Instead, we should hold in our minds the
branching
tree diagram which is the only illustration in The Origin ofSpecies.
Richard-Dawkins-The-Devil-s-Chaplain
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exposure of the nature/nurture cliche. A flurry of publicity in 1993 for a so-called 'gay gene' on the X chromosome led to an invitation from the Daily Telegraph to expose the myths of 'genetic determinism'. The result was the piece reproduced here as Genes Aren't Us (2. 4).
My literary agent John Brockman has the charisma to persuade his clients and others to drop everything and contribute to books of his own editing, even in the teeth of the better commercial judgement he might normally advise them to deploy. The distinction of his guest list flatters them in through the door of his salon (http://www. edge. org/) and before they know where they are they are correcting the proofs for a printed spin- off. Son of Moore's Law (2. 5) was my futurological contribution to a typically fascinating on-line symposium, The Next Fifty Years.
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Foreword to a new Student Edition of Darwin's Descent of Man "
"!
Light Will Be Thrown
Humanity is the missing guest at the feast of The Origin of Species. The famous 'Light will be thrown on the origin of man and his history' 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 a possible copying mechanism for the genetic material. ' By the time Darwin finally got around to throwing that light in 1871, others had been there before him. And the greater part of The Descent of Man is not about humans but about Darwin's 'other' theory, sexual selection.
The Descent of Man was conceived as a single book but ended up as three, two of them bound together under the same title, with the second topic signalled by the subtitle, Selection in Relation to Sex. The third was The Expression of the Emotions, not my concern here, but Darwin tells us that it grew out of the original Descent, and he began writing it immediately after finishing Descent. Given that the idea of splitting the book was in Darwin's mind, it is at first sight surprising that he didn't spin off sexual selection as well. It would have seemed natural to publish chapters 8 to 18 as Selection in Relation to Sex followed by a second book, The Descent of Man, consisting of the present Chapters 1 to 8, and 19 to 21. That's a neat split into eleven chapters
for each book, and many have wondered why he did not do this. I shall follow the same order - sexual selection followed by the descent of man - and then return at the end to the question of whether the two might have been split. In addition to discussing Darwin's book, I shall try to give some pointers to where the subject is moving today.
The ostensible connection between sexual selection and the descent of man is that Darwin believed the first was a key to understanding the second; especially to understanding human races, a topic which preoccupied Victorians more than it does us. But, as the historian and philosopher of science Michael Ruse has remarked to me, there was a
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tighter thread binding the two topics. They were the only two sources
of disagreement between Darwin and his co-discoverer of natural
selection. Alfred Russel Wallace never took kindly to sexual selection, at
least in its full-blooded Darwinian form. And Wallace, though he
coined the word Darwinism and described himself as 'more Darwinian
than Darwin', stopped short of the materialism implied by Darwin's
view of the human mind. These disagreements with Wallace were all
the more important to Darwin because these two great men agreed on
almost everything else. Darwin himself said, in a letter to Wallace of
28 1867:
The reason of my being so much interested just at present about sexual selection is, that I have almost resolved to publish a little essay on the origin of Mankind, and I still strongly think (though I failed to convince you, and this, to me is the heaviest blow possible) that sexual selection has been the main agent in forming the races of man.
The Descent ofMan and Selection in Relation to Sex could be seen, then, as Darwin's two-pronged answer to Wallace. But it's also possible - and anyone who reads those chapters would forgive him - that he just got carried away by his enthusiasm for sexual selection.
The disagreements between Darwin and Wallace over sexual selection have been teased out by the Darwinian philosopher and historian
29
Helena Cronin in her stylish book The Ant and the Peacock.
follows the two threads to the present day, classifying later theorists of sexual selection as 'Wallaceans' and 'Darwinians'. Darwin rejoiced in sexual selection. The naturalist in him loved the extravagant ostenta- tion of stag beetles and pheasants, while the theorist and teacher knew that survival is only a means to the end of reproduction. Wallace could not stomach aesthetic whim as a sufficient explanation for the evolu- tion of bright colours and the other conspicuous features for which Darwin invoked female (or in a few species male) choice. Even when persuaded that certain male features have evolved as advertisements aimed at females, Wallace insisted that the qualities they advertise must be utilitarian qualities. Females choose males not because they are pretty but because they are good providers, or something equally
30
worthy. Modern Wallaceans such as William Hamilton and Amotz
31
Zahavi see bright colours and other sexually selected advertisements as
honest and uncheatable badges of true quality: health, for example, or resistance to parasites.
Darwin would have no problem with that, but he also was prepared to countenance pure aesthetic whim as a selective force in nature.
64
She even
? Something about the female brain just likes bright coloured feathers, or whatever is the species equivalent, and that is a sufficient pressure for males to have evolved them, even if this is disadvantageous to the male's own survival. It was that leader among twentieth-century Darwinians, R. A. Fisher, who put the idea on a sound theoretical foundation by suggesting that female preference could be under genetic control and therefore subject to natural selection, in just the same way as the male
32
qualities preferred. The interaction between selection on female
preference genes (inherited by both sexes) and simultaneously on male advertisement genes (also inherited by both sexes) provides the coevolutionary driving force for the expansion of ever more extravagant sexual advertisements. I suspect that Fisher's elegant reasoning, supplemented by more recent theorists such as R. Lande, might have reconciled Wallace to Darwin, because Fisher did not leave female whim unexplained, as an arbitrary given. The key point is that female whims
33 of the future agree with those inherited from the past.
The divide between Darwinian and Wallacean sexual selection, then,
is one thing to bear in mind while reading the substantial middle section of The Descent of Man. Another is that Darwin made a clear distinction between sexual and natural selection, one which today is not always understood. Sexual selection is all about competition between members of the same sex for the opposite sex. It usually produces adaptations in males for outcompeting other males: either for fighting males or for attracting females. It does not include all the rest of the apparatus of sexual reproduction. A penis, in its capacity as an organ
of intromission, is a manifestation of natural selection, not sexual selection. A male needs a penis to reproduce, whether or not competing males are around. But male vervet monkeys (Cercopithecus aethiops) have a bright red penis set off by a sky-blue scrotum, which together are shown off in dominance displays to other males. It is for their colours, not the organs themselves, that Darwin would invoke sexual selection.
