The Blind Watchmaker (52 page)

Our refutation of Lamarckism, then, is a bit devastating. First, its key assumption, that of the inheritance of acquired characteristics, seems to be false in all life-forms that we have studied. Second, it not only is false but it
has to
be false in any life-form that relies upon an epigenetic (‘recipe’) rather than a preformationistic (‘blueprint’) kind of embryology, and this includes all life-forms that we have studied. Third, even if the assumptions of the Lamarckian theory were true, the theory is in principle, for two quite separate reasons, incapable of explaining the evolution of serious adaptive complexity, not j,ust on this earth but anywhere in the universe. So, it isn’t that Lamarckism is a rival to the Darwinian theory that happens to be wrong. Lamarckism isn’t a rival to Darwinism at all. It isn’t even a serious
candidate
as an explanation for the evolution of adaptive complexity. It is doomed from the start as a potential rival to Darwinism.

There are a few other theories that have been, and even occasionally still are, advanced as alternatives to Darwinian selection. Once again, I shall show that they are not really serious alternatives at all. I shall show (it is really obvious) that these ‘alternatives’ - ‘neutralism’, ‘mutationism’, and so on - may or may not be responsible for some proportion of observed evolutionary change, but they cannot be responsible for
adaptive
evolutionary change, that is for change in the direction of building up improved devices for survival like eyes, ears, elbow joints, and echo-ranging devices. Of course, large quantities of evolutionary change may be non-adaptive, in which case these alternative theories may well be important in parts of evolution, but only in the boring parts of evolution, not the parts concerned with what is special about life as opposed to non-life. This is especially clear in the case of the neutralist theory of evolution. This has a long history, but it is particularly easy to grasp in its modern, molecular guise in which it has been promoted largely by the great Japanese geneticist Motoo Kimura, whose English prose style, incidentally, would shame many a native speaker.

We have already briefly met the neutralist theory. The idea, you will remember, is that different versions of the same molecule, for instance versions of the haemoglobin molecule differing in their precise aminoacid sequences, are exactly as good as each other. This means that mutations from one alternative version of haemoglobin to another are
neutral
as far as natural selection is concerned. Neutralists believe that the vast majority of evolutionary changes, at the level of molecular genetics, are neutral -
random
with respect to natural selection. The alternative school of geneticists, called selectionists, believe that natural selection is a potent force even at the level of detail at every point along molecular chains.

It is important to distinguish two distinct questions. First is the question that is relevant to this chapter, whether neutralism is an alternative to natural selection as an explanation for adaptative evolution. Second, and quite distinct, is the question whether most of the evolutionary change that actually occurs is adaptive. Given that we are talking about an evolutionary change from one form of a molecule to another, how likely is it that the change came about through natural selection, and how likely is it that it is a neutral change which came about through random drift? Over this second question, a ding-dong battle has raged among molecular geneticists, first one side gaining the upper hand, then the other. But if we happen to be focusing our interest on adaptation - the first question - it is all a storm in a teacup. As far as we are then concerned, a neutral mutation might as well not exist because neither we, nor natural selection, can see it. A neutral mutation
isn’t
a mutation at all, when we are thinking about legs and arms and wings and eyes and behaviour! To use the recipe analogy again, the dish will taste the same even if some words of the recipe have ‘mutated’ to a different print font. As far as those of us who are interested in the final dish are concerned, it is still the same recipe, whether printed like this or
like this
or like this. Molecular geneticists are like pernickety printers. They care about the actual form of the words in which recipes are written down. Natural selection doesn’t care, and nor should we when we are talking about the evolution of adaptation. When we are concerned with other aspects of evolution, for instance rates of evolution in different lineages, neutral mutations will be of surpassing interest.

Even the most ardent neutralist is quite happy to agree that natural selection is responsible for all adaptation. All he is saying is that most evolutionary change is not adaptation. He may well be right, although one school of geneticists would not agree. From the sidelines, my own hope is that the neutralists will win, because this will make it so much easier to work out evolutionary relationships and rates of evolution. Everybody on both sides agrees that neutral evolution cannot lead to adaptive improvement, for the simple reason that neutral evolution is, by definition, random; and adaptive improvement is, by definition, nonrandom. Once again, we have failed to find any alternative to Darwinian selection, as an explanation for the feature of life that distinguishes it from non-life, namely adaptive complexity.

