notion at all satisfies me. (Darwin to Gray, 22 May 1860, in Darwin 1993, p. 224) He later chose to elaborate the point—about God leaving “the details” to chance—in the closing pages of Variation in Plants and Animals under Domestication. He began with the analogy about the architect that constructs a building out of stones fallen from a nearby cliff. Does anyone really think that God, foreseeing the architect’s situation, would have somehow ordained the coming-into-being of stones in just the right sizes and shapes, and at just the right time and place, for the builder’s convenience? If so, then one might also suppose that God, foreseeing the particular predilections of future breeders, ordained the existence of variations that they would artificially select. But now we have a theological problem even worse than the willful provision for ichneumonids—namely, the existence of pit bulls! As Darwin continued his reductio ad absurdum line of questioning, “Did He [God] cause the frame and mental qualities of the dog to vary in order that a breed might be formed of indomitable ferocity, with jaws fitted to pin down the bull for man's brutal sport” (Darwin, 1875, vol. 2, pp. 430-431)? (Pit bulls bring down their much larger opponents by clamping their jaws over the bull’s snout and not letting go until the bull suffocates.) This is partly (largely) God’s handiwork?! The alternative is to suppose that God leaves variation to chance, which raises other theological problems, especially concerning humans, but at least avoids a malicious conception of the creator. But if we give up the principle in one case [i.e., if we deny that there is any divine ordering of variation in the domesticated world],—no shadow of reason can be assigned for the belief that variations, alike in nature and the result of the same general laws, 11 which have been the groundwork through natural selection of the formation of the most perfectly adapted animals in the world, man included, were intentionally and specially guided. (1875, vol 2, pp. 431-432) 3. Simpson on “Evolutionary Opportunism” However, Darwin’s position on chance variation and evolutionary contingency did not win the day. Gray’s position—directed variation with predetermined evolutionary outcomes—became more and more popular in the late nineteenth and early twentieth centuries, at least in a somewhat more secular form known as “directed evolution” or “orthogenesis.” This view was especially popular among paleontologists; as Vernon Kellogg reported in his state-ofthe-art, Darwinism To-Day (1907): Paleontologists believe, practically as a united body, that variation has followed fixed lines through the ages; that there has been no such unrestricted and utterly free play of variational vagary as the Darwinian natural selection theory presupposes. (p. 33) Proponents of this view believed that directed variation was the only way to explain (what they perceived to be) the morphological directionality of evolution, together with the (supposed) fact that while the morphological changes were ultimately adaptive, the requisite initial stages were not (e.g., Osborn 1925, p. 750; Osborn 1926). One example worked out in detail by Henry Fairfield Osborn concerned the horns of the titanotheres (an extinct family of an order whose only three extant familes are the horses/zebras/asses, rhinoceroses, and tapirs). In lineage after lineage of titanothere, the same morphological progression was preserved in the fossil record: from no horns, to minimal protuberances from the snout, to somewhat larger protuberances, to massive horns. The larger horns, Osborn reasoned, were increasingly useful for fighting, but the 12 initial bumps were quite useless. Such a directional evolutionary change could not, therefore, be the result of natural selection. It must be due instead to some direction in the process of variation itself, which guaranteed the same evolutionary outcome time after time (Osborn 1929). Osborn was generally silent on the process by which variation is directed; he felt that it might possibly be “beyond human solution” (1926, p, 341), which suggests that he may, like Gray, have attributed it to Providence (who else, besides humans, did he think might know the answer?). The most determined critic of orthogenesis, George Simpson, ridiculed it for its mysteriousness: Most theories of this school . . . involve an element of predestination, of a goal, a perfecting principle, whether as a vitalistic urge, or a metaphysical necessity, or a frankly theological explanation of evolution according to which it is under divine or otherwise spiritual guidance. (1944, p. 152). He acknowledged that prominent proponents of directed evolution, like Osborn, did not explicitly invoke supernatural causes (1964, p. 200), but to Simpson, the reasons for its popularity were evident from the frankly theological versions of it defended by others like Pierre Lecomte de Noüy, Teilhard de Chardin, and Edmund Sinnott. Simpson was not only concerned about the mysteriousness of orthogenesis, and the possibly religious motives of its proponents, however. His critiques of directed evolution were more pointed than that. For each purported case of directional evolution, he either denied the phenomenon, or offered an alternative explanation that did not rely on any unknown causal processes (Simpson 1944, pp. 