Darwin's Theory of Natural Selection: Beyond Simple Foundations, Lecture notes of Biology

Download Darwin's Theory of Natural Selection: Beyond Simple Foundations and more Lecture notes Biology in PDF only on Docsity! 1 Cambridge Companion to the Origin of Species, eds. R. Richards and M. Ruse   Darwin’s Theory of Natural Selection and  Its Moral Purpose  Robert J. Richards Thomas Henry Huxley recalled that after he had read Darwin’s Origin of Species, he had exclaimed to himself: “How extremely stupid not to have thought of that!” (Huxley,1900, 1: 183). It is a famous but puzzling remark. In his contribution to Francis Darwin’s Life and Letters of Charles Darwin, Huxley rehearsed the history of his engagement with the idea of transmutation of species. He mentioned the views of Robert Grant, an advocate of Lamarck, and Robert Chambers, who anonymously published Vestiges of the Natural History of Creation (1844), which advanced a crude idea of transmutation. He also recounted his rejection of Agassiz’s belief that species were progressively replaced by the divine hand. He neglected altogether his friend Herbert Spencer’s early Lamarckian ideas about species development, which were also part of the long history of his encounters with the theory of descent. None of these sources moved him to adopt any version of the transmutation hypothesis. Huxley was clear about what finally led him to abandon his long-standing belief in species stability: 2 The facts of variability, of the struggle for existence, of adaptation to conditions, were notorious enough; but none of us had suspected that the road to the heart of the species problem lay through them, until Darwin and Wallace dispelled the darkness, and the beacon-fire of the “Origin” guided the benighted (Huxley, 1900, 1: 179-83). The elements that Huxley indicated—variability, struggle for existence, adaptation— form core features of Darwin’s conception of natural selection. Thus what Huxley admonished himself for not immediately comprehending was not the fact, as it might be called, of species change but the cause of that change. Huxley’s exclamation suggests—and it has usually been interpreted to affirm—that the idea of natural selection was really quite simple and that when the few elements composing it were held before the mind’s eye, the principle and its significance would flash out. The elements, it is supposed, fall together in this way: species members vary in their heritable traits from each other; more individuals are produced than the resources of the environment can sustain; those that by chance have traits that better fit them than others of their kind to circumstances will more likely survive to pass on those traits to offspring; consequently, the structural character of the species will continue to alter over generations until individuals appear specifically different from their ancestors. Yet, if the idea of natural selection were as simple and fundamental as Huxley suggested and as countless scholars have maintained, why did it take so long for the theory to be published after Darwin supposedly discovered it? And why did it then require a very long book to make its truth obvious? In this essay, I will try to answer 5 “Each species changes. Does it progress. Man gains ideas. The simplest cannot help.—becoming more complicated; & if we look to first origin there must be progress” (Notebooks, 175). Being the conservative thinker that he was, Darwin retained in the Origin the idea that some species, under special conditions, might alter through direct environmental impact as well as the conviction that modifications would be progressive. Darwin seems to have soon recognized that the direct influence of surroundings on an organism could not account for its more complex adaptations, and so he began constructing another causal device. He had been stimulated by an essay of Frédéric Cuvier, which suggested that animals might acquire heritable traits through exercise in response to particular circumstances. He rather quickly concluded that “all structures either direct effect of habit, or hereditary <& combined> effect of habit” (Notebooks, 259).1 Darwin, thus, assumed that new habits, if practiced by the population over long periods of time, would turn into instincts; and these latter would eventually modify anatomical structures, thus altering the species. Use-inheritance was, of course, a principal mode of species transformation for Lamarck. In developing his own theory of use-inheritance, Darwin carefully distinguished his ideas from those of his discredited predecessor—or at least he convinced himself that their ideas were quite different. He attempted to distance himself from the French naturalist by proposing that habits introduced into a population would first gradually become instinctual before they altered anatomy. And instincts—innate patterns of behavior—would be expressed automatically, without the intervention of conscious will- power, the presumptive Lamarckian mode (Notebooks, 292). By early summer of 1838, 6 Darwin thus had two devices by which to explain descent of species with modification: the direct effects of the environment and his habit-instinct device. Elements of the Theory of Natural Selection At the end of September 1838, Darwin paged through Thomas Malthus’s Essay on the Principle of Population. As he later recalled in his Autobiography, this happy event changed everything for his developing conceptions: I soon perceived that selection was the keystone of man’s success in making useful races of animals and plants. But how selection could be applied to organisms living in a state of nature remained for some time a mystery to me. In October 1838, that is, fifteen months after I had begun my systematic enquiry, I happened to read for amusement Malthus on Population, and being well prepared to appreciate the struggle for existence which everywhere goes on from long-continued observation of the habits of animals and plants, it at once struck me that under these circumstances favourable variations would tend to be preserved, and unfavourable ones to be destroyed. The result of this would be the formation of new species. Here, then, I had at last got a theory by which to work (Autobiography, 119-20). 