To decide whether something is a sexually selected adaptation or not, do the following thought experiment. Imagine that all competitors of the same sex could somehow be magicked away. If the pressure for the adaptation now disappears, it was sexually selected. In the case of the vervet monkeys it is reasonable to guess, as Darwin surely would, that if competition from rival males were removed by a magic wand, the penis and scrotum would remain, but their red and blue colour scheme would fade. The ornate colours are a product of sexual selection, the utilitarian organs of sperm production and intromission are manifesta- tions of natural selection. Darwin would have loved the baroque and
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spiky penises documented by W. G. Eberhard in his book, Sexual 3
Selection and Animal Genitalia. *
The distinguished American philosopher Daniel Dennett has credited
35
Darwin with the greatest idea ever to occur to a human mind. This was
natural selection, of course, and I would include sexual selection as part of the same idea. But Darwin was not only a deep thinker, he was a naturalist of encyclopaedic knowledge and (which by no means necessarily follows) the ability to hold it in his head and deploy it in constructive directions. He was a master encyclopaedist, who collated huge quantities of information and observations solicited from naturalists 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 in his Victorian prose style, quite apart from the feeling one gets of having been ushered into the presence of one of the great minds of all time.
Prescient as he was (Michael Ghiselin has said that he worked at least
36
a century ahead of his time ) Darwin was still a Victorian, and his book
must be read in the context of its age, warts and all. What will grate most irksomely on the modern ear is the unquestioned Victorian presumption that animals in general, and humans in particular, are disposed on a ladder of increasing superiority. Like all Victorians, Darwin happily referred to particular species as 'lowly in the scale of nature'. Even some modern biologists do this, though they should not, for all living species are cousins who have been evolving for exactly the
37
same length of time since the common ancestor. What educated
moderns never do, but equivalent Victorians always did, is think of human races in the same hierarchical way. It requires a special effort for us to read something like the following without distaste:
It seems at first sight a monstrous supposition that the jet blackness of the negro has been gained through sexual selection [i. e. is attractive to the opposite sex] . . . The resemblance of Pithecia satanas with his jet black skin, white rolling eyeballs, and hair parted on the top of the head, to a negro in
38 miniature, is almost ludicrous.
It is a mark of historical infantilism to view the writings of one century through the politically tinted glasses of another. The very title, Descent of Man, will raise hackles among those naively locked into the mores of our own time. It can be argued that reading historic documents that violate the taboos of one's own century gives valuable lessons in the ephemerality of such mores. Who knows how our descendants will judge us?
Less obvious, but as important to understand, are the changes in the
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? scientific climate. In particular, it is hard to overstate the fact that Darwin's genetics were pre-Mendelian. The intuitively plausible blending inheritance theory of his time was not just wrong, it was grievously wrong and especially grievous for natural selection. Darwinism's incompatibility with blending inheritance was pointed out in a hostile review of the Origin by the Scottish engineer Fleeming Jenkin. Variation tends to disappear with every blending generation, leaving not enough for natural selection to get its teeth into. What Jenkin should have realized is that blending inheritance is incompatible not just with Darwinian theory but with obvious fact. If it were really true that variation disappeared, every generation should be more uniform than the previous one. By now, all individuals should be as indistinguishable as clones. Darwin needed only to retort to Jenkin: Whatever the reason, it is obviously the case that there is plenty of inherited variation and that's good enough for my purposes.
It is often claimed that the answer to the riddle lay on Darwin's shelves, in the uncut pages of the proceedings of the Brunn Natural History Society, where nestled Gregor Mendel's paper on Versuche iiber Pflanzen-Hybriden. Unfortunately this poignant story seems to be an urban myth. The two scholars best placed (at Cambridge and at Down House) to know what was in Darwin's personal library can find no evidence that he ever subscribed to the proceedings, nor does it seem
39
likely that he would have 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. The whole affair would make a nice little project in memetic research, complementing that other popular urban legend, the agreeable falsehood that Darwin
40 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 in 1900 and inspired the Hardy-Weinberg Law in 1908, it was not until Fisher came along in 1930* that its supreme relevance to Darwinism was widely understood. If heredity is particulate, variation does not disappear but is reconstituted in every generation. Neo- Darwinian evolution precisely means change in gene frequencies in gene pools. What is genuinely poignant is that Darwin himself came
41
tantalizingly close. Fisher quotes him in a letter to Huxley of 1857:
I have lately been inclined to speculate, very crudely and indistinctly, that propagation by true fertilization will turn out to be a sort of mixture, and not true
'Actually rather earlier, but 1930 was when Fisher published his landmark book.
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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.
Fisher cleverly remarked that Mendelism has a kind of necessary plausibility
which could have led to its discovery by any thinker in a mid-Victorian
armchair (quoted on page 82). He might have added that particulate inheri-
tance stares us in the face whenever we contemplate sex itself (as we not
infrequently do). All of us have one female and one 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
42
to Wallace, which Fisher 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 certain varieties. It does not refer to fertility; an instance will explain. I crossed the Painted Lady and Purple sweetpeas, which are very differently 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 least with your butterflies & the three forms of Lythrum; tho' 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 . . .
Believe me, yours very sincerely Ch. Darwin
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? Here Darwin comes closer to anticipating Mendel than in the passage quoted by Fisher, and he even mentions his own Mendel-like experi- ments on sweet peas. I am extremely grateful to Dr Seymour J. Garte of New York University, who found this letter by chance in a volume of correspondence between Darwin and Wallace in the British Library in London, immediately recognized 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 of Man, in which Darwin prudently said:
I formerly thought that when a tendency to produce the two sexes in equal numbers was advantageous to the species, it would follow from natural selection, but I now see that the whole problem is so intricate that it is safer to leave its solution to the future.