149-179; 1949, pp. 130-159). For example, in the case of the titanotheres, he argued that even the most rudimentary of horns would have been better than nothing for fighting, so that the directionality in question might simply be the result of evolution 13 by natural selection of chance variations, leading to larger and larger horns in each lineage (1949, pp. 154-157). Mostly, though, he denied the supposed directionality of evolution. Closely related lineages do not follow the same trajectories to the same endpoints, he argued. “History does not repeat itself” in this way; it is not so predictable. His reasoning was similar to Darwin’s (e.g., in the case of orchids), though somewhat more elaborate. There were, according to Simpson, two main reasons that, together, explain why evolution is unpredictable. The first is that evolution is driven principally by natural selection of chance variations in changing environments, and is therefore—as Simpson put it—“opportunistic” (Simpson 1949, pp. 160-186). Consider that environmental changes and variational changes both present opportunities to adapt. Natural selection exploits these changes by perpetuating the most favorable of the existing or new variants in the existing or new environment. This is “opportunistic” in the sense that there is no plan or provision by which organisms automatically vary in ways that are adaptive (the OED definition of “opportunism” is “the practice or policy of exploiting circumstances or opportunities to gain immediate advantage, rather than following a predetermined plan”). The second reason why evolution is unpredictable is that, just as there are multiple means to any end, there are “multiple solutions” to any environmental “problem,” or multiple ways of exploiting any environmental opportunity. Thus, closely related and very similar (or even initially identical) lineages, inhabiting a similar or even an identical series of environments, may nonetheless generate different variations. And natural selection, opportunistically perpetuating whichever of those variations are most advantageous, may lead to different outcomes representing multiple solutions to the 14 environments faced. The outcomes are only as predictable as the variation on which natural selection acts. [T]he results of mutations do not tend to correspond at all closely with the needs or opportunities of the mutating organisms. It is a rather astonishing observation that the supply of this basic material for evolution seems to have no particular relationship to the demand. This accounts for much of the opportunism in evolution, and the nature of that opportunism in turn attests the random nature of mutation. Evolution works on the materials at hand: the groups of organisms as they exist at any given time and the mutations that happen to arise in them. The materials are the results of earlier adaptations plus random additions and the orienting factor in change is adaptation to new opportunities. If this view of evolution is correct, then we must expect to find similar opportunities exploited in different ways. The problems involved in performing certain functions should have multiple solutions. (1949, pp. 164-165) 15 Figure 2. Antelopes of the (then) Belgian Congo (from Simpson 1949, p. 166). See the text. Simpson illustrated the point with the existence of a wide variety of horns among antelopes in the Belgian Congo (see Fig. 2). Should these be seen as adaptations to different circumstances—e.g., different predators—within that region, or as alternative adaptations to the same or very similar circumstances? Don’t the doubly curved horns of the impala (11) and kob (15) serve basically the same function as the singly curved horns of the two reedbuck species (12 and 14)? Aren’t these are just two different means to the same end of having forward-pointing horns (much as a 360-degree twist and no twist at all are different means to the same end of 16 having the labellum petal on top)? In which case the difference is due to chance differences in the order in which variations appeared and were subsequently selected. Similarly, must we imagine that the antelopes with forward-pointing horns face a different range of threats than those with backward-pointing horns? Or are these both effective ways of dealing with the same or similar range of predators? In which case, again, the difference is due to chance differences in the order in which variations appeared and were subsequently selected. As Simpson summarized this case, and by extension many others, If evolution were really operating according to a fixed plan, surely these radical discrepancies would not arise. Nor would they arise if evolution were basically orthogenetic or if a rigid orientation were everywhere the rule. It looks as if there had been an orienting factor, indeed—all these animals do have horns that serve them well enough even if not perfectly—but as if what it had to orient was not uniform or completely responsive. As horns developed in the different lines different sorts of mutations affected them. As long as the mutations increased the development of a functional horn, however various in other respects, they served the adaptive end and were retained and promoted in the evolution of each line. The mutations that would have been mechanically the best, that an engineer would have chosen, simply