Información sobre lineamientos y normativa

For Darwin and his contemporaries, the question of adaptation was even more problematic than mode of transmission, as indeed it still is for some present-day fundamentalist religious critics of

evolutionary theory. The problem lies in an argument that precedes Darwin, and is often posed in the form it was originally given by the theologian William Paley in his book Natural Theology, written at the very beginning of the nineteenth century. If, on a walk through the countryside, you stumble across a watch lying on the ground, only the briefest examination is needed to convince you that it cannot have come about by chance. The watch and its inner mechanisms show clear evidence of design, and how can one have design without a designer? If this is so for the relatively crude mechanism of a watch, how much more for such marvellous structures as the eye. Darwin himself confessed his terror when trying to think about the possible evolution of the eye. On closer inspection, though, this apparent problem vanishes.

Dawkins confronts this question head-on in The Blind Watchmaker and its successors: 'What use is half in eye?' he asks, and answers, 'One per cent better than 49 per cent of an eye, and the difference is significant.' 22 The trouble with this argument is that there is no way of determining whether, among our evolutionary ancestors, 50 per cent of an eye ever proved significantly better in Darwinian terms - that is, whether it contributed significantly more to reproductive success -- than 49 per cent. It would depend on what other costs the organism accrued in achieving this 1 per cent advantage, and on how much having eyes contributed to its success in finding food, and avoiding predators so as to increase its chances of finding a mate and hence reproducing. Of course, no such evidence can be forthcoming, and so the claim must remain an undemonstrable assertion, although one which most biologists will find reasonably convincing. Dawkins

-193-

goes on to cite evidence that serviceable image-forming eyes have evolved independently at least forty times in different invertebrate groups, quite apart from 'the' eye, by which we mean the light- and image-detecting mechanisms which humans share with our more immediate evolutionary neighbours. 23

How many generations would it take to evolve such an eye from an initial flat retina, above a flat pigment layer surmounted by a protective transparent layer? Dawkins cites a computer model by Dan Nilsson and Susanne Pelger which could do it in under half a million. At the rate of one generation a year, this means just 500,000 years, easily attainable within the timespan of life on Earth. The required assumptions are that each step is heritable, digital in effect, and provides a selective advantage to the creature which carries the variation.

Accepting these assumptions unquestioningly requires something of an act of faith (and, as I argue below, there are grounds for rather less credulity than Dawkins affects to display), but even so I see no

problem with the general principle invoked here. In the classical Popperian sense, as we have seen, such evolutionary stories are unfalsifiable. All that we can do, all that we are required to do, is offer plausible accounts of how a process may have occurred or a structure may have evolved, in response to those who claim that it is impossible on a priori grounds. If I argue, as I do, that life is a good deal more complex than the computer-generated biomorphs that Dawkins has created as a spin-off from his writings, this should not be read as yielding any ground at all to those who would argue that life is a product of anything other than material forces operating in a material universe, potentially explicable by the methods of a (non-reductive) science.

Once again, as with genetic transmission, the problem of adaptation -- at least as it confronted Darwin - - is not an insoluble one. He surmised that it could be resolved, given enough evolutionary space and time, and he was surely right. Later on, I shall turn to how the problem recurs in its modern form.

-194- THE LIMITS TO NATURAL SELECTION

The third major problem that Darwin faced, and which his theory in its simple form was unable to resolve, is that of speciation. It may seem extraordinary, but the Darwinian syllogism of natural selection presented on page 181 provides no mechanism for the formation of new species, which was after all ostensibly what The Origin was all about. All it says is that, in any given circumstances, external conditions (the environment, nature) will favour the perpetuation of varieties which can do their species-thing a bit better than the rest. Antelopes, for example, are preyed upon by lions. Any antelope in a group which has been evolutionarily favoured by being able to run slightly faster than the others has a slightly better chance of avoiding lions, and is therefore that much more likely to survive. Similarly, lions which can run faster, or develop cooperative methods of stalking their prey in packs, will boost their chances of survival. But this won't in itself turn antelopes or lions, or their descendants, into new species.

There is a real-life example of this type of process in action. It is found in all the textbooks, if only because it is one of the bestdocumented examples of a change in the form of a species over time which can be attributed to natural selection (as opposed to some test-tube experiments with bacterial

populations). The peppered moth, widely distributed throughout Britain, spends much of its time clinging to tree trunks. As its name implies, the normal form of this species is a speckled brown, but a somewhat rarer, black (melanic) variety also occurs, first observed in Manchester in the middle of the nineteenth century. The British enthusiasm for nature study meant that the moth has been observed over many years, and records kept of the proportions of the two forms, which showed a steady increase in the proportion of the dark over the light form in industrial areas in the twentieth century. The moths are much preyed upon by birds, and an obvious interpretation is that in the absence of environmental pollution which darkens the tree bark, the light, speckled form is harder for birds to spot. Where pollution darkens the tree bark, the speckled form will stand out, while the dark form will be better concealed.