43
Fisher's own solution made no appeal to species advantage. Instead he
pointed out that, since every individual born has one father and one mother, the total male contribution to posterity must equal the total female contribution. If the sex ratio is anything other than 50/50,
LIGHT WILL BE THROWN
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therefore, an individual of the minority sex can expect, other things being equal, a greater share of descendants, and this will set up selection in favour of rebalancing the sex ratio. Fisher rightly used economic language to express the strategic decisions involved: they are decisions over how to allocate parental expenditure. Natural selection will favour parents who spend proportionately more food or other resources on offspring of the minority sex. Such correcting selection will continue until the total expenditure on sons in the population balances the total expenditure on daughters. This will amount to equal numbers of males and females, except in those cases where offspring of one sex cost more to rear than offspring of the other. If, for example, it costs twice as much food to rear a son than a daughter (perhaps to make sons big enough to compete effectively with rival males) the stable sex ratio will be twice as many females as males. This is because the strategic alternative to one son is not one daughter but two. Fisher's powerful logic has been extended and refined in various ways, for
44 45 example by W. D. Hamilton and E. L. Charnov .
Once again, and notwithstanding the quotation above from the Second Edition of The Descent of Man, Darwin himself, in the First Edition, came remarkably close to anticipating Fisher, although without the economic language of parental expenditure:
Let us now take the case of a species producing, from the unknown causes just alluded to, an excess of one sex - we will say of males - these being superfluous and useless, or nearly useless. Could the sexes be equalized through natural selection? We may feel sure, from all characters being variable, that certain pairs would produce a somewhat less excess of males over females than other pairs. The former, supposing the actual number of the off- spring to remain constant, would necessarily produce more females, and would therefore be more productive. On the doctrine of chances a greater number of the offspring of the more productive pairs would survive; and these would inherit a tendency to procreate fewer males and more females. Thus a tendency toward equalization of the sexes would be brought about.
Sadly, Darwin deleted this remarkable passage when he came to prepare the Second Edition, preferring the more cautious paragraph later to be quoted by Fisher. Darwin's partial anticipation of Fisher in the First Edition of Descent is all the more impressive because, as Alan Grafen points out to me, Fisher's argument depends crucially on a fact which was not available to Darwin, namely that the two parents make an equal genetic contribution to every offspring. Indeed, in historical times, different schools of thought (the spermists and the ovists respectively)
70
? had held that the male, or the female, sex had a monopoly on heredity. The whole question of Fisher's sources for the sex ratio theory has
been meticulously sleuthed by Professor A. W. F. Edwards of Cambridge
46
University, himself one of Fisher's most distinguished pupils. Edwards
not only notes Darwin's priority over the essential argument and the odd fact that he deleted it from the Second Edition. He also shows how Darwin's argument was taken up and developed by a series of other workers whose writings were probably known to Fisher. First Carl Diising of Jena, in 1884, reiterated and clarified Darwin's argument. Next, in 1908 the Italian statistician Corrado Gini discussed the argument more critically. Finally in 1914, the eugenicist J. A. Cobb gave a form of the argument which seems to have all the refinements of Fisher's own of 1930, including the economic idea of parental expenditure. Cobb seems to have been unaware of Darwin's priority, but Edwards is persuasive that Fisher was aware of Cobb's. Edwards remarks that:
commentators have assumed, and most have firmly stated, that the argument was original to Fisher, though he did not claim it to be, nor did he refer to it either before or after 1930 in any of his other publications. Indeed, there is no evidence that he saw it as particularly novel, remarkable, or likely to lead to major developments in evolutionary biology . . . he may well have regarded it as public property by 1930.
Edwards himself is one of those (I am another) who once overlooked the crucial difference between the First and Second Editions of The Descent.
Fisher's economic view of sex was developed further by Robert L. Trivers, writing in a volume published to commemorate the centenary of The Descent of Man. " Trivers's subtle application of the theory of parental investment (his name for what Fisher had called parental expenditure) to male and female roles in sexual selection greatly illuminates the facts collected by Darwin in the middle chapters of Descent. Trivers defines parental investment (PI) as (what economists would call) an opportunity cost. The cost to a parent of investing in a particular child is measured in correspondingly lost opportunity to invest in others, present or future. Sexual inequality is fundamentally economic. The mother typically invests more in any individual offspring than the father does, and this inequality has far-reaching consequences, which reach even further in a kind of self-feeding process. A member of the low-investing sex (usually male) who persuades a member of the high-investing sex (usually female) to mate with him has gained an economic prize worth fighting (or otherwise competing) for. This is why males typically devote more effort to competing with other males, while females typically shunt their
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effort away from competing with other females and into investing in offspring. It is why, when one sex is more brightly coloured than the other, it is typically the male. It is why, when one sex is more choosy in selecting a mate, it is typically the female. And it is why variance in reproductive success is typically higher among males than among females: the most successful male may have many times more descendants than the least successful male, where the most successful female is only somewhat more successful than the least successful female. The Fisher/ Trivers economic inequalities between the sexes should be kept in mind while reading Darwin's enthralling review of sexual selection through the animal kingdom. It is a most striking example of a single idea uniting and explaining, at one blow, a multitude of seemingly disparate facts.
Now, to the descent of man itself. Darwin's guess that our species arose in Africa was typically ahead of its 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 denned 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 book are concerned to narrow the perceived gap between ourselves and the quadrumana, a gap which Darwin's target audience would have seen as yawning between the top rung of a ladder and the next rung down. Today we would not (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 ofSpecies. Humanity is just one little twig, nestling among many others somewhere in the middle of a thicket of African apes.
Two vital techniques which were unavailable to Darwin are radio- active dating of rocks, and molecular evidence including the 'molecular clock'. Where Darwin, in his quest to demonstrate the similarity between ourselves and the quadrumana, could point to comparative anatomy supplemented by charming anecdotes of psychological and emotional resemblance (arguments extended in The Expression of the Emotions), we are privileged to know the exact letter-by-letter sequence of massive DNA texts. It is claimed that more than 98 per cent of the human genome, when measured in this way, is identical with chimpanzees'. Darwin would have been spellbound. Such closeness of resemblance, and such precision in measuring it, would have delighted him beyond dreams.