We now come to another historical rival to Darwinism the theory of ‘mutationism’. It is hard for us to comprehend now but, in the early years of this century when the phenomenon of mutation was first named, it was regarded not as a necessary part of Darwinian theory but as an
alternative
theory of evolution! There was a school of geneticists called the mutationists, which included such famous names as Hugo de Vries and William Bateson among the early rediscoverers of Mendel’s principles of heredity, Wilhelm Johannsen the inventor of the word gene, and Thomas Hunt Morgan the father of the chromosome theory of heredity. De Vries in particular was impressed by the magnitude of the change that mutation can wreak, and he thought that new species always originated from single major mutations. He and Johannsen believed that most variation
within
species was non-genetic. All the mutationists believed that selection had at best a minor weeding-out role to play in evolution. The really creative force was mutation itself. Mendelian genetics was thought of, not as the central plank of Darwinism that it is today, but as antithetical to Darwinism.

It is extremely hard for the modern mind to respond to this idea with anything but mirth, but we must beware of repeating the patronizing tone of Bateson himself: ‘We go to Darwin for his incomparable collection of facts (but…] for us he speaks no more with philosophical authority. We read his scheme of Evolution as we would those of Lucretius or Lamarck.’ And again, ‘the transformation of masses of populations by imperceptible steps guided by selection is, as most of us now see, so inapplicable to the fact that we can only marvel both at the want of penetration displayed by the advocates of such a proposition, and at the forensic skill by which it was made to appear acceptable even for a time.’ It was above all R.A.Fisher who turned the tables and showed that, far from being antithetical to Darwinism, Mendelian paniculate heredity was actually essential to it.

Mutation is necessary for evolution, but how could anybody ever have thought it was sufficient? Evolutionary change is, to a far greater extent than chance alone would expect,
improvement
. The problem with mutation as the sole evolutionary force is simply stated: how on earth is mutation supposed to
know
what will be good for the animal and what will not? Of all possible changes that might occur to an existing complex mechanism like an organ, the vast majority will make it worse. Only a tiny minority of changes will make it better. Anybody who wants to argue that mutation, without selection, is the driving force of evolution, must explain how it comes about that mutations tend to be for the better. By what mysterious, built-in wisdom does the body choose to mutate in the direction of getting better, rather than getting worse? You will observe that this is really the same question, in another guise, as we posed for Lamarckism. The mutationists, needless to say, never answered it. The odd thing is that the question hardly seems to have occurred to them.

Nowadays, and unfairly, this seems all the more absurd to us because we are brought up to believe that mutations are ‘random’. If mutations are random, then, by definition, they cannot be biased towards improvement. But the mutationist school did not, of course, regard mutations as random. They thought that the body had a built-in tendency to change in certain directions rather than others, though they left open the question of how the body ‘knew’ what changes would be good for it in the future. While we write this off as mystical nonsense, it is important for us to be clear exactly what we mean when we say that mutation is random. There is randomness and randomness, and many people confuse different meanings of the word. There are, in truth, many respects in which mutation is not random. All I would insist on is that these respects do
not
include anything equivalent to anticipation of what would make life better for the animal. And something equivalent to anticipation would indeed be needed if mutation, without selection, were to be used to explain evolution. It is instructive to look a little further at the senses in which mutation is, and is not, random.

The first respect in which mutation is nonrandom is this. Mutations are caused by definite physical events; they don’t just spontaneously happen. They are induced by so-called ‘mutagens’ (dangerous because they often start cancers): X-rays, cosmic rays, radioactive substances, various chemicals, and even other genes called ‘mutator genes’. Second, not all genes in any species are equally likely to mutate. Every locus on the chromosomes has its own characteristic
mutation rate
. For instance, the rate at which mutation creates the gene for the disease Huntington’s chorea (similar to St Vitus’s Dance), which kills people in early middle age, is about 1 in 200,000. The corresponding rate for achondroplasia (the familiar dwarf syndrome, characteristic of basset hounds and dachsunds, in which the arms and legs are too short for the body) is about 10 times as high. These rates are measured under normal conditions. If mutagens like X-rays are present, all normal mutation rates are boosted. Some parts of the chromosome are so-called ‘hot spots’ with a high turnover of genes, a locally very high mutation rate.

