Richard Dawkins delivered this year's Open University lecture at the Natural History Museum on Tuesday 17th March 2009. Dawkins’ presented to an invited audience at the Natural History Museum, and investigated if Darwin was the most revolutionary scientist ever, and examined the evolutionary theories of his contemporaries.

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Was Darwin the most revolutionary scientist ever? If by revolutionary we mean the scientist whose discovery initiated the most seismic overturning of pre-existing science, the honour would at least be contested by Newton, Einstein and the architects of Quantum Theory. Those same physicists might have outclassed Darwin in sheer intellectual fire-power but Darwin probably did revolutionise the world view of people outside science more comprehensively than any other scientist. He may be only one plausible candidate for the most important or most revolutionary scientist ever, but Darwin has a strong claim to be the most seditious.

Before Darwin it took a philosopher of the calibre of David Hume to rumble the illogic of if a thing looks designed it must have been designed. And even Hume, though he could see that the argument to design was a bad argument, couldn’t think of a good alternative. Darwin provided the alternative. How Hume would have relished the ‘I told you so’ moment that Darwin handed him. The argument to design was familiar to Darwin, for whose cohort of Cambridge Undergraduates the Reverend William Paley was compulsory reading. If it looks designed it was designed, and the more designed it looks the stronger the argument.

Looks designed means something along the lines of statistically improbable in a previously specified direction. Paley’s famous watch and the vertebrate eye are both statistically improbable, in that if you take their parts and scramble them into random combinations a million times not once will you hit upon a combination that tells the time to the nearest second or that sees in full colour stereoscopically and with instantaneous light metering and auto-focus. We have to add in a previously specified direction because with hindsight every random combination can be made to seem as improbable as any other.

How astounding that of all the blades of grass on the golf course the ball landed on this particular blade and no other. The reason a hole-in-one is so rare is that the hole is specified in advance as the target. If you specified any particular blade of grass in advance and the ball landed on it, it would be as remarkable as a hole-in-one. Actually more so because the hole is larger than a blade of grass. Watches and eyes have their functions, telling the time and seeing respectively, specified in advance, and both are functions that are difficult to achieve.

So a random scrambling of parts is exceedingly unlikely to perform either function with any efficiency. The fact that a watch does tell the time accurately, and with at least two hands to accommodate two conveniently related timescales, correctly indicates to any reasonable person that it is not the product of random chance. Before Darwin the only known alternative to random chance was design. Everybody could intuitively see the force of the argument that Paley generalised from watch to eye, and to every other part of every living body. There must have been a designer. And yet intuition was wrong. It’s the unholy juxtaposition of commonsensically true with now known to be false that singles out Darwin’s great idea as seditious.

Darwin discovered the alternative to chance and design that had eluded everybody, even Hume. The answer is cumulative natural selection. Provided that a smoothly cumulative gradient of improvement exists, not a difficult condition to realise, natural selection is likely to find it and will propel evolution up the slopes of mount improbable to apparently limitless heights of perfection which, if you overlook the smooth cumulative gradients, you would think were too improbable to countenance.

Darwin’s dangerous idea was seditious, revolutionary, deeply surprising and yet, having eluded Hume in the 18th Century and every great philosopher and scientist before him, it was an idea that came independently into the prepared minds of at least two naturalists in the 19th Century, Charles Darwin and Alfred Russel Wallace. I’m not talking about evolution itself for that idea had occurred to many, including Lamarck and Darwin’s Grandfather Erasmus. Nor am I talking about natural selection itself for that too, as we shall see, had crossed other minds than Darwin’s and Wallace’s. I’m talking about the idea that natural selection is powerful enough to drive evolution in such a way as to explain everything about life, including that illusion of design that, in Hume’s own words, ravishes into admiration all men who have ever contemplated them.