Nevertheless, we must beware of being carried away by the euphoria of it all. That 98 per cent doesn't mean we are 98 per cent chimpanzees. And
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? it really matters which unit you choose to make your comparison. If you count the number of whole genes that are identical, the figure for humans and chimpanzees would be close to zero. This is not a paradox. Think of the human genome and the chimpanzee genome as two editions of the same book, say the first and second editions of The Descent ofMan. If you count the number of letters that are identical to their opposite numbers in the other edition, it is probably well over 90 per cent. But if you count the number of chapters that are identical, it may well be zero. This is because it takes only one letter to be different, anywhere in a chapter, for the whole chapter to be judged different between the two editions. When you are measuring the percentage similarity between two texts, whether two editions of a book or two editions of an African ape, the unit of comparison you choose (letter or chapter, DNA base pair or gene) makes
a huge difference to the final percentage similarity.
The point is that we should use such percentages not for their
absolute value but in comparisons between animals. The 98 per cent figure for humans and chimpanzees starts to make sense when we compare it with the 96 per cent resemblance between humans and orang utans (it is the same 96 per cent between chimpanzees and orang utans, and the same between gorillas and orang utans, because all the African apes are connected to the Asian orang utans via a shared African ancestor). For the same kind of reason, all the great apes share 95 per cent of their genomes with the gibbons and siamangs. And all the apes share 92 per cent of their genomes with all Old World monkeys.
The hypothesis of a molecular clock allows us to use such percentage figures to put a date on each of the splits in our family tree. It assumes that evolutionary change, at the molecular genetic level, proceeds at an approximately fixed rate for each gene. This is in accordance with the widely accepted neutral theory of the Japanese geneticist Motoo Kimura. Kimura's neutral theory is sometimes seen as anti-Darwinian but it is not. It is neutral with respect to Darwinian selection. A neutral mutation is one that makes no difference to the functioning of the protein produced. The post-mutation version is no better and no worse than the pre-mutation version, where both may be vital to the life of the organism.
From a Darwinian point of view, neutral mutations are not mutations at all. But from a molecular point of view they are extremely useful mutations because their fixed rate makes the clock reliable. The only point of controversy introduced by Kimura is how many mutations are neutral. Kimura thought it was the great majority which, if true, is very nice for the molecular clock. Darwinian selection remains the only explanation for adaptive evolution and it is arguable (I would argue)
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that most if not all of the evolutionary changes we actually see in the macroscopic world (as opposed to those concealed among the molecules) are adaptive and Darwinian.
As so far described, the molecular clock gives relative timings but not absolute ones. We can read off timings of evolutionary splits, but only in arbitrary units. Fortunately, in another great advance that would have entranced Darwin, various absolute clocks are available for dating fossils. These include the known rates of radioactive decay of isotopes in volcanic rocks sandwiching the sedimentary strata in which fossils are found. By taking a group of animals with a rich fossil record and dating the splits in their family tree two ways - by the molecular genetic clock and by radioactive clocks - the arbitrary units of the genetic clock can be validated, and simultaneously calibrated in real millions of years. This is how we can estimate that the split between humans and chimpanzees occurred between 5 and 8 million years ago, the split between African apes and orang utans about 14 million years ago, and the split between apes and Old World monkeys about 25 million years ago.
Fossils, all discovered after Descent was published, provide us with a sporadic picture of some possible intermediates connecting us to our common ancestor with chimpanzees. Unfortunately, there are no fossils connecting modern chimpanzees to that shared ancestor, but on our side of the split reports of new fossil finds are coming in at a rate which
I find exciting and surely Darwin would have too. Going back in steps
of roughly one million years we find: Homo erectus, Homo habilis, Australopithecus afarensis, Australopithecus anamensis, Ardipithecus, Orrorin and, a recent discovery which may date from as long ago as 7 million years, Sahelanthropus. That last find is from Chad, far to the west of the great Rift Valley which had hitherto been thought to constitute
a geographic barrier dividing our lineage from that of the chimpanzees. It is good for our orthodoxies to be upset from time to time.
We must beware of assuming that this temporal series of fossils represents an ancestor/descendant series. It is always safer to assume that fossils are cousins rather than ancestors, but we need not be shy of guessing that earlier cousins may tell us at least something about the true ancestors among their contemporaries.
What are the main changes that occurred since our split from the chimpanzees? Some, such as our loss of body hair, are interesting, but fossils can tell us nothing directly about them. The two main changes that fossils can help us with, and where we therefore have a big advantage over Darwin, are that we rose up on our hind legs, and that our brains got rather dramatically bigger. Which of these changes came
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? first, or did they happen together? All three views have been supported,
and controversy has gone back and forth over the decades. Darwin
thought the two big changes happened in concert, and he makes out a
plausible case. But this is a rare instance where Darwin's tentative guess
has turned out wrong. The fossils give a satisfyingly decisive and clear
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answer. Bipedality came first, and its evolution was more or less
complete before the brain started to swell. Three million years ago, Australopithecus was bipedal and had feet like ours, although it probably still retreated up trees. But its brain, relative to its body size, was the same size as a chimpanzee's, and presumably the same as the shared ancestor with chimpanzees. Nobody knows whether the bipedal gait set up new selection pressures that encouraged the brain to grow, but Darwin's original arguments for simultaneous evolution can be adapted to make that plausible. Perhaps the enlargement of the brain had some- thing to do with language, but here nobody knows and disagreements abound. There is evidence that particular parts of the human brain are uniquely pre-wired to handle specific universals of language, although
49 the particular language spoken is, of course, locally learned.
Another twentieth-century idea which is probably important in human evolution, and which again would have intrigued Darwin, is neoteny: evo- lutionary infantilization. The axolotl, an amphibian living in a Mexican lake, looks just like the larva of a salamander, but it can reproduce, and has chopped off the adult, salamander stage of the life history. It is a sexually mature tadpole. Such neoteny has been suggested as a way in which a lineage can suddenly initiate an entirely new direction of evolution, at a stroke. Apes don't have a discrete larval stage like a tadpole or a caterpillar, but a more gradualistic version of neoteny can be discerned in human evolution. Juvenile chimpanzees resemble humans far more than adult chimpanzees do. Human evolution can be seen as infantilism. We are apes
50 that became sexually mature while still morphologically juvenile. If
humans could live for 200 years, would we finally 'grow up', drop on all fours and develop huge prognathous chimpanzee-like jaws? The possibility has not been lost on writers of ironic fiction, notably Aldous Huxley in After Many a Summer. He presumably learned about neoteny from his elder brother Julian, who was one of the pioneers of the idea and did amazing research on axolotls, injecting hormones to make them turn into salamanders never before seen.