7 Darwin’s description provides the classic account of his discovery, and it does capture a moment of that discovery, though not the complete character or full scope of his mature conception. The account in the Autobiography needs to be placed against the notebooks, essays, and various editions of the Origin and the Descent of Man. These comparisons will reveal many moments of discovery, and a gradual development of his theory of natural selection from 1838 through the next four decades. In the Autobiography, Darwin mentioned two considerations that had readied him to detect in Malthus a new possibility for the explanation of species development: the power of artificial selection and the role of struggle. Lamarck had suggested domestic breeding as the model for what occurred in nature. Undeterred by Lyell’s objection that domestic animals and plants were specially created for man (Lyell, 1830-33, 2: 41), Darwin began reading in breeders’ manuals, such as those by John Sebright (1809) and John Wilkinson (1820). This literature brought him to understand the power of domestic “selection” (Sebright’s term) but he was initially puzzled, as his Autobiography suggests, about what might play the role of the natural selector or “picker.” In mid summer of 1838, he observed: The Varieties of the domesticated animals must be most complicated, because they are partly local & then the local ones are taken to fresh country & breed confined, to certain best individuals.—scarcely any breed but what some individuals are picked out.—in a really natural breed, not one is picked out . . . (Notebooks, 337). 10 Darwin found in those passages from Malthus a propulsive force that had two effects: it would cause the death of the vast number in the population by reason of the better adapted pushing out the weaker, and thus it would sort out, or transform, the population. On September 28, 1838, Darwin phrased it this way in his Notebook D: Even the energetic language of <Malthus> <<Decandoelle>> does not convey the warring of the species as inference from Malthus. . . population in increase at geometrical ratio in FAR SHORTER time than 25 years—yet until the one sentence of Malthus no one clearly perceived the great check amongst men. . . One may say there is a force like a hundred thousand wedges trying force <into> every kind of adapted structure into the gaps <of> in the oeconomy of Nature, or rather forming gaps by thrusting out weaker ones. <<The final cause of all this wedging, must be to sort out proper structure & adapt it to change (Notebooks, 375-76). All the “wedging” caused by population pressure would have the effect, according to Darwin, of filtering out all but the most fit organisms and thus adapting them (actually, leaving them pre-adapted) to their circumstances. Though natural selection is the linchpin of Darwin’s theory of evolution, his notebooks indicate only the slow emergence of its ramifying features. He reflected on his burgeoning notions through the first week of October 1838, but then turned to other matters. Through the next few months, here and there, the implications became more prominent in his thought. In early December, for instance, he explicitly drew for the first time the analogy between natural selection and domestic selection: “It is a beautiful 11 part of my theory, that <<domesticated>> races . . . are made by percisely [sic] same means as species” (Notebooks, 416). But the most interesting reflections, which belie the standard assumptions about Darwin’s theory, were directed to the final cause or purpose of evolution. This teleological framework would help organize several other elements constituting his developing notion. The Purpose of Progressive Evolution: Human Beings and Morality The great peroration at the very end of the Origin of Species asserts a long- standing and permanent conviction of Darwin, namely that the “object,” or purpose, of the “war of nature” is “the production of the higher animals” (Origin, 490). And the unspoken, but clearly intended, higher animals are human beings with their moral sentiments. Darwin imbedded his developing theory of natural selection in a decidedly progessivist and teleological framework, a framework quite obvious when one examines the initial construction of his theory. At the end of October 1838, he focused on the newly formulated device: My theory gives great final cause <<I do not wish to say only cause, but one great final cause . . .>> of sexes. . . for otherwise there would be as many species, as individuals, &. . . few only social . . . hence not social instincts, which as I hope to show is <<probably>> the foundation of all that is most beautiful in the moral sentiments of the animated beings (Notebooks, 409). 