I singled out Darwin and Wallace as the two 19th Century naturalists who independently solved the riddle of life, but claims of priority have been made on behalf of at least two other 19th Century writers; Patrick Matthew and Edward Blyth. If those claims are upheld, it should be a matter of some national pride that all four independent discoverers of natural selection were British. But should they be upheld?

Edward Blyth, 1810-73, was Darwin’s near contemporary. Like Darwin and Wallace he was a naturalist and collector of specimens in the tropics, in his case India. He really did hit upon the idea of natural selection, publishing it in 1835, but his version is only what we would today call stabilising selection, that is natural selection preserving the original type, not natural selection driving evolutionary change to ever new types. No wonder Blyth was a staunch creationist, he thought of natural selection as preserving God’s original creations in their pristine archetypal state. He was the very opposite of an evolutionist. Natural selection in his formulation would amount of a force of resistance against evolutionary change.

Patrick Matthew, 1790-1874, used his experience of growing apple and pear trees in his Scottish orchard to write a book in 1831 on Naval Timber and Arboriculture. In an appendix to this work, Matthew recognised that the principles of artificial selection, which he advocated for growing good quality timber for the Navy, could be generalised to natural selection. Unlike Blyth, Matthew didn’t see natural selection purely as a stabilising force preserving the original form of the species. He even went so far as to speculate that ‘the progeny of the same parents under great differences of circumstance might in several generations even become distinct species incapable of co-reproduction’.

When The Origin of Species was first published, Matthew protested at Darwin’s failure to cite him, and Darwin punctiliously did so in later editions of his book. The passage that immediately follows the sentence I just quoted seems to bear out Darwin’s acknowledgement that Matthew clearly saw the full force of the principle of natural selection. Like Blyth, indeed Darwin seems to have been indebted to Blyth’s observations, Matthew saw the importance of over-production and the consequent struggle for existence.

“The self-regulating adaptive disposition of organised life may in part be traced to the extreme fecundity of nature who, as before stated, has in all the varieties of her offspring a prolific power much beyond, in many cases a thousand-fold, what is necessary to fill up the vacancies caused by senile decay. As the field of existence is limited and preoccupied, it is only the hardier, more robust, better suited to circumstance individuals who are able to struggle forward to maturity, these inhabiting only the situations to which they have superior adaptation and greater power of occupancy than any other kind, the weaker, less circumstance-suited being prematurely destroyed. This principle is in constant action, it regulates the colour, the figure, the capacities and instincts. Those individuals of each species whose colour and covering are best suited to concealment or protection from enemies, or defence from vicissitude and inclemencies of climate whose figure is best accommodated to health, strength, defence and support, whose capacities and instincts can best regulate the physical energies to self-advantage according to circumstances in such immense waste of primary and youthful life, those only come forward to maturity from the strict ordeal by which nature tests their adaptation, their standard of perfection and fitness to continue their kind by reproduction.”

Well, I’m still left wondering did Matthew really grasp the immense power of the discovery that he had made. Did he appreciate that natural selection is the answer to the great riddle of existence? Did he see it as the explanation for all of life, the destroyer of the argument from design? If he had, wouldn’t he have published it in a more prominent place than the appendix to a manual on naval timber? Wouldn’t he have trumpeted it from the rooftops as arguably the most important idea anyone ever had? On the contrary, Matthew seems to have found the idea so obvious, almost trivial, as to need no discovery.

In a letter to the Gardeners’ Chronicle of 12th May 1860, he wrote, “To me the conception of this law of nature came intuitively as a self-evident fact, almost without an effort of concentrated thought. Mr Darwin seems to have had more merit in the discovery than I have had. To me it did not appear a discovery. He seems to have worked it out by inductive reason, slowly and with due caution to have made his way synthetically from fact to fact onwards, while with me it was by a general glance at the scheme of nature that I estimated this select production of species as an a priori recognisable fact, an axiom requiring only to be pointed out to be admitted by unprejudiced minds of sufficient grasp.”

