Charles Darwin's theory of human evolution was published long before knowledge of genes was available. But an obscure letter found in a library proves Darwin was already doing research into heredity which anticipated the breakthroughs of the next cen
Humanity is the missing guest at the feast of Charles Darwin's Origin of Species, published in 1859. The famous "light will be thrown on the origin of man and hishistory" is a calculated understatement matched, in the annals of science, only by Watson and Crick's"it has not escaped our notice that the specific pairing we have postulated immediately suggests apossible copying mechanism for the genetic material".
By the time Darwin finally got around to throwing that light with the publication of The Descent of Man in1871, others had been there before him and the greater part of his book is not about humans but about Darwin's"other" theory, sexual selection. It might have seemed a good idea to separate it into two books: Sexual Selection followed by the Descent of Man. But Darwin knew what he was doing.
The distinguished American philosopher Daniel Dennett has credited Darwin with the greatest idea ever tooccur to a human mind. This was natural selection, the survival of the fittest, of course, and I would includesexual selection as part of the same idea. But Darwin was not only a deep thinker, he was a naturalist ofencyclopaedic knowledge and (which by no means necessarily follows) the ability to hold it in his head anddeploy it in constructive directions.
He was a master encyclopaedist, who collated huge quantities of information and observations solicited fromnaturalists all around the world, each gentleman meticulously acknowledged for having "attended to"the subject and sometimes complimented as a "reliable observer". I find an addictive fascination inhis Victorian prose style, quite apart from the feeling one gets of having been ushered into the presence ofone of the great minds of all time.
Prescient as he was (Michael Ghiselin, author of The Triumph of the Darwinian Method, University of ChicagoPress, has said that he worked at least a century ahead of his time) Darwin was still a Victorian, and hisbook must be read in the context of its age, warts and all. What will grate most irksomely on the modern earis the unquestioned Victorian presumption that animals in general, and humans in particular, are disposed on aladder of increasing superiority.
Like all Victorians, Darwin happily referred to particular species as "lowly in the scale ofnature". Even some modern biologists do this, though they should not, for all living species are cousinswho have been evolving for exactly the same length of time since the common ancestor. What educated modernsnever do, but equivalent Victorians always did, is think of human races in the same hierarchical way. Itrequires a special effort for us to read something like the following without distaste: "It seems atfirst sight a monstrous supposition that the jet blackness of the negro has been gained through sexualselection [ie, is attractive to the opposite sex]..."
But it is a mark of historical infantilism to view the writings of one century through the politicallytinted glasses of another. The very title, The Descent of Man, will raise hackles among those naively lockedinto the mores of our own time. It can be argued that reading historic documents that violate the taboos ofone's own century gives valuable lessons in the ephemerality of such mores. Who knows how our descendants willjudge us?
Less obvious, but as important to understand, are the changes in the scientific climate. In particular, itis hard to overstate the fact that Darwin's genetics were pre-Mendelian (Gregor Mendel, 1822-84, did not liveto see himself revered as the father of genetics). The intuitively plausible blending inheritance theory, thatvariation disappears over generations, of Darwin's time was not just wrong, it was grievously wrong andespecially grievous for natural selection.
The fact that Darwinism could not work under the assumption of blending inheritance was pointed out in ahostile review of the Origin by the Scottish engineer Fleeming Jenkin. Variation tends to disappear with everyblending generation, leaving not enough for natural selection to get its teeth into.
What Jenkin should have realised is that blending inheritance is incompatible not just with Darwiniantheory but with obvious fact. If it were really true that variation disappeared, every generation should bemore uniform than the previous one. By now, all individuals should be as indistinguishable as clones. Darwinneeded only to retort to Jenkin: whatever the reason, it is obviously the case that there is plenty ofinherited variation and that's good enough for my purposes.
It is often claimed that the answer to the riddle of the allegedly disappearing variation lay on Darwin'sshelves, in the uncut pages of the proceedings of the Brunn Natural History Society where nestled Mendel'spaper on "Versuche über Pflanzen-Hybriden". Unfortunately this poignant story seems to be an urbanmyth. The two scholars best placed (at Cambridge and at Down House) to know what was in Darwin's personallibrary can find no evidence that he ever subscribed to the proceedings, nor does it seem likely that he wouldhave done so. They have no idea where the legend of the "uncut pages" originated.
Once originated, however, it is easy to see that its very poignancy might speed its proliferation. Thewhole affair would make a nice little project in memetic research [the theory, outlined in The SelfishGene,that ideas are "viral"] complementing that other popular urban legend, the agreeable falsehood thatDarwin turned down an offer from Marx to dedicate Das Kapital to him.