Let me end by bringing together once again the two halves of Darwin's book. He went to town on sexual selection in The Descent of Man because he thought it was important in human evolution, and especially because he thought it was the key to understanding the
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differences among human races. Race, in Victorian times, was not the political and emotional minefield it is today, when one can give offence j by so much as mentioning the word. I shall tread carefully, but I cannot I ignore the topic because it 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 than most of his contemporaries, emphasized the fundamental unity of our species. In Descent he carefully considered, and decisively rejected, the idea, rather favoured in his own time, that different human races should be regarded as separate species. Today we know that, at the genetic level, our species is more than usually uniform. It has been said that there is more genetic variation among the chimpanzees of a small region of Africa than among the entire world population of humans (suggesting that we have been through a bottleneck in the past hundred thousand years or so). Moreover, the great majority of human genetic variation is to be found within races, not between them. This means that if you were to wipe out all human races except one, the I great majority of human genetic variance would be preserved. The variance between races is just a bit extra, stuck on the top of the greater quantity of variation within all races. It is for this reason that many geneticists advocate the complete abandonment of the concept of race.
At the same time - the paradox is similar to one recognized by Darwin - the superficially conspicuous features characteristic of local populations around the world seem very different. A Martian taxonomist who didn't know that all human races happily interbreed with one another, and didn't know that most of the underlying genetic variance in our species is shared by all races, might be tempted by our regional differences in skin colour, facial features, hair, body size and proportions to split us into more than one species. What is the resolution of the paradox? And why did such pronounced superficial differences evolve in different geographical areas, while most of the less conspicuous variation is dotted around across all geographical areas? Could Darwin have been right all along? Is sexual selection the answer to the paradox? The distinguished
51
biologist Jared Diamond thinks so, and I am inclined to agree.
Utilitarian answers have been suggested to the question of the evolution of racial differences, and there may well be some truth in them. Dark skin may protect against skin cancer in the tropics, light skin admit beneficial rays in sun-starved latitudes where there is a danger of Vitamin D deficiency. Small stature probably is of benefit to hunters in dense forest, such as the pygmies of central Africa, and various independently evolved hunter gatherers of Amazon and South East Asian
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1
? forests. The ability to digest milk when adult seems to have evolved in peoples who, for cultural reasons, prolong the use of this primitively juvenile food. But I am impressed by the diversity of features that are superficial and conspicuous, while deeper differences are so slight.
What sexual selection explains, better than natural selection, is diversity that seems arbitrary, even driven by aesthetic whim. Especially if the variation concerned is geographical. And also especially if some of 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 fashions like headdresses, body paint, penis sheaths, ritual mutilations or ornamental clothes. Given that cultural differences such as those of language, religion, manners and customs certainly provide resistance to interbreeding and gene flow, I think it is entirely plausible that genetic differences between peoples of different regions, at least where superficial, externally prominent features are concerned, have evolved through sexual selection. Our species really does seem to have unusually conspicuous, even ostentatious, superficial differences between local populations, coupled with unusually low levels of overall genetic variation. This double circumstance carries, to my mind, the stamp of sexual selection.
52
In this respect, human races seem a lot like breeds of dog, another
favourite topic of Darwin. Superficially, the domestic breeds of dogs are astonishingly varied, even more so than human races, yet the under- lying genetic differences are slight, and they are all clearly descended
53
from wolves within the past few thousand years. Reproductive
isolation is today maintained by disciplined pedigree breeders, and the shapes and colours of the dogs themselves are steered through their rapid evolution by the whim of the human eye rather than the whim of female dogs. But the essential features of the situation, as Darwin realized, are similar to those of sexual selection.
In this, as in so much else, I suspect that Darwin was right. Sexual
selection really is a good candidate for explaining a great deal about the
unique evolution of our species. It may also be responsible for some
unique features of our species which are shared equally by all races, for
54
example our enormous brain. Geoffrey Miller, in The Mating Mind, strongly developed precisely this case, and Darwin would have loved it
no less because Miller takes a Wallacean view of sexual selection. It is starting to look as though, despite initial appearances, Darwin really was right to bring together, in one volume, Selection in Relation to Sex and The Descent ofMan.
LIGHT WILL BE THROWN
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has
? ? En ? AMI
Darwin Triumphant55 Darwinism as a Universal Truth
If we are visited by superior creatures from another star system - they will have to be superior if they are to get here at all - what common ground shall we find for discussion with them? Shall we overcome the barriers simply by learning one another's language, or will the subjects that interest our two cultures be so divergent as to preclude serious conversation? It seems unlikely that the star travellers will want to talk about many of our intellectual stocks-in-trade: about literary criticism or music, religion or politics. Shakespeare may mean nothing to those without human experiences and human emotions, and if they have a literature or an art these will probably be too alien to excite our sensi- bilities. To name two thinkers who have more than once been promoted as Darwin's equals, I rather doubt whether our visitors will have much interest in talking about Marx or Freud, other than perhaps as anthropological curiosities. We have no reason to suppose that these men's works are of more than local, parochial, human, earthly, post- Pleistocene (some would add European and male) significance.
Mathematics and physics are another matter. Our guests may find our level of sophistication quaintly low, but there will be common ground. We shall agree that certain questions about the universe are important, and we shall almost certainly agree on the answers to many of these questions. Conversation will flourish, even if most of the questions flow one way and most of the answers the other. If we discuss the histories of our respective cultures, our visitors will surely point with pride, however far back in time, to their equivalents of Einstein and Newton, of Planck and Heisenberg. But they won't point to an equivalent of Freud or Marx any more than we, visiting a hitherto undiscovered tribe in a remote forest clearing, would nominate our civilization's equiva- lent of the local rainmaker or gully-gully man. One does not have to disparage the local achievements of Freud and Marx on this planet to agree that their findings have no universality.