With hindsight we may be tempted to sympathise, but where Huxley on closing The Origin movingly said, “How extremely stupid of me not to have thought of that”, Matthew’s response would seem to have been the Victorian equivalent of big deal, so what else is new. Is this the response of a man who seven years before Darwin and twenty-seven before Wallace found himself in possession of the central unifying idea that dominates all biology and explains everything about life?

As a fair parallel, imagine that a 17th Century ancestor of Patrick Matthew saw an apple fall, perhaps in the very same orchard where the Matthews had been farming in the Carse of Gowrie since the 16th Century. Our earlier Matthew I imagine to have been a physicist, and as he watched his apple fall he conjectured that the earth exerted an attractive force on apples pulling them towards it. If this hypothetical horticulturalist had later written to Isaac Newton and indignantly claimed priority for the theory of gravitation Newton, a less generous man than Darwin, would rightly have given him short shrift. The physicist Matthew let’s suppose confined his theory to apples, or at best to objects falling towards the earth. He lacked Newton’s grand vision of the same force acting throughout the universe, responsible for the elliptical orbits of the planets, for the stars in their courses, ultimately for the very structure of the universe itself.

I agree with WJ Dempster, Patrick Matthew’s modern champion, that Matthew has been unkindly treated by history. But, unlike Dempster, I hesitate to assign full priority to him. Partly it's because he wrote in a much more obscure style than either Darwin or Wallace, which makes him hard to understand. Darwin himself noted this. But mostly it's because he seems to have underestimated it to an extent where we have to doubt whether he really understood how important it was.

And the same could be said even more strongly - which is why I haven't treated his case in the same detail as Matthews - of WC Wells, whom Darwin also scrupulously acknowledged in the fourth and subsequent editions of The Origin. Wells made the leap to generalise from artificial to natural selection, but he applied it only to humans, and he thought of it as choosing among races of people rather than individuals (as Darwin and Wallace did). Wells then seems to have arrived at a form of group selection, rather than true Darwinian natural selection (as Matthew did) which selects individual organisms for their reproductive success.

And Darwin also lists other partial predecessors who had shadowy inklings of natural selection. Like Patrick Matthew, none of them seems to have grasped the earth-shattering significance of the idea they’d lit upon, and I shall use Matthew’s name to represent them all. I am increasingly inclined to agree with Matthew that natural selection itself scarcely needed discovering. What needed discovering was the significance of natural selection for the evolution of all life.

Alfred Russel Wallace (1823-1913) was different. Although he discovered natural selection later than Patrick Matthew and later than Darwin’s unpublished manuscripts, he has a genuine claim to be up there with Darwin and Newton among the immortals. When Wallace hit upon natural selection, he was in no doubt of its immense importance for the whole history of life. The very title of his paper, the one he sent to Darwin and which set the cat among Darwin’s pigeons says it all: "On the Tendency of Varieties to Depart Indefinitely from the Original Type". "Depart indefinitely" - that was the key phrase. If they departed indefinitely from the original type, they can branch and eventually spawn all of life, and Wallace made that explicit.

The drama of how Wallace’s letter arrived at Down House on the 17th of June 1858, casting Darwin into an agony of indecision and worry, is too well known for me to retell it. In my view, the whole episode is one of the more creditable and agreeable in the history of scientific priority disputes, precisely because it wasn’t a dispute, although it so easily could have become one. It was resolved amicably and with heart-warming generosity on both sides, especially Wallace’s.

As Darwin later wrote: “Early in 1856 Lyell advised me to write out my views pretty fully, and I began at once to do so on a scale three or four times as extensive as that which was afterwards followed by my Origin of Species, yet it was only a abstract of the materials which I had collected, and I had got through about half the work on this scale. But my plans were overthrown, for early in the summer of 1858 Mr Wallace, who was then in the Malay Archipelago, sent me an essay, "On the Tendency of Varieties to Depart Indefinitely from the Original Type", and this essay contained exactly the same theory as mine. Mr Wallace expressed the wish that if I thought well of this essay, I should send it to Lyell for perusal.