did not happen to occur in all, or perhaps in any, of these lines. There are two aspects of opportunism: to seize such diverse opportunities as occur, and when a single opportunity or need occurs, to meet it with what is available, even if this is not the best possible. The antelopes, and many other groups of animals, well illustrate this second sort of evolutionary opportunism. (1949, pp. 167-168) 17 So it is not the case that evolution is directed toward particular adaptive outcomes by some preordained or otherwise built-in order of variation. But Simpson went further with this line of reasoning, critiquing not only directed evolution, but also every attempt to generalize about evolutionary trajectories, other than to say that they are adaptive. The opportunism of natural selection, together with the possibility of multiple solutions to any environmental problem, render it highly unlikely that evolution will ever repeat itself. So we should not expect to find any generalizations, and definitely not any “laws” about the course of evolution, again with the exception that evolution is generally adaptive ([1950] 1964, pp. 176-189). There is just one other exception to the rule that there are no such rules: a rule that, like the general adaptiveness of evolutionary outcomes, makes sense in light of the opportunism of natural selection and the possibility of multiple solutions. This is the rule that “evolution is irreversible.” Simpson understood this to be just a special case of the principle that evolutionary history does not repeat itself. In other words, it is just as unlikely that evolution would exactly reverse itself given the same series of environmental circumstances, except in reverse, as that evolution would exactly repeat itself under the same series of environments (Simpson [1950], 1964, pp. 186-187). Simpson’s concerns about directed evolution and “laws” of evolution, however, were not entirely biological. It was fairly common at the time to argue not only that evolution is directional, but that the direction must be good. And not only should we not interfere to divert evolution from its natural course, but we should even intervene to hasten it along his proper trajectory. This line of reasoning was often used to argue for the inevitability and desirability of a highly collectivist organization of society. Simpson (1941) singled-out the physiologist Ralph Gerard, who had argued that evolution proceeded in the direction of greater and greater “integration” on all levels, from the organs of an organism, to the organisms that compose a 18 species. This involved decreased totipotency and self-sufficiency, and increased specialization and reliance on the other parts of the whole. And this was all for the good. To be sure, it involved a loss of freedom. But Gerard argued that his fellow Americans had perhaps oversold the importance of autonomy and freedom: Men born into a caste, apprenticed into a guild, trained in a craft are proud to do their jobs well and do not feel forlorn at the impossibility of becoming king. The relative fluidity of our own early culture and the “freedom” that American youth has to “rise to the top” has certainly produced little contentment—European visitors and local psychiatrists are impressed with the restless striving which is almost a national psychosis. No, it is possible for men to be part of a highly integrated society and yet feel, as individuals, more free, actually to have more avenues open for satisfying self-expression, than when they are epiorganisms of their own, like single-celled organisms. Which of us would exchange our present state for the privilege of roaming the woods naked and unarmed, without language or fire? (Gerard, 1940, pp. 411-412) Simpson could also have referred to Conrad Waddington, who had argued that one could and should look to see if there is an overall direction to evolution, and then do what one can to promote that outcome. And not surprisingly for the time, the alternatives that most interested him were whether evolution tends toward greater autonomy among species members, or toward a more highly organized form of interaction. Having discovered which way evolution (including human evolution) is headed, we should then act on that knowledge. Once we have decided from evolutionary data whether the continuation of man's progress demands a high degree of respect for the individual rather than his thorough subjection to a group organization, for instance, we can work out the implications of that directive in 19 present political terms. Only when we have found our Ten Commandments in general evolution, can we discover our Deuteronomy in political analysis. (Waddington 1942, p. 125) That is as openly as Waddington ever posed the question. Elsewhere, he expressed his view that evolution leads to greater control over environmental circumstances, which in turn favors more highly integrated social relations, and in humans to more collectivist political economies. He concluded that “totalitarianism . . . does seem to be inevitable,” and that “the application of conscious control to the economic functioning of society is actually a step along the path of man's evolutionary advance” (1948, pp. 22, 171). He also wrote in carefully chosen Marxist terms of the importance of “assisting” the “birth pangs” of the new economic order. In