12 In this intricate cascade of ideas, Darwin traced a path from sexual generation to its consequences: the establishment of stable species, then the appearance of social species, and finally the ultimate purpose of the process, the production of human beings with their moral sentiments. This trajectory needs further explication. When Darwin opened his first transmutation notebook in spring 1837, he began with his grandfather’s reflections on the special value of sexual generation over a-sexual kinds of reproduction. The grandson supposed that sexually produced offspring would, during gestation, recapitulate the forms of ancestor species. As he initially put the principle of recapitulation: “The ordinary kind [i.e., sexual reproduction], which is a longer process, the new individual passing through several stages (typical, <of the> or shortened repetition of what the original molecule has done)” (Notebooks, 170). Darwin retained the principle of embryological recapitulation right through the several editions of the Origin (Nyhart, this volume). Recapitulation produced an individual that gathered in itself all the progressive adaptations of its ancestors. But the key to progressive adaptation was the variability that came with sexual reproduction (Notebooks, 171). In spring of 1837, he still did not understand exactly how variability might function in adaptation; he yet perceived that variable offspring could adjust to a changing environment in ways that clonally reproducing plants and animals could not. Moreover, in variable offspring accidental injuries would not accumulate as they would in continuously reproducing a-sexual organisms. Hence stable species would result from sexual generation. For “without sexual crossing, there would be endless changes . . . & hence there could not be improvement . <<& hence not <<be>> higher animals” 15 those “necessary and sufficient” axioms advanced by contemporary evolutionary theorists: variation, heritability, and differential survival (Lewontin, 1978). Such analytic reduction does appear to render evolution by natural selection a quite simple concept, as Huxley supposed. However, these bare principles do not identify a causal force that might scrutinize the traits of organisms to pick out just those that could provide an advantage and thus be preserved. Darwin would shortly construct that force as both a moral and an intelligent agent, and the structure of that conception would sink deeply into the language of the Origin. Natural Selection as an Intelligent and Moral Force In 1842, Darwin roughly sketched out the outlines of his theory, and two years later he enlarged the essay to compose a more complete and systematic version. In the first section of both essays, as in the first chapter of the Origin, Darwin discussed artificial selection. He suggested that variations in traits of plants and animals occurred as the result of the effects of the environment in two different ways: directly on features of the malleable body of the young progeny; but also indirectly by the environment’s affecting the sexual organs of the parents (Foundations, 1-2). Typically a breeder would examine variations in plant or animal offspring; and if any captured his fancy, he would breed only from those suitable varieties and prevent back-crosses to the general stock. Back- crosses, of course, would damp out any advantages the selected organisms might possess. In the next section of the essays, Darwin inquired whether variation and selection could be found in nature. Variations in the wild, he thought, would occur much as they 16 did in domestic stocks. But the crucial, two-pronged issue was: “is there any means of selecting those offspring which vary in the same manner, crossing them and keeping their offspring separate and thus producing selected races” (Foundations, 5)? The first of these problems might be called the problem of selection, the second that of swamping out. In beginning to deal with these difficulties (and more to come), Darwin proposed to himself a certain model against which he would construct his device of natural selection. This model would control his language and the concepts deployed in the Origin. In the 1844 Essay, he described the model this way: Let us now suppose a Being with penetration sufficient to perceive the differences in the outer and innermost organization quite imperceptible to man, and with forethought extending over future centuries to watch with unerring care and select for any object the offspring of an organism produced under the foregoing circumstances; I can see no conceivable reason why he could not form a new race (or several were he to separate the stock of the original organism and work on several islands) adapted to new ends. As we assume his discrimination, and his forethought, and his steadiness of object, to be incomparably greater than those qualities in man, so we may suppose the beauty and complications of the adaptations of the new races and their differences from the original stock to be greater than in the domestic races produced by man’s agency (Foundations, 85). The model Darwin had chosen to explain to himself the process of selection in nature was that of a powerfully intelligent being, one that had foresight and selected animals to 17 produce beautiful and intricate structures. This prescient being made choices that were “infinitely wise compared to those of man” (Foundations, 21). As a wise breeder, this being would prevent back-crosses of his flocks. Nature, the analogue of this being, was thus conceived not as a machine but as a supremely intelligent force. In the succeeding sections of both essays, Darwin began specifying the analogs for the model, that is, those features of nature that operated in a fashion comparable to the imaginary being. He stipulated, for instance, that variations in nature would be very slight and intermittent due to the actions of a slowly changing