The circumstances under which I consented at the request of Lyell and Hooker to allow an extract from my manuscript, together with a letter to Asa Gray, dated September the 5th, 1857, to be published at the same time with Wallace’s essay, are given in the Journal of the Proceedings of the Linnean Society. I was at first very unwilling to consent, as I thought that Mr Wallace might consider my doing so unjustifiable, for I did not then know how generous and noble was his disposition. The extract from my manuscript and the letter to Asa Gray had neither been intended for publication and were badly written.

Mr Wallace’s essay, on the other hand, was admirably expressed and quite clear. Nevertheless, our joint productions excited very little attention, and the only published notice of them which I can remember was by Professor Horton of Dublin whose verdict was that all that was new in them was false and what was true was old. This shows how necessary it is that any new view should be explained at considerable length in order to arouse public attention.”

Darwin was over-modest about his own two papers. Both are models of the explainer’s art. Wallace’s paper is also very clearly argued. His ideas were indeed remarkably similar to Darwin’s, and there’s no doubt at all that Wallace arrived at them independently. In my opinion, the Wallace paper needs to be read in conjunction with his earlier paper in the Annals and Magazine of Natural History.

Darwin read this paper when it came out in 1855 and, in fact, it led to Wallace joining his large circle of correspondents and to Darwin’s engaging Wallace’s services as a collector. But, oddly, Darwin didn’t see in the 1855 paper any warning that Wallace was by then a convinced evolutionist of a very Darwinian stamp. I mean as opposed to the Lamarckian view of evolution which saw modern species as all on a ladder, changing into one another as they moved up the ladder.

By contrast, Wallace, in 1855, had a clear view of evolution as a branching tree, exactly like Darwin’s famous diagram which became the only illustration in The Origin of Species. The 1855 paper, however, makes no mention of natural selection or the struggle for existence. That was left to Wallace’s 1858 paper, the one that hit Darwin like a lightning bolt. Here Wallace even used the phrase "struggle for existence." Wallace devoted considerable attention to the exponential increase in numbers, another key Darwinian point.

Wallace wrote: “The greater or less fecundity of an animal is often considered to be one of the chief causes of its abundance or scarcity. But a consideration of the facts will show us that it really has little or nothing to do with a matter, even the least prolific of animals would increase rapidly if unchecked, whereas it is evident that the animal population of the globe must be stationary or perhaps decreasing.” And Wallace deduced from this that “the numbers that die annually must be immense and as the individual existence of each animal depends upon itself, those that die must be the weakest.”

Wallace’s peroration could have been Darwin himself writing.

“The powerful retractile talons of the falcon and the cat tribes have not been produced or increased by the volition of those animals; but among different varieties which occurred in the earlier and less highly organised forms of these groups, those always survived longest which had the greatest facilities for seizing their prey. Neither did the giraffe acquire its long neck by desiring to reach the foliage of the more lofty shrubs, and constantly stretching its neck for the purpose, but because any varieties which occurred among its antitypes with a longer neck than usual at once secured a fresh range of pasture over the same ground as their shorter-necked companions, and on the first scarcity of food were thereby enabled to outlive them.

"Even the peculiar colours of many animals, especially insects, so closely resembling the soil or the leaves or the trunks on which they habitually reside, are explained on the same principle; for though in the course of ages varieties of many tints may have occurred, yet those races having colours best adapted to concealment from their enemies would inevitably survive the longest. We have also here an acting cause to account for that balance so often observed in nature; a deficiency in one set of organs always being compensated by an increased development of some others, powerful wings accompanying weak feet, or great velocity making up for the absence of defensive weapons.

"For it has been shown that all varieties in which an unbalanced deficiency occurred could not long continue their existence. The action of this principle is exactly like that of the centrifugal governor of the steam engine, which checks and corrects any irregularities almost before they become evident.”

The image of the steam governor is a powerful one which I can’t help feeling Darwin might have envied.

