The Blind Watchmaker (53 page)

The element of caricature comes in the portrayal of what the Darwinian means when he says that there is no bias in the mutational variation that is offered up for selection. To me, as a real-life Darwinian, it means only that mutation is not systematically biased in the direction of adaptive improvement. But to the larger-than-life caricature of a Darwinian, it means that all conceivable changes are ‘equally likely’. Setting aside the logical impossibility of such a belief, already noted, the caricature of a Darwinian is thought to believe that the body is infinitely malleable clay, ready to be shaped by all-powerful selection into any form that selection might favour. It is important to understand the difference between the real-life Darwinian and the caricature. We shall do so in terms of a particular example, the difference between the flight techniques of bats and of angels.

Angels are always portrayed as having wings sprouting from their backs, leaving their arms unencumbered by feathers. Bats, on the other hand, along with birds and pterodactyls, have no independent arms. Their ancestral arms have become incorporated into wings, and cannot’ be used, or can only be used very clumsily, for other purposes such as picking up food. We shall now listen in on a conversation between a real-life Darwinian and an extreme caricature of a Darwinian.

Real-life
. I wonder why bats didn’t evolve wings like angels. You’d think that they could use a free pair of arms. Mice use their arms all the time for picking up food and nibbling it, and bats look terribly clumsy on the ground without arms. I suppose one answer might be that mutation never provided the necessary variation. There just never were any mutant ancestral bats that had wing buds sticking out of the middle of their backs.

Caricature
. Nonsense. Selection is everything. If bats haven’t got wings like angels, this can only mean that selection didn’t favour wings like angels. There certainly were mutant bats with wing buds sticking out of the middle of their backs, but selection just didn’t favour them.

Real-life
. Well, I quite agree that selection might not have favoured them if they
had
sprouted. For one thing they would have increased the weight of the whole animal, and surplus weight is a luxury no aircraft can afford. But surely you don’t think that,
whatever
selection might in principle favour, mutation will always come up with the necessary variation?

Caricature
. Certainly I do. Selection is everything. Mutation is random.

Real-life
. Well yes, mutation is random, but this only means that it can’t see into the future and plan what would be good for the animal. It doesn’t mean that absolutely
anything
is possible. Why do you think that no animal breathes fire out of its nostrils like a dragon, for instance? Wouldn’t it be useful for catching and cooking prey?

Caricature
. That’s easy. Selection is everything. Animals don’t breathe fire, because it wouldn’t pay them to do so. Fire-breathing mutants were eliminated by natural selection, perhaps because making fire was too costly in energy.

Real-life
. I don’t believe there ever were fire-breathing mutants. And if there had been, presumably they would have been in grave danger of burning themselves!

Caricature
. Nonsense. If that was the only problem, selection would have favoured the evolution of asbestos-lined nostrils.

Real-life
. I don’t believe any mutation ever produced asbestos-lined nostrils. I don’t believe mutant animals could secrete asbestos, any more than mutant cows could jump over the moon.

Caricature
. Any moon-jumping mutant cow would promptly be eliminated by natural selection. There’s no oxygen up there you know.

Real-life
. I’m surprised you don’t postulate mutant cows with genetically determined space-suits and oxygen masks.

Caricature
. Good point! Well, I suppose the real explanation must be that it just wouldn’t pay cows to jump over the moon. And we mustn’t forget the energetic cost of reaching escape velocity.

Real-life
. This is absurd.

Caricature
. You are obviously not a true Darwinian. What are you, some kind of crypto-mutationist deviationist?

Real-life
. If you think that, you should meet a real mutationist.

Mutationist
. Is this a Darwinian in-group argument, or can anyone join in? The trouble with both of you is that you give far too much prominence to selection. All that selection can do is weed out gross deformities and freaks. It can’t produce really constructive evolution. Go back to the evolution of bat wings. What really happened is that in an ancient population of grounddwelling animals, mutations started turning up with elongated fingers and webs of skin between. As the generations went by, these mutations became more and more frequent until, eventually, the whole population had wings. It had nothing to do with selection. There was just this built-in tendency in the ancestral bat constitution to evolve wings.