explaining his concerns about this use of evolutionary thinking, Simpson switched from talking about directed evolution and laws of evolution, to “evolutionary fatalism.” Biological justification for the totalitarian development of society has also been sought in the doctrines of evolutionary fatalism. Regardless of such labels as “right,” “wrong,” “good,” or “bad,” it is argued, this is the inevitable future. Mankind is going this way just as horsekind was going toward Equus throughout the Tertiary. Opposition is as futile and foolish as if the little Eohippus had said, “I am going to be a dinosaur,” instead of, “—a horse.” Even aside from the fact that this is another false use of analogy, it has been shown that a fatalistic view of evolution has little scientific support. (Simpson 1941, p. 20) 20 Simpson complained that biology, in the form of evolutionary fatalism, had become more dangerous than physics—with all its firepower—because of the way it had been “used to provide the more insidious and still more menacing moral implementation of totalitarianism.” If this use is wrong, scientifically, and if free biologists support it or even tacitly permit it, then they will deserve an accusation stronger than any that can be brought against physical science, and they will be contributing to their own destruction. (1941, p. 15) Simpson proceeded to apply his own expertise in issues of macroevolution and long-term evolutionary trends to undermine the view that evolution is directional and fatalistic. Proponents of such a deterministic conception of evolution, he argued, overlooked the extent to which “the products of evolution are the results of a sequence of accidents” (1941, p. 8). Evolution may sometimes lead to the extreme integration of individual organisms, so that they become more like parts of a whole, as for example in the case of colonial organisms like corals. But, he argued, this is rare. In considering Simpson’s worries, it is also worth recalling Karl Popper’s initial concerns about “Darwinism,” which were based mainly on historically deterministic (mis)renderings of Darwinian evolutionary thought, and connections drawn to Marxist and fascist theories of social and racial history. When Popper’s first extended discussion of Darwinism appeared in book form, as Poverty of Historicism (1957), it was dedicated to “the countless men and women of all creeds or nations or races who fell victim to the fascist and communist belief in Inexorable Laws of Historical Destiny.” Simpson argued that, in light of the contingency of evolutionary outcomes, it was up to us to take control of our own fates: 21 Man has risen, not fallen. He can choose to develop his capacities as the highest animal and to try to rise still farther, or he can choose otherwise. The choice is his responsibility, and his alone. There is no automatism that will carry him upward without choice or effort and there is no trend solely in the right direction. Evolution has no purpose; man must supply this for himself. (ME, 310) 4. Gould on “Replaying Life’s Tape” Gould read Simpson’s Meaning of Evolution at the age of ten, and later in life could not recall what impression it had on him as a boy, though he had in the interim come to regard it as “the finest introduction to deeper issues of life's meaning, insofar as biology can approach these questions” (Gould 1985, p. 230). He was also deeply influenced by Simpson’s collection of essays, This View of Life, the title of which was derived from the last passage of the Origin—where Darwin reflected that “There is grandeur in this view of life . . .”—and which in turn served as the title of Gould’s own long-running column in Natural History. Either book could have started Gould thinking about what would happen if evolutionary history could be rerun. In Meaning, Simpson elaborated his views of evolutionary opportunism. In This View, Simpson had defended the indeterminism of evolution by natural selection, and had posed the question, what would become of the Cambrian trilobites if they could be put back in place, and the environmental conditions of the Cambrian and subsequent Ordovician periods could be repeated (Simpson, [1950] 1964, p. 187)? But alas, evolution is not repeatable. Gould would rewind the videotape of life back to the Cambrian, and then replay it for his generation. It got people thinking, in a way. In one of the most popular episodes of The Simpsons (no. 606, 1994), Homer tried to fix his toaster and accidentally turned it into a time machine. It 22 transported him back to the age of the dinosaurs, over and over, with a different evolutionary outcome each time. Homer rejected each world in turn (the first one he rejected was, interestingly, a totalitarian world in which Ned Flanders was the dictator and molded everyone into his personality—very scary for Homer; equally scary was a world in which everything was better than Homer could imagine, except that no one had ever heard of donuts), before finally settling on a world in which everyone had reptilian tongues and could get their food from plate to mouth without the need of a fork. (Gould later appeared as himself in a segment—no. 908, 1997—in which he was consulted about a fossilized, winged protohuman—allegedly an angel; you have to see it!). Gould never, to my knowledge, discussed the replay scenario without also