Mendel did indeed have exactly the insight Darwin needed. Its relationship to the Jenkin critique, however,would not have been immediately obvious to the Victorian mind. Even after Mendel's work was rediscovered in1900, it was not until RA Fisher, founder of modern statistics and of British population genetics, came alongin 1930 that its supreme relevance to Darwinism was widely understood. If heredity is particulate - if, asMendel showed, a gene is an individisible entity such that you either have it or you don't, with no halfmeasures - variation does not disappear but is reconstituted in every generation. Neo-Darwinian evolutionprecisely means change in gene frequencies in gene pools.
What is genuinely poignant is that Darwin himself came tantalisingly close to Mendel's conclusion. Fisherquotes him in a letter to Huxley of 1857:
"I have lately been inclined to speculate, very crudely andindistinctly, that propagation by true fertilisation will turn out to be a sort of mixture, and not truefusion, of two distinct individuals, or rather of innumerable individuals, as each parent has its parents andancestors. I can understand on no other view the way in which crossed forms go back to so large an extent toancestral forms. But all this, of course, is infinitely crude."
Fisher cleverly remarked that Mendelism has a kind of necessary plausibility, which could have led to itsdiscovery by any thinker in a mid-Victorian armchair. He might have added that particulate inheritance staresus in the face whenever we contemplate sex itself (as we not infrequently do). All of us have one female andone male parent, yet each of us is either male or female, not an intermediate hermaphrodite. Fascinatingly,Darwin himself made this very point, clearly, in an 1866 letter to fellow naturalist Alfred Wallace, whichFisher would surely have quoted had he known of it.
"My dear Wallace...
I do not think you understand what I mean by the non-blending of certainvarieties. It does not refer to fertility; an instance will explain. I crossed the Painted Lady and Purplesweetpeas, which are very differently coloured varieties, and got, even out of the same pod, both varietiesperfect but none intermediate. Something of this kind I should think must occur at least with your butterflies& the three forms of Lythrum; tho' these cases are in appearance so wonderful, I do not know that they arereally more so than every female in the world producing distinct male and female offspring...
Believe me,
yours very sincerely
Ch. Darwin"
Here Darwin comes closer to anticipating Mendel than in the passage quoted by Fisher, and he even mentionshis own Mendel-like experiments on sweet peas. I am extremely grateful to Dr Seymour J Garte of New YorkUniversity, who found this letter by chance in a volume of correspondence between Darwin and Wallace in theBritish Library in London, immediately recognised its significance and sent a copy to me.
Another piece of Darwin's unfinished business later sorted out by Fisher was the matter of the sex ratio,and how it evolves under natural selection. Fisher begins by quoting the second edition of The Descent ofMan,in which Darwin prudently said:
"I formerly thought that when a tendency to produce the two sexes inequal numbers was advantageous to the species, it would follow from natural selection, but I now see that thewhole problem is so intricate that it is safer to leave its solution to the future."
Fisher's own solution made no appeal to species advantage. Instead he pointed out that, since everyindividual born has one father and one mother, the total male contribution to posterity must equal the totalfemale contribution. If the sex ratio is anything other than 50/50, therefore, an individual of the minoritysex can expect, other things being equal, a greater share of descendants, and this will set up selection infavour of rebalancing the sex ratio.
Fisher rightly used economic language to express the strategic decisions involved: they are decisions overhow to allocate parental expenditure. Natural selection will favour parents who spend proportionately morefood or other resources on offspring of the minority sex. Such correcting selection will continue until thetotal expenditure on sons in the population balances the total expenditure on daughters. This will amount toequal numbers of males and females, except in those cases where offspring of one sex cost more to rear thanoffspring of the other.
If, for example, it costs twice as much food to rear a son than a daughter (perhaps to make sons big enoughto compete effectively with rival males) the stable sex ratio will be twice as many females as males. This isbecause the strategic alternative to one son is not one daughter but two. Fisher's powerful logic has beenextended and refined in various ways, for example by WD Hamilton and EL Charnov.
Once again, and notwithstanding the quotation above from the second edition, Darwin himself, in the firstedition, came remarkably close to anticipating Fisher, although without the economic language of parentalexpenditure:
"Let us now take the case of a species producing, from the unknown causes just alluded to, an excessof one sex - we will say of males - these being superfluous and useless, or nearly useless. Could the sexes beequalised through natural selection? We may feel sure, from all characters being variable, that certain pairswould produce a somewhat less excess of males over females than other pairs. The former, supposing the actualnumber of the offspring to remain constant, would necessarily produce more females, and would therefore bemore productive. On the doctrine of chances, a greater number of the offspring of the more productive pairswould survive; and these would inherit a tendency to procreate fewer males and more females. Thus a tendencytoward equalisation of the sexes would be brought about."