Rank mysticism! Get back in the last century where you belong.

I hope I am not being presumptuous when I take it that the reader’s sympathies are with neither the Mutationist nor with the caricature of a Darwinian. I assume that the reader agrees with the real-life Darwinian, as, of course, do I. The caricature does not really exist.

Unfortunately some people
think
he exists, and think that, since they disagree with him, they are disagreeing with Darwinism itself. There is a school of biologists who have taken to saying something like the following. The trouble with Darwinism is that it neglects the constraints imposed by embryology. Darwinians (this is where the caricature comes in) think that, if selection would favour some conceivable evolutionary change, then the necessary mutational variation will turn out to be available. Mutational change in any direction is equally likely: selection provides the only bias.

But any real-life Darwinian would acknowledge that, although any gene on any chromosome may mutate at any time, the consequences of mutation on
bodies
are severely limited by the processes of embryology. If I ever doubted this (I didn’t), my doubts would have been dispelled by my biomorph computer simulations. You can’t just postulate a mutation ‘for’ sprouting wings in the middle of the back. Wings, or anything else, can only evolve if the process of development allows them to. Nothing magically ‘sprouts’. It has to be made by the processes of embryonic development. Only a minority of the things that conceivably could evolve are actually permitted by the status quo of existing developmental processes. Because of the way arms develop, it is possible for mutations to increase the length of fingers and cause webs of skin to grow between them. But there may not be anything in the embryology of backs that lends itself to ‘sprouting’ angel wings. Genes can mutate till they are blue in the face, but no mammal will ever sprout wings like an angel unless mammalian embryological processes are susceptible to this kind of change.

Now as long as we don’t know all the ins and outs of how embryos develop, there is room for disagreement over how likely it is that particular imagined mutations have or have not ever existed. It might turn out, for instance, that there is nothing in mammalian embryology to forbid angel wings, and that the caricature Darwinian was right, in this
particular
case, to suggest that angel wing-buds arose but were not favoured by selection. Or it might turn out that when we know more about embryology we shall see that angel wings were always a nonstarter, and that therefore selection never had a chance to favour them. There is a third possibility, which we should list for completeness, that embryology never allowed the possibility of angel wings and that selection would never have favoured them even if it had. But what we must insist on is that we can’t afford to ignore the constraints on evolution that embryology imposes. All serious Darwinians would agree about this, yet some people portray Darwinians as denying it. It turns out that people who make a lot of noise about ‘developmental constraints’ as an alleged anti-Darwinian force are confusing Darwinism with the caricature of Darwinism that I parodied above.

This all began with a discussion over what is meant when we say that mutation is ‘random’. I listed three respects in which mutation is not random: it is induced by X-rays, etc.; mutation rates are different for different genes; and forward mutation rates do not have to equal backward mutation rates. To this, we have now added a fourth respect in which mutation is not random. Mutation is nonrandom in the sense that it can only make alterations to
existing
processes of embryonic development. It cannot conjure, out of thin air, any conceivable change that selection might favour. The variation that is available for selection is constrained by the processes of embryology, as they actually exist.

There is a fifth respect in which mutation
might
have been nonrandom. We can imagine (just) a form of mutation that was systematically biased in the direction of improving the animal’s adaptedness to its life. But although we can imagine it, nobody has ever come close to suggesting any means by which this bias could come about. It is only in this fifth respect, the ‘mutationist’ respect, that the true, real-life Darwinian insists that mutation is random. Mutation is not systematically biased in the direction of adaptive improvement, and no mechanism is known (to put the point mildly) that could guide mutation in directions that are nonrandom in this fifth sense. Mutation is random with respect to adaptive advantage, although it is nonrandom in all sorts of other respects. It is selection, and only selection, that directs evolution in directions that are nonrandom with respect to advantage. Mutationism is not just wrong in fact. It never could have been right. It is not in principle capable of explaining the evolution of improvement. Mutationism belongs with Lamarckism, not as a disproved rival to Darwinism but as no rival at all.