-195-

In 1955 H. B. D. Kettlewell checked this hypothesis, and showed that the dark form of the moth was indeed at a selective advantage (being less preyed upon) in soot-blackened areas. 24 As expected, the reverse is the case in unpolluted areas. Admittedly the example is not really of natural selection by competition for scarce resources, the original Darwinian motor, but we can allow it none the less. What

makes it particularly instructive is that as the shift to less polluting energy sources reduces the amount of soot in the air around Manchester, and trees suffer less blackening, so the melanic form of the moth is decreasing and the speckled form is increasing. Whereas a few years ago blacks outnumbered speckleds by more than 2 to 1, the proportions are now reversed, and the dark form's days seem numbered.

So natural selection can work to change populations, increasing the adaptiveness of individuals within them; favoured varieties are preserved and therefore their distribution in the population changes with time. Furthermore, the example of the peppered moth demonstrates another fundamental point about natural selection. By definition, a 'more favoured variety' is one which is favoured under current circumstances. Evolution by natural selection can respond only to the current situation -- it cannot predict the future. At one point of the species' trajectory in time, it is the speckled form which has the greater survival value, then the melanic, and at a later time the speckled form again. The environmental change occurs, and natural selection trails along behind, following, responding, but never leading -- and never predicting.

This inability to predict future advantage, and therefore to adapt in advance, holds even in the lifetime of an individual. A mutation which resulted in the adult antelope being able to run faster, but which also meant that it took longer to mature and was therefore more vulnerable to attack by lions for longer periods, would scarcely have much chance of spreading in the antelope population.

-196- SEXUAL SELECTION

There are two further important twists in the tale/tail of the evolutionary adaptation story. The first concerns sex. If all adaptation serves the function of enhancing survival, how come so many animals -- especially males -- have traits which seem on the face of it to be inimical to a long and efficient life? The peacock's train is the classic case. How and why does there evolve such an apparently

dysfunctional object, of such startling beauty to human eyes? The question vexed Darwin so much that he was led to develop an entire supplementary theory of selection -- sexual selection. To pass on their genes, males and females need to mate, and in such animal species that have been studied, given conditions where choice is possible (which means outside the standard laboratory cage), mating is non- random. Potential mates compete in various ways with members of their own sex, and choose a partner of the opposite sex from among a range of potential candidates. What determines success in these two ventures?

Darwin's view was that, by and large, it is the female of the species that does the choosing. He went so far as to postulate that animals have an aesthetic sense, and tend to choose the most beautiful of the potential mates. If peahens regarded the peacock's tail as humans did, they would tend to choose the mate with the most striking tail. Even if one discounts the possibility of aesthetic judgement (or at least an aesthetic judgement which coincides with that of humans, for the sexual adornments carried by the males of many species often strike human observers as more absurd or extraordinary than beautiful), one has only to make the assumption that at some past time, for whatever reason, peahens were attracted to peacocks with bright fan-like tails; this trait would then be selected, and would spread in the male population, and tails would evolve into more and more splendid objects as a result.

According to a slightly different version of sexual selection theory, to grow an elaborate tail requires a considerable expenditure of energy, and, because the tail is manifestly a handicap to normal survival -- it makes the bird more conspicuous to predators, and less able to move

-197-

fast to escape -- then any male which survives to adulthood bearing such a burden must be particularly fit in other ways. In this picture the tail becomes a sort of marker, indicating that its possessor is a genetically good bet for a potential partner.

There has been no lack of those who have sought to take the theory, in whichever version, and press it into service to provide an evolutionary 'Darwinian' explanation for human sexual preferences. The general procedure, in this as in so much of the reductive approach offered by the new genetics and sociobiology, is to treat metaphor as if it were homology. For example, competition for mates among human males is discussed as the macro-version of what is said to be the micro-level competition among individual sperms to be 'the one' to successfully penetrate and fertilize the egg. Males and their sperm compete, females and their ova quiescently await their fate.

The problem is that, as with most human extensions of evolutionary mechanisms, but in an even more extreme form, such accounts simply cannot encompass the rich diversity of human experience. Instead they fall back on traditional and often sexist caricatures so crude as to make cheap romantic novels read like sociological essays. Thus the sociobiologists largely ignore the historical and anthropological evidence of variation in social practices across time and space, 25 and instead treat current Western norms (or rather, assertive restatements of what they perceive to be those norms, for they show as little respect for sociology as they do for history or anthropology) as if they were human universals. For example, there have been widely publicized claims that there are universal human standards of beauty. These are based on a cross-cultural comparison of ratings given by Japanese and Western males to computer-generated faces. 26 That the two civilizations have been approaching one another culturally for several generations, and share visual images transmitted via cinema, television and advertising, is not allowed to stand in the way of this drive to evolutionary universalism. Symmetry of feature is apparently highly regarded, and we have even been regaled with tales, based on evidence which would be laughed out of court did it not have the fascination of prurience, that women have more orgasms during sex with men whose bodies are symmetrical. 27 It has to be said that the relevance