Historians of science have raised the suggestion that Wallace’s version of natural selection was not quite so Darwinian as Darwin himself believed. Wallace persistently used the word "variety" as the level of entity at which natural selection acts, and there was an example in this long passage I just read, an example of Wallace’s using the word "race’" in a similar sense. Some people have suggested that Wallace, unlike Darwin, who clearly saw selection as choosing among individuals, was proposing what nearly all modern theorists rightly denigrate as group selection.

This would be true if by "varieties" or "races" Wallace meant geographically separated groups of individuals or races in the more usual sense of the word. At first, I wondered myself whether Wallace meant that. But a careful reading of his paper rules it out. By "variety" and "race", Wallace meant what we would nowadays call genetic type, even what a modern population geneticist might mean by an allele. To Wallace, in this paper, "variety" meant not a local race of eagles, for example, but that set of individual eagles whose talons, say, were hereditarily sharper than usual.

If I'm right, it's a similar misunderstanding to the one suffered rather grievously by Darwin whose use of the word "race" in the subtitle of The Origin of Species is all too often misread as supporting group selection or even racialism. That subtitle or rather alternative title is "the preservation of favoured races in the struggle for life".

Once again, Darwin was using "race" to mean that set of individuals who share a particular hereditary characteristic such as sharp talons, not a geographically distinct race such as the hooded crow. If he had meant that, Darwin too would have been guilty of the group selection confusion. I believe that neither Darwin nor Wallace was, and by the same token, I do not believe that Wallace’s conception of natural selection was different from Darwin’s.

I should add, by the way, that Darwin did countenance a form of group selection on one occasion when discussing the evolution of co-operation in humans. I think that was the only occasion.

As for the calumny that Darwin plagiarised Wallace, that’s rubbish. The evidence is very clear that Darwin did think of natural selection before Wallace, although he didn’t publish it. We have his abstract of 1842, his longer essay of 1844, both of which establish his priority clearly, as did his letter to Asa Gray of 1857, read at the Linnean Society in 1858.

Why Darwin delayed so long before publishing is one of the great mysteries in the history of science. Some historians have suggested that he was afraid of the religious implications, others the political ones. Perhaps he was afraid of upsetting his devout wife. Maybe he was just a perfectionist keen to have all his evidence lined up and in place before going public. Or did he just get distracted by barnacles?

When Wallace’s letter arrived, Darwin was more surprised than we moderns might think he had any right to be. He wrote to Lyell: “I never saw a more striking coincidence. If Wallace had had my manuscript sketch written out in 1842, he could not have made a better short abstract of it. Even his terms now stand as heads of my chapters.”

The coincidence extended to both Darwin and Wallace being inspired by Thomas Malthus on population. Darwin, by his own account, was immediately struck by Malthus’ emphasis on overpopulation and competition. He wrote in his autobiography: “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 continuous 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.”

Wallace’s epiphany after reading Malthus took longer to happen but was more dramatic when it came to his overheated brain in the midst of a malarial fever on the Island of Ternata in the Moluccas Archipelago.

“I was suffering from a sharp attack of intermittent fever, and every day during the cold and succeeding hot fits had to lie down for several hours, during which time I had nothing to do but to think over any subjects then particularly interesting me. One day something brought to my recollection Malthus’ principles of population. I thought of his clear exposition of the positive checks to increase, disease, accidents, war and famine which keep down the population of savage races to so much lower on average than that of more civilised peoples. It then occurred to me…” and Wallace proceeds to his own admirably clear exposition of natural selection as the guiding principle of all evolution.

I want to recognise four bridges to evolutionary understanding, and I can illustrate them with our four claimants to independent discovery of natural selection. Blyth crossed the first of Darwin’s four bridges, Matthew the first two, Wallace the first three and Darwin all four.