discussing the Darwin-Gray correspondence and Darwin’s views on ichneumonids and “the details left to chance” (see, e.g., 1989, pp. 290-291; 1990, p. 21; 1995; 1999, pp. 173-207). He liked to confront readers who might find the contingency of evolutionary outcomes—especially the contingency of human existence—“depressing.” He always countered that he found the thought “exhilarating,” for reasons similar to those that Simpson raised: Homo sapiens, I fear, is a “thing so small” in a vast universe, a wildly improbable evolutionary event well within the realm of contingency. Make of such a conclusion what you will. Some will find the prospect depressing; I have always regarded it as exhilarating, and a source of both freedom and consequent moral responsibility. (1989, p. 291) And elaborating on the last point: We are the offspring of history, and must establish our own paths in this most diverse and interesting of conceivable universes—one indifferent to our suffering, and therefore 23 offering us maximal freedom to thrive, or to fail, in our own chosen way. (1989, p. 323; see also Gould 1999, pp. 191-207) Of course, Gould did not promote evolutionary contingency for the sole reason that he found it morally satisfying, nor defend it solely by arguing that it was not as morally repugnant as one might think. He felt that it needed to be promoted/defended on other grounds. While Simpson had championed evolutionary contingency in response to proponents of directed variation, Gould aimed his case at proponents of the “all importance” of natural selection (see also Sterelny and Griffiths 1999, pp. 296-302). Such “pan-selectionists” assume, at least as a working hypothesis, that the traits that predominate in a population are optimal for the environment inhabited by the population, and hence if different traits prevail in different populations, this must reflect environmental differences. I am not aware of any pan-selectionist arguments explicitly to the effect that chance variation is irrelevant to evolutionary outcomes. But it is as if chance variation can safely be ignored—the optimal traits will always arise. Evolution is predictable on the basis of selective considerations alone. It was during the late forties, fifties, and sixties that natural selection came to be regarded as all-important. Gould referred to this development as the “hardening of the evolutionary synthesis,” meaning that earlier evolutionary biologists had been more pluralistic with regard to the agents and factors of evolution (e.g., Gould 1983). The increasing importance attributed to natural selection had to do with the reinterpretation of some phenomena that had previously been thought to be inexplicable in selectionist terms. It is important to note that these phenomena had been accounted for in terms of “random drift” in particular, rather than natural selection. Proponents of the importance of random drift had argued that many or most traits do not confer any selective advantage or disadvantage, and hence do not increase or decrease in frequency as a 24 result of selection for or against them. Rather, their frequencies fluctuate by “chance,” not in the sense of chance origin of variation but rather in the sense of chance perpetuation of variation—e.g., when a parent has two different genes for a trait, but just happens to pass down the same one to all of its offspring, or one organism is struck by lightning while another is spared and lives to reproduce, etc. This position was quite popular in the nineteen thirties and forties. It seemed to be the only way to account for many otherwise senseless differences—e.g., that different blood groups prevail in different populations of humans, different shell colors and patterns prevail in different populations of snails, and different chromosomal inversions prevail in different populations of fruitflies. There was no conceivable selective basis for such differences. But as time went on, the traits that had been singled-out as paradigm cases of selective neutrality—whose frequencies were supposedly entirely a matter of chance—were oneafter-another shown to confer selective advantages or disadvantages in particular environments, and to have frequencies that reflected the degree of selection for or against them in those environments. It was especially in the context of this reversal of fortune that natural selection came to be regarded, by Gould’s elders and many of his contemporaries, as the paramount, and even all-important agent of evolution (Beatty 1984). Somehow in the course of these debates, random drift came to be seen as the, rather than a source of the unpredictability of evolutionary outcomes, and evolution by natural selection came to be seen, in contrast, as eminently predictable. But to identify evolution by natural selection with evolutionary predictability was to miss the point that Darwin and Simpson had made earlier. As long as natural selection acts on chance variation, and as long as there are multiple possible solutions to any adaptive problem, then the outcomes of evolution by natural selection are going to depend on which variations happen to arise. Gould and Richard Lewontin 25 felt the need to remind enthusiasts of natural selection that when different traits prevail in different populations or species, this need not reflect differences in their environments. Evolution