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? What about Darwin? Will our guests revere another Darwin as one of their greatest thinkers of all time? Shall we be able to have a serious conversation with them about evolution? I suggest that the answer is yes (unless, as a colleague suggests to me, their Darwin is on the expedi- tion and we are her Galapagos*). Darwin's achievement, like Einstein's, is universal and timeless, whereas that of Marx is parochial and ephemeral. That Darwin's question is universal, wherever there is life, is surely undeniable. The feature of living matter that most demands explanation is that it is almost unimaginably complicated in directions that convey a powerful illusion of deliberate design. Darwin's question, or rather the most fundamental and important of Darwin's many questions, is the question of how such complicated 'design' could come into being. All living creatures, everywhere in the universe and at any time in history, provoke this question. It is less obvious that Darwin's answer to the riddle - cumulative evolution by nonrandom survival of random hereditary changes - is universal. It is at first sight conceivable that Darwin's answer might be valid only parochially, only for the kind of life that happens to exist in our own little clearing in the universal
56
forest. I have previously made the case that this is not so, that the
general form of Darwin's answer is not merely incidentally true of our kind of life but almost certainly true of all life, everywhere in the universe. Here, let me for the moment make the more modest claim that, at the very least, Darwin's bid for immortality is closer to the Einstein end of the spectrum than to the Marx end. Darwinism really matters in the universe.
When I was an undergraduate in the early nineteen sixties, we
were taught that although Darwin was an important figure in his own
time, modern neo-Darwinism was so much further advanced that it
hardly deserved the name Darwinism at all. My father's generation of
biologist undergraduates read, in an authoritative Short History of 57
. . . the struggle of living forms leading to natural selection by the survival of the fittest, is certainly far less emphasized by naturalists now than in the years that immediately followed the appearance of Darwin's book. At the time, however, it was an extremely stimulating suggestion.
And the generation of biologists before that could read, in the words of William Bateson, perhaps the dominant British geneticist of the time,
This is how my friend worded her suggestion. The joke was rather ruined by the political scruples of the original article's copy-editor, who changed 'her Galapagos' to 'his or her Galapagos'.
Biology , that
DARWIN TRIUMPHANT
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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 . . . 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 . . . at the want of
58 penetration displayed by the advocates of such a proposition.
And yet the editors of this volume can commission an article with the title 'Darwin Triumphant'. I do not normally like writing to titles that others have proposed, but I can accept this one without reservation. In the last quarter of the twentieth century, it seems to me that Darwin's standing among serious biologists (as opposed to nonbiologists influenced by religious preconceptions) is rightly as high as it has been at any time since his death. A similar story, of even more extreme eclipse in earlier years followed by triumphant recent rehabilitation, can be told of Darwin's 'other theory', that of sexual selection. *
It is only to be expected that, a century and a quarter on, the version
of his theory that we now have should be different from the original. Modern Darwinism is Darwinism plus Weismannism plus Fisherism plus Hamiltonism (arguably plus Kimuraism and a few other isms). But when I read Darwin himself, I am continually astonished at how modern
he sounds. Considering how utterly wrong he was on the all-important topic of genetics, he showed an uncanny gift for getting almost every- thing else right. Maybe we are neo-Darwinists today, but let us spell the neo with a very small n\ Our neo-Darwinism is very much in the spirit of Darwin himself. The changes that Darwin would see if he came back today are in most cases changes that, I venture to suggest, he would instantly approve and welcome as the elegant and obviously correct answers to riddles that troubled him in his own time. Upon learning that evolution is change in frequencies within a pool of particulate hereditary elements, he might even quote T. H. Huxley's alleged remark upon reading the Origin itself: 'How extremely stupid not to have thought of thatl't
*See 'Light Will Be Thrown' (pp. 63-77).
tOf the two stories about Huxley that have become chestnuts, I greatly prefer this to the one about his so-called 'debate' with the Bishop of Oxford, Sam Wilberforce. There is something admirably honest about Huxley's exasperation at not having thought of such a simple idea. I have long found it a complete mystery why it had to wait until the nineteenth century before anyone thought of it. Archimedes' and Newton's achievements seem, on the face of it, far more difficult. But the fact that nobody did think of natural selection before the nineteenth century clearly shows that I am wrong. As does the fact that so many people, even today, don't get it.
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? I referred to Darwin's gift for getting things right, but surely this can only mean right as we see it today. Shouldn't we be humble enough to admit that our right may be utterly wrong in the sight of future scientific generations? No, there are occasions when a generation's humility can be misplaced, not to say pedantic. We can now assert with confidence that the theory that the Earth moves round the Sun not only is right in our time but will be right in all future times even if flat- Earthism happens to become revived and universally accepted in some new dark age of human history. We cannot quite say that Darwinism is in the same unassailable class. Respectable opposition to it can still be mounted, and it can be seriously argued that the current high standing of Darwinism in educated minds may not last through all future generations. Darwin may be triumphant at the end of the twentieth century, but we must acknowledge the possibility that new facts may come to light which will force our successors of the twenty-first century to abandon Darwinism or modify it beyond recognition. But is there, perhaps, an essential core of Darwinism, a core that Darwin himself might have nominated as the irreducible heart of his theory, which we might set up as a candidate for discussion as potentially beyond the reach of factual refutation?
Core Darwinism, I shall suggest, is the minimal theory that evolution is guided in adaptively nonrandom directions by the nonrandom survival of small random hereditary changes. Note especially the words small and adaptively. Small implies that adaptive evolution is gradualistic, and we shall see why this must be so in a moment. Adaptive does not imply that all evolution is adaptive, only that core Darwinism's concern is limited to the part of evolution that is. There is no reason to assume
59
that all evolutionary change is adaptive.
exposure of the nature/nurture cliche. A flurry of publicity in 1993 for a so-called 'gay gene' on the X chromosome led to an invitation from the Daily Telegraph to expose the myths of 'genetic determinism'. The result was the piece reproduced here as Genes Aren't Us (2. 4).
My literary agent John Brockman has the charisma to persuade his clients and others to drop everything and contribute to books of his own editing, even in the teeth of the better commercial judgement he might normally advise them to deploy. The distinction of his guest list flatters them in through the door of his salon (http://www. edge. org/) and before they know where they are they are correcting the proofs for a printed spin- off. Son of Moore's Law (2. 5) was my futurological contribution to a typically fascinating on-line symposium, The Next Fifty Years.
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? 2
Foreword to a new Student Edition of Darwin's Descent of Man "
"!