Sadly, Darwin deleted this remarkable passage when he came to prepare the second edition, preferring themore cautious paragraph later to be quoted by Fisher. Darwin's partial anticipation of Fisher in the firstedition of Descent is all the more impressive because, as my colleague Alan Grafen points out to me, Fisher'sargument depends crucially on a fact that was not available to Darwin, namely that the two parents make anequal genetic contribution to every offspring. Indeed, in historical times, different schools of thought (thespermists and the ovists respectively) had held that the male, or the female, sex had a monopoly on heredity.
Now, to the descent of man itself. Darwin's guess that our species arose in Africa was typically ahead ofhis time, amply confirmed today by numerous fossils, none of which was available to him. We are African apes,closer cousins to chimpanzees and gorillas than they are to orang-utans and gibbons, let alone monkeys.
Darwin's "quadrumana" were defined so as to exclude humans: they were all the apes and monkeys,with a hand bearing an opposable digit on the hindlegs as well as the forelegs. The early chapters of his bookare concerned to narrow the perceived gap between ourselves and the quadrumana, a gap that Darwin's targetaudience would have seen as yawning between the top rung of a ladder and the next rung down. Today we wouldnot (or should not) see a ladder at all. Instead, we should hold in our minds the branching tree diagram,which is the only illustration in The Origin of Species. Humanity is just one little twig, nestling among manyothers somewhere in the middle of a thicket of African apes.
Darwin went to town on sexual selection in The Descent of Man because he thought it was important in humanevolution, and especially because he thought it was the key to understanding the differences among humanraces. The topic is prominent in Darwin's book and especially germane to the unification of its two parts.
Darwin, like all Victorians, was intensely aware of the differences among humans but he also, more thanmost of his contemporaries, emphasised the fundamental unity of our species. In Descent he carefullyconsidered, and decisively rejected, the idea, rather favoured in his own time, that different human racesshould be regarded as separate species.
Today we know that, at the genetic level, our species is more than usually uniform. It has been said thatthere is more genetic variation among the chimpanzees of a small region of Africa than among the entire worldpopulation of humans (suggesting that we have been through a bottleneck in the past 100,000 years or so).Moreover, the great majority of human genetic variation is to be found within races, not between them. Thismeans that if you were to wipe out all human races except one, the great majority of human genetic variancewould be preserved. The variance between races is just a bit extra, stuck on the top of the greater quantityof variation within all races. It is for this reason that many geneticists advocate the complete abandonmentof the concept of race.
At the same time - the paradox is similar to one recognised by Darwin - the superficially conspicuousfeatures characteristic of local populations around the world seem very different. Why did such pronouncedsuperficial differences evolve in different geographical areas, while most of the less conspicuous variationis dotted around across all geographical areas? Could Darwin have been right all along? Is sexual selectionthe answer to the paradox? The distinguished biologist Jared Diamond thinks so, and I am inclined to agree.
What sexual selection explains, better than natural selection, is diversity that seems arbitrary, evendriven by aesthetic whim. Especially if the variation concerned is geographical. And also especially if someof the features concerned, for example beards and the distribution of body hair and subcutaneous fat deposits,differ between the sexes.
Most people have no problem in accepting an analogue of sexual selection for culturally mediated fashionslike headdresses, body paint, penis sheaths, ritual mutilations or ornamental clothes. Given that culturaldifferences such as those of language, religion, manners and customs certainly provide resistance tointerbreeding and gene flow, I think it is entirely plausible that genetic differences between peoples ofdifferent regions, at least where superficial, externally prominent features are concerned, have evolvedthrough sexual selection.
Our species really does seem to have unusually conspicuous, even ostentatious, superficial differencesbetween local populations, coupled with unusually low levels of overall genetic variation. This doublecircumstance carries, to my mind, the stamp of sexual selection.
In this, as in so much else, I suspect that Darwin was right. Sexual selection really is a good candidatefor explaining a great deal about the unique evolution of our species. It may also be responsible for someunique features of our species that are shared equally by all races, for example our enormous brain. It isstarting to look as though, despite initial appearances, Darwin really was right to bring together, in onevolume, Selection in Relation to Sex and the Descent of Man.
© Richard Dawkins, 2003. This is an edited version of Richard Dawkins' introduction to a new edition ofCharles Darwin's The Descent ofMan, published by Gibson Square Books. This piece also appears in A Devil's Chaplain: Selected Essays by Richard Dawkins, published by Weidenfeld.