by natural selection can lead to different outcomes even from the same starting point and in the same environments: When “multiple adaptive peaks” are occupied, we usually have no basis for asserting that one solution is better than another. The solution followed in any spot is a result of history; the first steps went in one direction, though others would have led to adequate prosperity as well. Every naturalist has his favourite illustration. In the West Indian land snail Cerion, for example, populations living on rocky and windy coasts almost always develop white, thick and relatively squat shells for conventional adaptive reasons. We can identify at least two different developmental pathways to whiteness from the mottling of early whorls in all Cerion, two paths to thickened shells and three styles of allometry leading to squat shells. All 12 combinations can be identified in Bahamian populations, but would it be fruitful to ask why—in the sense of optimal design rather than historical contingency—Cerion from eastern Long Island evolved one solution, and Cerion from Acklins Island another? (Gould and Lewontin 1979, p. 593) As we have seen, the unpredictability of evolutionary outcomes has been stressed from time to time ever since Darwin. But Gould’s particular version of the thesis struck a chord and was followed-up by a flurry articles purporting to demonstrate or refute his position. These included macroevolutionary “natural experiments;” laboratory-controlled, microevolutionary experiments; and artificial-life simulation-experiments (see Beatty 2006, n. 4, p. 337). The results have been mixed. For example, Jonathan Losos et al. argued that the same four ecomorphs (adaptive types) of lizard had evolved independently at least four times on islands of 26 the Caribbean, a seeming devastating blow to Gould’s view that “any replay would lead evolution down a pathway radically different from the road actually taken.” While Michael Travisano et al. (1995a) and Travisano et al. (1995b) cultivated twelve genetically identical clones of E. coli in identical environments for two thousand generations, and found that the twelve evolutionary outcomes were similar in terms of their fitness, but were quite different genetically. Gould intended for the replay metaphor to cover numerous sources of evolutionary contingency, not just chance variation. But his initial articulation of the metaphor coincided with and reinforced a burst of interest in the evolutionary significance of “mutational order,” as opposed to random drift. For example, Mani and Clarke (1990) emphasized that random drift was not the only source of evolutionary stochasticity: It is common to consider only a single stochastic process in evolution, random genetic drift, yet there are at least three others that have often been neglected because of a failure to recognize their distinctive properties, and a tendency to confuse them with each other and with drift itself. The three processes are: 1. Random fluctuations in selective values. 2. The random consequences of gene conversion and other mechanisms ‘homogenizing’ the DNA. 3. The random order of mutations. (1990, p. 29) It was the latter that they focused on. In order to distinguish the effects of mutational order from those of random drift, they simulated evolution in two groups of identical populations, inhabiting identical environments. In the first group, mutations were introduced in 27 the same order—i.e., the same mutations arose in each population at the same time. Any divergence between these populations must be due to random drift alone. In the second group of populations, mutations were introduced randomly. In both groups, there was selection, and the same selection regimes held. The investigators measured the evolutionary divergence within each group in terms of genetic distance. Under every scenario that they studied, the mean genetic distance of the random-mutation group was considerably greater than that of the orderedmutation group. And not too surprisingly, the difference was greater, the larger the population size (see Fig. 3). This is partly because the larger the population size, the less important are the sorts of fortuities associated with random drift. Figure 3. Mean genetic distance (y-axis) vs. 10-2 x number of generations (x-axis) for populations in which mutations are ordered (continuous line) vs. random (dotted line); population sizes (a) 5000, (b) 2000, (c) 500, (d) 100 (from Mani and Clarke 1990, p. 32) 28 Others pointed out that large population size in combination with high mutation rates would increase the chance of coincident mutations, and hence reduce the impact of mutational order. In introducing their discussion of this issue, Johnson et al. (1995; see also Wahl and Krakauer 2000) invoked Gould’s replay metaphor, and the body of literature that had recently addressed the role of chance variation in the unpredictability of evolutionary outcomes: How reproducible is evolution, and in particular adaptive evolution? Gould (1989) proposed a thought experiment of ‘replaying life’s tape’ to address this question. . . . Rigorous experiments on the predictability of evolution using organisms with short generations, such as fruit flies and bacteria, demonstrate that: (i) the random origin of mutations, either alone or in concert with genetic