environment. But, looking to his model, he supposed that nature would compensate for very gradually appearing variations by acting in a way “far more rigid and scrutinizing” (Foundations, 9). He then brought to bear the Malthusian idea of geometrical increase of offspring, and the consequent struggle for existence that would cull all but those having the most beneficial traits. Many difficulties in the theory of natural selection were yet unsolved in the essays. Darwin had not really dealt with the problem of swamping. Nor had he succeeded in working out how nature might select social, or altruistic, instincts, the ultimate goal of evolution. And as he considered the operations of natural selection, it seemed improbable that it could produce organs of great perfection, such as the vertebrate eye. His strategy for solving this last problem, however, did seem ready to hand—namely, to find a graduation of structures in various different species that might illustrate how organs like the eye might have evolved over long periods of time. Moreover, if natural 20 exclude a Lamarckian explanation of the wonderful instincts of the social insects—since no acquired habits could be passed to offspring—and simultaneously he could overcome a potentially fatal objection to his theory (see Lustig, this volume). But second, this theory of family selection (or community selection as he came to call it) would enable him to solve the like problem in human evolution, namely the origin of the altruistic instincts. In the Descent of Man, Darwin would mobilize the model of the social insects precisely to construct a theory of human moral behavior that contained a core of pure, unselfish altruism—that is, acts that benefited others at cost to self, something that could not occur under individual selection (Richards, 1987, 206-19). Hence, the final goal of evolution, as he originally conceived its telic purpose, could be realized: the production of the higher animals having moral sentiments. Yet not only did Darwin construe natural selection as producing moral creatures, he conceived of natural selection itself as a moral and intelligent agent. The model of an intelligent and moral selector, which Darwin cultivated in the earlier essays, makes an appearance in the Big Species Book. In the chapter “On Natural Selection,” he contrasted man’s selection with nature’s. The human breeder did not allow “each being to struggle for life”; he rather protected animals “from all enemies.” Further, man judged animals only on surface characteristics and often picked countervailing traits. He also allowed crosses that reduced the power of selection. And finally, man acted selfishly, choosing only that property which “pleases or is useful to him.” Nature acted quite differently: 21 She cares not for mere external appearances; she may be said to scrutinize with a severe eye, every nerve, vessel & muscle; every habit, instinct, shade of constitution,—the whole machinery of the organization. There will be here no caprice, no favouring: the good will be preserve & the bad rigidly destroyed (Species Book, 224). Nature thus acted steadily, justly, and with divine discernment, separating the good from the bad. Nature, in this conception, was God’s surrogate, which Darwin signaled by penciling in his manuscript above the quoted passage: “By nature, I mean the laws ordained by God to govern the Universe” (Species Book, 224; see also Brooke, this volume). As Darwin pared away the overgrowth of the Big Species Book, the intelligent and moral character of natural selection stood out even more boldly in the précis, that is, in the Origin of Species. Natural Selection in the Origin of Species In the first edition of the Origin, Darwin approached natural selection from two distinct perspectives, conveyed in two chapters whose titles suggest the distinction: “Struggle for Existence” and “Natural Selection” (chapters 3 and 4). Though their considerations overlap, the first focuses on the details of the operations of selection and the second contains the more highly personified re-conceptualization of its activities. In chapter 3, Darwin proposed that small variations in organisms would give some an advantage in the struggle for life. He then defined natural selection: 22 Owing to this struggle for life, any variation, however slight and from whatever cause proceeding, if it be in any degree profitable to an individual of any species, . . . will tend to the preservation of that individual, and will generally be inherited by its offspring. The offspring, also, will thus have a better chance of surviving. . . I have called this principle, by which each slight variation, if useful, is preserved by the term Natural Selection (Origin, 61). Darwin would explain what he meant by “struggle” a bit later in the chapter, and I will discuss that in a moment. Here, I would like to note several revealing features of his definition. First, selection is supposed to operate on all variations, even those produced by the inheritance of acquired characters and not just those that arise accidentally from the environment acting on the sex organs of parents. Second, Darwin believed that virtually all traits, useful or not, would be heritable—what he called the “strong principle of inheritance” (Origin, 5). Third, though the initial part of the definition indicates it is the individual that is preserved, in the second part it is the slight variation that is preserved—which latter is the meaning of the phrase “natural selection” (Origin, 61 and 81). The passage draws out “the chicken and egg” problem for Darwin: a trait gives an individual an advantage in its struggle, so that the individual is preserved, who, in turn, preserves the trait by passing it on to offspring. Finally, the definition looks to the future, when useful traits will be sifted out and the non-useful extinguished, along with their carriers. In the short run, individuals are preserved; in the long run, it is their morphologies that are both perpetuated and slowly change as the result of continued selection. 