Light Will Be Thrown
Humanity is the missing guest at the feast of The Origin of Species. The famous 'Light will be thrown on the origin of man and his history' 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 a possible copying mechanism for the genetic material. ' By the time Darwin finally got around to throwing that light in 1871, others had been there before him. And the greater part of The Descent of Man is not about humans but about Darwin's 'other' theory, sexual selection.
The Descent of Man was conceived as a single book but ended up as three, two of them bound together under the same title, with the second topic signalled by the subtitle, Selection in Relation to Sex. The third was The Expression of the Emotions, not my concern here, but Darwin tells us that it grew out of the original Descent, and he began writing it immediately after finishing Descent. Given that the idea of splitting the book was in Darwin's mind, it is at first sight surprising that he didn't spin off sexual selection as well. It would have seemed natural to publish chapters 8 to 18 as Selection in Relation to Sex followed by a second book, The Descent of Man, consisting of the present Chapters 1 to 8, and 19 to 21. That's a neat split into eleven chapters
for each book, and many have wondered why he did not do this. I shall follow the same order - sexual selection followed by the descent of man - and then return at the end to the question of whether the two might have been split. In addition to discussing Darwin's book, I shall try to give some pointers to where the subject is moving today.
The ostensible connection between sexual selection and the descent of man is that Darwin believed the first was a key to understanding the second; especially to understanding human races, a topic which preoccupied Victorians more than it does us. But, as the historian and philosopher of science Michael Ruse has remarked to me, there was a
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tighter thread binding the two topics. They were the only two sources
of disagreement between Darwin and his co-discoverer of natural
selection. Alfred Russel Wallace never took kindly to sexual selection, at
least in its full-blooded Darwinian form. And Wallace, though he
coined the word Darwinism and described himself as 'more Darwinian
than Darwin', stopped short of the materialism implied by Darwin's
view of the human mind. These disagreements with Wallace were all
the more important to Darwin because these two great men agreed on
almost everything else. Darwin himself said, in a letter to Wallace of
28 1867:
The reason of my being so much interested just at present about sexual selection is, that I have almost resolved to publish a little essay on the origin of Mankind, and I still strongly think (though I failed to convince you, and this, to me is the heaviest blow possible) that sexual selection has been the main agent in forming the races of man.
The Descent ofMan and Selection in Relation to Sex could be seen, then, as Darwin's two-pronged answer to Wallace. But it's also possible - and anyone who reads those chapters would forgive him - that he just got carried away by his enthusiasm for sexual selection.
The disagreements between Darwin and Wallace over sexual selection have been teased out by the Darwinian philosopher and historian
29
Helena Cronin in her stylish book The Ant and the Peacock.
follows the two threads to the present day, classifying later theorists of sexual selection as 'Wallaceans' and 'Darwinians'. Darwin rejoiced in sexual selection. The naturalist in him loved the extravagant ostenta- tion of stag beetles and pheasants, while the theorist and teacher knew that survival is only a means to the end of reproduction. Wallace could not stomach aesthetic whim as a sufficient explanation for the evolu- tion of bright colours and the other conspicuous features for which Darwin invoked female (or in a few species male) choice. Even when persuaded that certain male features have evolved as advertisements aimed at females, Wallace insisted that the qualities they advertise must be utilitarian qualities. Females choose males not because they are pretty but because they are good providers, or something equally
30
worthy. Modern Wallaceans such as William Hamilton and Amotz
31
Zahavi see bright colours and other sexually selected advertisements as
honest and uncheatable badges of true quality: health, for example, or resistance to parasites.
Darwin would have no problem with that, but he also was prepared to countenance pure aesthetic whim as a selective force in nature.
64
She even
? Something about the female brain just likes bright coloured feathers, or whatever is the species equivalent, and that is a sufficient pressure for males to have evolved them, even if this is disadvantageous to the male's own survival. It was that leader among twentieth-century Darwinians, R. A. Fisher, who put the idea on a sound theoretical foundation by suggesting that female preference could be under genetic control and therefore subject to natural selection, in just the same way as the male
32
qualities preferred. The interaction between selection on female
preference genes (inherited by both sexes) and simultaneously on male advertisement genes (also inherited by both sexes) provides the coevolutionary driving force for the expansion of ever more extravagant sexual advertisements. I suspect that Fisher's elegant reasoning, supplemented by more recent theorists such as R. Lande, might have reconciled Wallace to Darwin, because Fisher did not leave female whim unexplained, as an arbitrary given. The key point is that female whims
33 of the future agree with those inherited from the past.
The divide between Darwinian and Wallacean sexual selection, then,
is one thing to bear in mind while reading the substantial middle section of The Descent of Man. Another is that Darwin made a clear distinction between sexual and natural selection, one which today is not always understood. Sexual selection is all about competition between members of the same sex for the opposite sex. It usually produces adaptations in males for outcompeting other males: either for fighting males or for attracting females. It does not include all the rest of the apparatus of sexual reproduction. A penis, in its capacity as an organ
of intromission, is a manifestation of natural selection, not sexual selection. A male needs a penis to reproduce, whether or not competing males are around. But male vervet monkeys (Cercopithecus aethiops) have a bright red penis set off by a sky-blue scrotum, which together are shown off in dominance displays to other males. It is for their colours, not the organs themselves, that Darwin would invoke sexual selection.
To decide whether something is a sexually selected adaptation or not, do the following thought experiment. Imagine that all competitors of the same sex could somehow be magicked away. If the pressure for the adaptation now disappears, it was sexually selected. In the case of the vervet monkeys it is reasonable to guess, as Darwin surely would, that if competition from rival males were removed by a magic wand, the penis and scrotum would remain, but their red and blue colour scheme would fade. The ornate colours are a product of sexual selection, the utilitarian organs of sperm production and intromission are manifesta- tions of natural selection. Darwin would have loved the baroque and
LIGHT WILL BE THROWN
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spiky penises documented by W. G. Eberhard in his book, Sexual 3
Selection and Animal Genitalia. *
The distinguished American philosopher Daniel Dennett has credited
35
Darwin with the greatest idea ever to occur to a human mind. This was
natural selection, of course, and I would include sexual selection as part of the same idea. But Darwin was not only a deep thinker, he was a naturalist of encyclopaedic knowledge and (which by no means necessarily follows) the ability to hold it in his head and deploy it in constructive directions. He was a master encyclopaedist, who collated huge quantities of information and observations solicited from naturalists 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 in his Victorian prose style, quite apart from the feeling one gets of having been ushered into the presence of one of the great minds of all time.