The same is true of my next alleged rival to Darwinian selection, championed by the Cambridge geneticist Gabriel Dover under the odd name ‘molecular drive’ (since everything is made of molecules it is not obvious why Dover’s hypothetical process should deserve the name
molecular
drive any more than any other evolutionary process; it reminds me of a man I knew who complained of a gastric stomach, and worked things out using his mental brain). Motoo Kimura and the other proponents of the neutralist theory of evolution do not, as we saw, make any false claims for their theory. They have no illusions about random drift being a rival to natural selection as an explanation for adaptive evolution. They recognize that only natural selection can drive evolution in adaptive directions. Their claim is simply that a lot of evolutionary change (as a molecular geneticist sees evolutionary change) is not adaptive. Dover makes no such modest claims for his theory. He thinks that he can explain
all
of evolution without natural selection, although he generously concedes that there may be
some
truth in natural selection as well!

Throughout this book, our first recourse when considering such matters has been to the example of the eye, although it has, of course, been only a representative of the large set of organs that are too complex and well designed to have come about by chance. Only natural selection, I have repeatedly argued, even comes close to offering a plausible explanation for the human eye and comparable organs of extreme perfection and complexity. Fortunately, Dover has explicitly risen to the challenge, and has offered his own explanation of the evolution of the eye. Assume, he says, that 1,000 steps of evolution are needed to evolve the eye from nothing. This means that a sequence of 1,000 genetic changes were needed to transform a bare patch of skin into an eye. This seems to me to be an acceptable assumption for the sake of argument. In the terms of Biomorph Land, it means that the bare-skin animal is 1,000 genetic steps distant from the eyed animal.

Now, how do we account for the fact that just the right set of 1,000 steps were taken to result in the eye as we know it? Natural selection’s explanation is well known. Reducing it to its simplest form, at each one of the 1,000 steps, mutation offered a number of alternatives, only one of which was favoured because it aided survival. The 1,000 steps of evolution represent 1,000 successive choice points, at each of which most of the alternatives led to death. The adaptive complexity of the modem eye is the endproduct of 1,000 successful unconscious ‘choices’. The species has followed a particular path through the labyrinth of all possibilities. There were 1,000 branch-points along the path, and at each one the survivors were the ones that happened to take the turning that led to improved eyesight. The wayside is littered with the dead bodies of the failures who took the wrong turning at each one of the 1,000 successive choice points. The eye that we know is the endproduct of a sequence of 1,000 successful selective ‘choices’.

That was (one way of expressing) natural selection’s explanation of the evolution of the eye in 1,000 steps. Now, what of Dover’s explanation? Basically, he argues that it wouldn’t have mattered which choice the lineage took at each step: it would retrospectively have found a use for the organ that resulted. Each step that the lineage took, according to him, was a random step. At Step 1, for example, a random mutation spread through the species. Since the newly evolved characteristic was functionally random, it didn’t aid the animals’ survival. So the species searched the world for a new place or new way of life in which they could use this new random feature that had been imposed upon their bodies. Having found a piece of environment that suited the random part of their bodies, they lived there for a while, until a new random mutation arose and spread through the species. Now the species had to scour the world for a new place or way of life where they could live with their new random bit. When they found it. Step 2 was completed. Now the Step 3 random mutation spread through the species, and so on for 1,000 steps, at the end of which the eye as we know it had been formed. Dover points out that the human eye happens to use what we call ‘visible’ light rather than infrared. But if random processes had happened to impose an infrared sensitive eye upon us, we would, doubtless, have made the most of it, and found a way of life that exploited infrared rays to the full.