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OU Lecture 2009: The fifth bridge

Updated Monday, 27th April 2009

Dawkins explores the ideas of Darwin’s peers including Fleeming Jenkin and Sewall Wright, and discusses how close Darwin came to Mendelism.

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Darwin never crossed the digital bridge. If he had, he would have had a ready answer to Fleeming Jenkin, the Scottish engineer who, independently of his colleague Lord Kelvin with who he collaborated on the transatlantic cable, gave Darwin a hard time over matters of theory.

Jenkin pointed out that, on the current non-digital blending view of heredity, variation would be swamped by successive sexual crossings and, after a few generations, would disappear. There’d be no hereditary variation for natural selection to work on. Blending inheritance would be like mixing black and white paint, you get grey, and no amount of subsequent mixing of grey with grey will give you back the original black and white.

As a matter of fact, any fool could have seen that Jenkin’s premise must be wrong. Variation does not dissolve away as the generations go by; we are not more uniform than our grandparents were, and our grandchildren will retain the same level of variation as we possess. Jenkin thought he was doubting Darwin. Actually, he was doubting observable facts. Nevertheless, his criticism worried Darwin.

Enlightened by Mendel’s 19th Century peas and building on Hardy and Weinberg’s elementary algebra, the 20th Century founders of population genetics, RA Fisher, JBS Haldane and Sewall Wright buried Fleeming Jenkin.

“If genes are countable digital entities that don’t blend, their frequencies have no inherent tendency to change. If they do change, that is evolution, and it happens for a reason. The most interesting reason is non-random selection but random drift also occurs. To an extent, disputed among the founding fathers but now widely admitted among molecular geneticists.”

Sewall Wright, by the way, did research on the genetics of guinea pigs, and you see him holding one there in front of a blackboard covered with mathematical genetics. There’s a persistent rumour that immediately after this picture was taken, Wright picked up the guinea pig and absentmindedly used it to clean off the blackboard!

Even the three founding fathers of population genetics never knew quite how digital it really is. In the light of the Watson-Crick revolution, we now see the very genes themselves, within themselves, as digitally coded messages, digital in exactly the same sense and in the same way to an astonishing level of detail as computer information is digital.

Of the three founding fathers, it was Fisher who, in his great book of 1930, The Genetical Theory of Natural Selection, most clearly expressed the evolutionary significance of blending inheritance and its Mendelian antithesis. If genes did indeed blend, the variance available for selection would be halved in every generation. It's the grey paint all over again but Fisher proved it mathematically. Mutation rates would have to be colossal, utterly unrealistic, to maintain the variation.

Fisher quotes a letter from Darwin to Huxley, tentatively dated to 1857 before the Origin which shows how tantalisingly close Darwin himself came to Mendelism.

“I have lately been inclined to speculate very crudely and indistinctly that propagation by true fertilisation will turn out to be a sort of mixture and not true fusion of two distinct individuals or rather of innumerable individuals as each parent has its parents and ancestors. I can understand on no other view the way in which crossed forms go back to so large an extent to ancestral forms. But all this, of course, is infinitely crude.”

Even Fisher didn’t know how breathtakingly near Darwin really was to discovering Mendelian genetics, even working on sweet peas.

In 1867, Darwin wrote a letter to Wallace that began as follows: “My dear Wallace. I do not think you understand what I mean by the non-blending of certain varieties. It does not refer to fertility; an instance will explain. I crossed the painted lady and purple sweet peas, which are very different coloured varieties, and got even out of the same pod both varieties perfect but none intermediate. Something of this kind, I should think, must occur at first with your butterflies. Though these cases are in appearance so wonderful, I do not know that they are really more so than every female in the world producing distinct male and female offspring.”

That last sentence is a beautiful example of the power of reason and the importance of seeing through the obvious. When a male mates with a female, you do not get a hermaphrodite, you get either a male or a female with approximately equal probability. In a way, Mendel never needed to go into his monastery garden. All he had to do was take the inheritance of sex itself and generalise it to all other cases of inheritance. Digital heredity was staring us in the face in the most obvious way you could imagine. The trouble was, it was too obvious to be noticed.

Darwin noticed it and he came close to making the connection but, just as Patrick Matthew didn’t quite cross the bridge that Darwin and Wallace crossed, so Darwin didn’t quite manage to cross the Mendel-Fisher bridge, at least not decisively enough to answer Fleeming Jenkin.

I distinguished bridge one from bridge two as stabilising selection versus directional selection, but there’s more to it than that, or perhaps the distinction I'm about to make really separates Matthew’s bridge two from Darwin and Wallace’s bridge three. I'm talking about the distinction between selection as a negative force and selection as a positive, constructive force that puts together complex new designs. My own preferred way, the selfish gene way, of explaining this, is again to deploy digital genes, so perhaps we really have to cross bridge five in order to paint the full picture.

In modern genetic terms, and not Darwin’s own, natural selection may be defined as the non-random survival of randomly varying coded instructions for how to survive. We see and admire the product, the phenotypes of the good ones. The instructions are DNA and their most visible products are bodies that survive by doing something impressive such as flying, swimming, running, digging or climbing - all in the service of reproduction, which means they also tend to be good at attracting a mate and warding off rivals.

An important part of the environment that each gene must exploit, if it's to ensure its survival in the form of copies of itself, is the other genes it encounters in the genomes of a succession of bodies which, because of sexual recombination, means the other genes in the gene pool of the species. As a result of this, cartels of mutually supportive genes co-operate to build bodies that specialise in some particular method of surviving, such as grazing or hunting. Different cartels are the gene pools of different species bound together by the remarkable phenomenon of sexual recombination and separated from all other cartels, for it is part of the definition of species that they can’t interbreed.

Occasionally, often through accidents of geography, gene pools find themselves subdivided for long enough to become sexually incompatible and the subdivisions are then free to go their separate evolutionary ways as distinct species. Eventually, separate ways can mean very separate indeed, for animals as different as vertebrates and molluscs originally split apart as members of the same species. Successive branchings of this kind have given rise to hundreds of millions of species over thousands of millions of years.

At least in sexually reproducing species, evolution consists of changes in gene frequencies in gene pools. I stipulate sexual reproduction because, without it, we have no clear idea what gene pool even means. Where there is sexual reproduction, the gene pool is the set of available alleles from which the individual members of a species draw their genomes, draw as in a lottery, the lottery of sex. Each individual genome is like a shuffled pack of cards. The available cards to be shuffled are sampled from the gene pool.

The statistical frequencies of these available cards change as the generations go by and that is evolution. We can monitor evolution by measuring a sample of the phenotypes, the anatomy, physiology of typical members of the population. As the average phenotype changes, as legs get shorter, horns longer, coats shaggier, whatever happens to be evolving at the time, it is tempting to see natural selection as a sculptor’s chisel carving the bones and flesh of the animals themselves.

But if we want to talk chisels, a sharper representation of evolution sees them as working not on the bodies of animals but on the statistical structure of the gene pool. As crests get longer or eyes rounder or tails gaudier, what is really being carved by natural selection is the gene pool. As mutation and sexual recombination enrich the gene pool, the chisels of natural selection carve it into shape.

We observe the results in the form of changes in the average phenotype and it's phenotypes that serve as the proxies for genes. As the external and visible manifestations of genes, they determine whether those genes are eliminated, or whether they persist in the gene pool.

Natural selection carves and whittles gene pools into shape, working away through geological time. This is an image that might have seemed strange to Darwin, but I think he would have come to love it. Thank you very much.

This clip is from the Professor Richard Dawkins lecture given on Tuesday 17th March 2009 at the Natural History Museum, London.





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