drift, can cause unpredictability; and (ii) substantial divergence of (initially identical) replicate populations in identical environments can occur even for traits that are subject to strong selection. (p. 125) The importance of mutational order vs. random drift was also demonstrated in a continuation of the study previously mentioned, in which twelve identical (cloned) populations of E. coli were grown in identical media for two thousand generations, and had similar fitnesses at the end. Lenski and Travisano (1994) extended the experiment from two thousand to ten thousand generations. Somewhat surprisingly, the fitness values of the twelve populations were still only similar after ten thousand generations. Why after so long in exactly the same medium had not all of the populations evolved the same ability to compete for resources within that environment? Lenski and Travisano argued that the residual differences in fitness reflected underlying genetic differences, which had in turn resulted largely from the different order of mutations occurring in the twelve populations: 29 Evolutionary biologists usually regard diversification as being caused by either (i) adaptation to different environments, which often produces conspicuous phenotypic variation, or (ii) random genetic drift, which is usually seen in molecular genetic variation. Yet our experiments demonstrate diversification, in identical environments and with very large populations, of no less selected a trait than fitness itself. Someone confronted with the variability among our derived populations (and unaware of the experimental design) might attribute this diversity to environmental heterogeneity or phylogenetic constraints , but any such “just-so story” would clearly be misguided in this case. Instead, or experiment demonstrates the crucial role of chance events (historical accidents) in adaptive evolution. (Lenski and Travisano 1994, p. 6813) Demonstrating the actual vs. the theoretical importance of evolutionary contingency (due to whatever source of unpredictability) is difficult for a number of reasons discussed by Kim Sterelny and Paul Griffiths (1999, pp. 301-302) and by Sterelny (2005). Take for instance “convergence,” which is generally regarded as the strongest form of evidence against the thesis that evolution is intrinsically contingent (and which is often cited in critiques of Gould—e.g., by Dennett 1995, pp. 305-308; Conway Morris 2003). Convergence occurs when distantly related lineages face the same or similar adaptive problems, and reach the same or similar solutions. However, inasmuch as lineages can be more or less related, convergence is a matter of degree, and hence there is no straightforward way to tally cases of convergence. An even more important problem, I think, arises in spades in the case of the twelve cloned populations inhabiting identical environments and evolving toward similar overall fitnesses by genetically diverse means, has to do with what Sterelny (2005) calls the “courseness” of description of the trait being studied. What bears more on Gould’s thesis? The 30 fact that the twelve populations were, after however many generations, similar with regard to a course-grained trait like fitness? Or that they had diverged with regard to the more fine-grained, underlying genetics? Is the case of the twelve cloned populations a victory for evolutionary contingency, or evolutionary predictability? Again, it is very difficult to count evidence for and against. And then there is the usual problem involved in adjudicating relative significance controversies in biology—namely, extrapolating to overall frequencies when the number of cases studied is so small. What does one study, or one hundred, or even one thousand tell us about the overall importance of evolutionary contingency? By way of conclusion, I would like to return briefly to the moral message that Gould drew from the contingency of evolutionary outcomes: that it is “a source of both freedom and consequent moral responsibility”—that it offers us “maximal freedom to thrive, or to fail, in our own chosen way.” Given that Gould also argued strongly against attempts to derive ethical norms from nature it may seem, as Daniel Dennett has suggested, that Gould was “trying to deny to others what he allow[ed] himself” (1995, p. 311). It does look that way. But there is a different way to make sense of Gould’s point, and in order to see that, it is important first of all to take his intended audiences into account. Dennett remarks that “Gould . . . seems to think that the view he is combating so vigorously is deterministic and ahistorical, in conflict with this creed of freedom” (p. 311). Gould was indeed responding to those who believe that the fixed direction of evolution provides moral direction. As we have seen, that was not a made-up audience (and there is much more that could be said about these sorts of believers; see Ruse 1996). The other equally real (and overlapping) audience that he was addressing consisted of those who believe that God built the 31 world in such a way as to guarantee our existence, along with our rational faculties and moral sense. According to Gould, evolutionary contingency undermines both positions. There is no direction to evolution, hence no moral guidance to be had there. 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