-198-

of this observation to the question of whether there are, as a result of these joyous matings, more offspring -- which is after all the only relevant Darwinian criterion -- is not specified. On the other hand, adulterous matings are said to have a greater chance of resulting in pregnancy than those within marriage. 28

As for sexual display, are not the Porsche and the Rolex, still overwhelmingly the appurtenances of financially successful males, the equivalent of the peacock's tail, demonstrating the genetic fitness of their owners to admiring females? The trouble is that wealth is no measure of genetic fitness, and although it may be inherited, the mode of transmission is not via the genes, nor is there much evidence that its possession results in a greater number of offspring. Once again, the supposed Darwinian imperative is negated at its most fundamental level. Sexual selection may be -- probably is -- an important mechanism by which to account for otherwise improbable features varying from the

we should not let its enthusiasts blind us to the more obvious explanations for the complexity of human sexual arrangements.

ALTRUISM

We now come to the claims for the genetic mechanism and evolutionary significance of altruistic behaviour, and here we are at the heart of sociobiological thinking. The problem for evolutionists is straightforwardly stated. Students of behaviour have described many examples of animals acting in ways which appear not to be in what may be interpreted as their genetic interest. That is, if we assume that organisms seek to maximize their reproductive success, and to pass on as many of their genes as possible to a succeeding generation, then how do we account for birds which, on detecting a predator, draw attention to it and simultaneously to themselves by uttering warning cries to alert the remainder of the flock? Ought they not instead try to make themselves as inconspicuous as possible, so as to

diminish the chance of being picked off?

-199-

Back in the 1960s, V. C. Wynne-Edwards attempted to account for a different example of seemingly altruistic behaviour. How are the numbers of a particular population of animals regulated, when their territory and food supplies are limited? One argument would be that they all breed to their maximum capacity, and that only the 'fittest' survive the subsequent struggle to obtain adequate food. Wynne Edwards offered an alternative explanation, based on studies of, among other species, grouse. He suggested that grouse have evolved a special display behaviour which informs them of the size of the population, and that individuals then respond to the problem of overpopulation by a sort of self-denial, limiting the numbers of their own offspring for the good of the community as a whole. He called this type of behaviour group selection. 29 Evolutionary biologists were quick to point out what they saw as the flaws in his proposed mechanism. Selection, they argued, could act only at the level of the

individual, and, for any individual, maximizing the number of its own offspring is the Darwinian driving force. So if most of the grouse were limiting the numbers of offspring they produced, then selection would favour any variant which 'cheated' by trading on the virtuous self-sacrifice of the remainder. So the number of 'cheats' would soon spread through the population, while the numbers who deliberately restrained themselves would fall. Group selection on this basis was a non-starter, although Wynne-Edwards continued to argue his case, against the prevailing climate of opinion.

So how could seemingly altruistic behaviour evolve? The clue is supposed to have been provided in an offhand remark by J. B. S. Haldane, who pointed out that on the basis of the proportion of genes he shared with his closer relatives, he ought to be prepared to sacrifice himself for two brothers, or eight cousins. That is, on the assumption that the life process is all about passing on one's genes to the next generation, then there is a genetic rationality about the individual risking its own life if by so doing it can ensure the survival, and presumably the reproductive success, of a sufficient number of those who share a proportion of its genes. This was a typically bravura statement by Haldane, who throughout his life was very proud of the fact that he had conducted many of his more hazardous physiological

-200-

experiments -- from ingesting excessive quantities of ammonium chloride to measure the effect of changing the acidity of the blood, to testing survival times in the restricted atmosphere of submarines -- using himself as a human guinea pig.

However, Haldane's offhand remark was given serious mathematical form by William Hamilton in 1964, 30 and termed kin selection. It was E. O. Wilson who, in 1975, brought the argument to the attention not merely of mainstream biologists but of a much wider public as well, when, in a deliberate evocation of the Darwin -- Mendel 'modern synthesis' of the 1930s, he called a book of his

Sociobiology: The New Synthesis. 31 However, the term that was to take hold in the popular imagination was due not to Wilson but to Dawkins, when the following year he published his evangelizing version of ultra-Darwinian and sociobiological theory: The Selfish Gene. 32 (It is worth making clear yet again that Dawkins' genes aren't selfish in the sense in which we might refer to 'gay' or 'aggression' genes. Dawkins' genes do not necessarily confer selfishness on their possessor; they are intended to ensure that their possessor does what is necessary in order that his or her genes are able to replicate and copies