25 and multiply” (Origin, 79). Darwin’s model of moral agency mitigated the force of Malthusian pitilessness. Conclusion I have argued that Darwin did not come to his conception of natural selection in a flash that yielded a fully formed theory. What appears as the intuitive clarity of his device is, I believe, quite deceptive. I have tried to show that his notions about the parameters of natural selection, what it operates on and its mode of operation, gradually took shape in Darwin’s mind, and hardly came to final form even with the publication of the first edition of the Origin of Species. In this gradual evolution of a concept—actually a set of concepts—I have emphasized the way Darwin characterized selection as a moral and intelligent agent. Most contemporary scholars have described Darwinian nature as mechanical, even a-moral in its ruthlessness. To be sure, when Wallace and others pointed out what seemed the misleading implications of the device, Darwin protested that, of course, he did not mean to argue that natural selection was actually an intelligent or moral agent. But even Darwin recognized, if dimly, that his original formulation of the device and the cognitively laden language of his writing carried certain consequences with which he did not wish to dispense—and, indeed, could not do so without altering his deeper conception of the character and goal of evolution. Darwin’s language and metaphorical mode of thought gave his theory a meaning resistant to any mechanistic interpretation and unyielding even to his later, more cautious reflections. 26 Let me spell out some of those consequences to make clear how markedly Darwin’s original notion of evolution by natural selection differs from what is usually attributed to him. Natural selection, in Darwin’s view, moved very slowly and gradually, operating at a stately Lyellian pace (perhaps seizing on useful variations that might occur only after thousands of generations; Origin, 80 and 82). It compensated for meager variability by daily and hourly scrutinizing every individual, for even the slightest and most obscure variation, to select just those that gave the organism an advantage. A nineteenth- century machine could not be calibrated to operate on such small variations or on features that might escape human notice. If natural selection clanked along like a Manchester spinning loom, one would not have fine damask—only a skillful and intelligent hand could spin that—or the fabric of the eye. Second, Darwin frequently remarked in the Origin that selection operated more efficiently on species with a large number of individuals in an extensive, open area (Origin, 41, 70, 102, 105, 125, 177, 179). He presumed that, as in the case of the human breeder, a large number of individual animals or plants would produce more favorable variations upon which selection might act. The greater quantities would also create Malthusian pressure. Yet in the wild, this scenario for selection could only occur if the watchful eye of an intelligent selector somehow gathered the favored varieties together and isolated them so as to prevent back-cross into the rest of the stock. When Fleeming Jenkin, in his review of the Origin, pointed out the problem of swamping of single variations, Darwin suggested in the fifth edition that groups of individuals would all vary in the same way due to the impact of the local environment (Variorum, 179). Thus when the implications of his model of intelligent nature were recognized, Darwin 27 had to invoke as analogue a Lamarckian scenario. Today, we assume that small breeding groups isolated by physical barriers would more likely furnish the requisite conditions for natural selection. Third, a wise selector that has the good of creatures at heart would produce a progressive evolution, one that created ever more improved organization, which Darwin certainly thought to be the case. He believed that more recent creatures had accumulated progressive traits and would triumph over more ancient creatures regardless of the environments in which they might compete (Origin, 336-37). He summed up his view in the last section of the Origin: “And as natural selection works solely by and for the good of each being, all corporeal and mental endowments will tend to progress towards perfection” (Origin, 489). This passage, which remains unchanged through the several editions of the Origin, is an index of both Darwin’s moral conception of nature and of its progressive intent. The moral overlay of the passage has blotted out the winnowing force of selection, which hardly works for the benefit of every creature. And, as Darwin made clear in the third edition, the “improvements” wrought by selection will “inevitably lead to the gradual advancement of the organization of the greater number of living beings throughout the world” (Variorum, 221; my emphasis). Fourth, such an intelligent agency would not merely select for each creature’s good, but also for that of the community. Darwin, in the fifth and sixth editions of the Origin, extended his model of family selection to one that operated simply on a community: “In social animals it [natural selection] will adapt the structure of each individual for the