The first bridge is to natural selection as a force for weeding out the unfit. I’ve used Blyth as my example of a 19th century writer who crossed this bridge, but really the only reason to single him out is that he has been championed by Loren Eisely as a predecessor and even a possible source of Darwin’s ideas. As Stephen Gould has argued, however, the idea of natural selection as a weeder out, a purely negative force, was already widespread.

“Yes, Blyth had discussed natural selection but Eisely didn’t realise, thus committing the usual and fateful error in this common line of argument, that all good biologists did so in the generations before Darwin. Natural selection ranked as a standard item in biological discourse but with a crucial difference from Darwin’s version. The usual interpretation invoked natural selection as part of a larger argument for created permanency. Natural selection in this negative formulation acted only to preserve the type constant and inviolate by eliminating extreme variants and unfit individuals who threaten to degrade the essence of created form.”

Gould even quotes William Paley himself as setting out this purely negative version of natural selection. As I remarked before, it's almost an anti-evolution argument, for it uses natural selection to explain the fixity of species rather than their changing into other species.

Darwin’s second bridge is the recognition that natural selection can drive evolutionary change. In modern jargon, it amounts to the difference between stabilising selection and directional selection. Matthew, Wallace and Darwin all crossed this second bridge.

Bridge number three leads to the imaginative grasp of the importance of natural selection in explaining all of life, in all of its speciose richness and especially to dispel the illusion of design. Wallace and Darwin certainly crossed it, maybe Matthew did too, but I’ve given reasons for doubting that he developed the full imaginative vision of the "constructive power" of Darwinism, as Wallace in a generous gesture was later to dub it.

Bridge four is the bridge to public understanding and appreciation. Darwin crossed it alone in 1859 by writing The Origin of Species. It's a striking fact, remarked by Darwin himself among others, that when the Darwin/Wallace papers were read to the Linnean Society in 1858, nobody took a blind bit of notice, even among the professional biologists of that august body. The end of year clanger of the hapless President of the Linnean, Thomas Bell, has become notorious and will ring on down the ages.

In his review of the Society’s transactions during 1858 he said that “the year had not been marked by any of those striking discoveries which at once revolutionise the Department of Science on which they bear.” The end of 1859 would have to be reviewed very differently.

The Origin of Species struck the Victorian solar plexus like a steam hammer. The world of the mind would never be the same again, neither science nor anthropology, psychology, sociology, even - and here we come close to the dark side - politics. This book, which Darwin always described as the abstract of the great book that he intended to write but never completed, achieved what the 1858 papers did not.

It isn't that The Origin explained the theory particularly more clearly than Darwin’s and Wallace’s brief offerings of 1858. The difference was that a book length treatment was required to muster all the evidence and lay it out for all to see. One long argument, as Darwin himself called it. And I quoted before Darwin’s own recognition when the joint papers of 1858 fell flat, that “this shows how necessary it is that any new view should be explained at considerable length in order to arouse public attention.”

And is there a fifth bridge which Darwin himself never crossed? Inevitably, a hundred and fifty years later, there are several, but the one that I shall single out is the bridge to the so-called Neo-Darwinism of the modern synthesis. Neo-Darwinism is a union of Darwinian evolution with Mendelian genetics. But the trouble with a word like "neo" is that what is "neo" changes all the time. What comes after nouvelle vague? We don’t want to get into a sort of infinite progress in the way that modernism gives rise to postmodernism and then neo-postmodernism and then what? I shall rename neo-Darwinism, digital-Darwinism.

There may be other things more "neo" than the neo-Darwinian modern synthesis of the 1930s, but digital-Darwinism is here to stay. The essence of Mendelian genetics is that it is digital. Mendelian genes are all or none, they don’t blend. Genes are things you can count in a population’s gene pool. Evolution consists of changing frequencies, of discrete digital countable entities. Not of changing quantities of substances or changing measurements of dimensions. Changing quantities and measurements apply at the organism level but not at the gene level. What happens in natural selection is that successful genes become more frequent in the gene pool and unsuccessful genes become less frequent - frequent as in counted.

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 Fleming 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.