Third, at each locus on the chromosomes, whether it is a hot spot or not, mutations in certain directions can be more likely than mutations in the reverse direction. This gives rise to the phenomenon known as ‘mutation pressure’ which can have evolutionary consequences. Even if, for instance, two forms of the haemoglobin molecule. Form 1 and Form 2, are selectively neutral in the sense that both are equally good at carrying oxygen in the blood, it could still be that mutations from 1 to 2 are commoner than reverse mutations from 2 to 1. In this case, mutation pressure will tend to make Form 2 commoner than Form 1. Mutation pressure is said to be zero at a given chromosomal locus, if the forward mutation rate at that locus is exactly balanced by the backward mutation rate.

We can now see that the question of whether mutation is really random is not a trivial question. Its answer depends on what we understand random to mean. If you take ‘random mutation’ to mean that mutations are not influenced by external events, then X-rays disprove the contention that mutation is random. If you think ‘random mutation’ implies that all genes are equally likely to mutate, then hot spots show that mutation is not random. If you think ‘random mutation’ implies that at all chromosomal loci the mutation pressure is zero, then once again mutation is not random. It is only if you define ‘random’ as meaning ‘no general bias towards bodily improvement’ that mutation is truly random. All three of the kinds of real nonrandomness we have considered are powerless to move evolution in the direction of adaptive improvement as opposed to any other (functionally) ‘random’ direction. There is a fourth kind of nonrandomness, of which this is also true but slightly less obviously so. It will be necessary to spend a little time on this because it is still muddling even some modern biologists.

There are people for whom ‘random’ would have the following meaning, in my opinion a rather bizarre meaning. I quote from two opponents (P.Saunders and M.W.Ho) of Darwinism, on their conception of what Darwinians believe about ‘random mutation’: ‘The neo-Darwinian concept of random variation carries with it the major fallacy that everything conceivable is possible’. ’
All
changes are held to be possible and all
equally likely
‘ (my emphasis). Far from holding this belief, I don’t see how you would begin to set about making such a belief even
meaningfuD
. What could it possibly mean to hold that ‘all’ changes are equally likely?
All
changes? In order for two or more things to be ‘equally likely’, it is necessary that those things should be definable as discrete events. For instance, we can say ‘Heads and Tails are equally likely’, because Heads and Tails are two discrete events. But ‘all possible’ changes to an animal’s body are not discrete events of this type. Take the two possible events: ‘Cow’s tail lengthens by one inch’; and ‘Cow’s tail lengthens by two inches’. Are these two separate events, and therefore ‘equally likely’? Or are they just quantitative variants of the same event?

It is clear that a kind of caricature of a Darwinian has been set up, whose notion of randomness is an absurd, if not actually meaningless, extreme. It took me a while to understand this caricature, for it was so foreign to the way of thinking of the Darwinians that I know. But I think I do now understand it, and I shall try to explain it, as I think it helps us to understand what lies behind quite a lot of alleged opposition to Darwinism.

Variation and selection work together to produce evolution. The Darwinian says that variation is random in the sense that it is not directed towards improvement, and that the tendency towards improvement in evolution comes from selection. We can imagine a kind of continuum of evolutionary doctrines, with Darwinism at one end and Mutationism at the other. The extreme mutationist believes that selection plays no role in evolution. The direction of evolution is determined by the direction of the mutations that are offered. For instance, suppose we take the enlargement of the human brain that has occurred during the last few million years of our evolution. The Darwinian says that the variation that was offered up by mutation for selection included some individuals with smaller brains and some individuals with larger brains; selection favoured the latter. The mutationist says that there was a bias in favour of larger brains in the variation that was offered up by mutation; there was no selection (or no need for selection) after variation was offered up; brains got bigger because mutational change was biased in the direction of bigger brains. To summarize the point: in evolution there was a bias in favour of larger brains; this bias could have come from selection alone (the Darwinian view) or from mutation alone (the mutationist view); we can imagine a continuum between these two points of view, almost a kind of trade-off between the two possible sources of evolutionary bias. A middle view would be that there was some bias in mutations towards enlargement of the brain, and that selection increased the bias in the population that survived.