Prescient as he was (Michael Ghiselin has said that he worked at least
36
a century ahead of his time ) Darwin was still a Victorian, and his book
must be read in the context of its age, warts and all. What will grate most irksomely on the modern ear is the unquestioned Victorian presumption that animals in general, and humans in particular, are disposed on a ladder of increasing superiority. Like all Victorians, Darwin happily referred to particular species as 'lowly in the scale of nature'. Even some modern biologists do this, though they should not, for all living species are cousins who have been evolving for exactly the
37
same length of time since the common ancestor. What educated
moderns never do, but equivalent Victorians always did, is think of human races in the same hierarchical way. It requires a special effort for us to read something like the following without distaste:
It seems at first sight a monstrous supposition that the jet blackness of the negro has been gained through sexual selection [i. e. is attractive to the opposite sex] . . . The resemblance of Pithecia satanas with his jet black skin, white rolling eyeballs, and hair parted on the top of the head, to a negro in
38 miniature, is almost ludicrous.
It is a mark of historical infantilism to view the writings of one century through the politically tinted glasses of another. The very title, Descent of Man, will raise hackles among those naively locked into the mores of our own time. It can be argued that reading historic documents that violate the taboos of one's own century gives valuable lessons in the ephemerality of such mores. Who knows how our descendants will judge us?
Less obvious, but as important to understand, are the changes in the
66
? scientific climate. In particular, it is hard to overstate the fact that Darwin's genetics were pre-Mendelian. The intuitively plausible blending inheritance theory of his time was not just wrong, it was grievously wrong and especially grievous for natural selection. Darwinism's incompatibility with blending inheritance was pointed out in a hostile review of the Origin by the Scottish engineer Fleeming Jenkin. Variation tends to disappear with every blending generation, leaving not enough for natural selection to get its teeth into. What Jenkin should have realized is that blending inheritance is incompatible not just with Darwinian theory but with obvious fact. If it were really true that variation disappeared, every generation should be more uniform than the previous one. By now, all individuals should be as indistinguishable as clones. Darwin needed only to retort to Jenkin: Whatever the reason, it is obviously the case that there is plenty of inherited variation and that's good enough for my purposes.
It is often claimed that the answer to the riddle lay on Darwin's shelves, in the uncut pages of the proceedings of the Brunn Natural History Society, where nestled Gregor Mendel's paper on Versuche iiber Pflanzen-Hybriden. Unfortunately this poignant story seems to be an urban myth. The two scholars best placed (at Cambridge and at Down House) to know what was in Darwin's personal library can find no evidence that he ever subscribed to the proceedings, nor does it seem
39
likely that he would have 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. The whole affair would make a nice little project in memetic research, complementing that other popular urban legend, the agreeable falsehood that Darwin
40 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 in 1900 and inspired the Hardy-Weinberg Law in 1908, it was not until Fisher came along in 1930* that its supreme relevance to Darwinism was widely understood. If heredity is particulate, variation does not disappear but is reconstituted in every generation. Neo- Darwinian evolution precisely means change in gene frequencies in gene pools. What is genuinely poignant is that Darwin himself came
41
tantalizingly close. Fisher quotes him in a letter to Huxley of 1857:
I have lately been inclined to speculate, very crudely and indistinctly, that propagation by true fertilization will turn out to be a sort of mixture, and not true
'Actually rather earlier, but 1930 was when Fisher published his landmark book.
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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.
Fisher cleverly remarked that Mendelism has a kind of necessary plausibility
which could have led to its discovery by any thinker in a mid-Victorian
armchair (quoted on page 82). He might have added that particulate inheri-
tance stares us in the face whenever we contemplate sex itself (as we not
infrequently do). All of us have one female and one 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
42
to Wallace, which Fisher 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 certain varieties. It does not refer to fertility; an instance will explain. I crossed the Painted Lady and Purple sweetpeas, which are very differently 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 least with your butterflies & the three forms of Lythrum; tho' 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 . . .
Believe me, yours very sincerely Ch. Darwin
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? Here Darwin comes closer to anticipating Mendel than in the passage quoted by Fisher, and he even mentions his own Mendel-like experi- ments on sweet peas. I am extremely grateful to Dr Seymour J. Garte of New York University, who found this letter by chance in a volume of correspondence between Darwin and Wallace in the British Library in London, immediately recognized 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 of Man, in which Darwin prudently said:
I formerly thought that when a tendency to produce the two sexes in equal numbers was advantageous to the species, it would follow from natural selection, but I now see that the whole problem is so intricate that it is safer to leave its solution to the future.
43
Fisher's own solution made no appeal to species advantage. Instead he
pointed out that, since every individual born has one father and one mother, the total male contribution to posterity must equal the total female contribution. If the sex ratio is anything other than 50/50,
LIGHT WILL BE THROWN
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therefore, an individual of the minority sex can expect, other things being equal, a greater share of descendants, and this will set up selection in favour of rebalancing the sex ratio. Fisher rightly used economic language to express the strategic decisions involved: they are decisions over how to allocate parental expenditure. Natural selection will favour parents who spend proportionately more food or other resources on offspring of the minority sex. Such correcting selection will continue until the total expenditure on sons in the population balances the total expenditure on daughters. This will amount to equal numbers of males and females, except in those cases where offspring of one sex cost more to rear than offspring of the other. If, for example, it costs twice as much food to rear a son than a daughter (perhaps to make sons big enough to compete effectively with rival males) the stable sex ratio will be twice as many females as males. This is because the strategic alternative to one son is not one daughter but two. Fisher's powerful logic has been extended and refined in various ways, for
44 45 example by W. D. Hamilton and E. L. Charnov .
Once again, and notwithstanding the quotation above from the Second Edition of The Descent of Man, Darwin himself, in the First Edition, came remarkably close to anticipating Fisher, although without the economic language of parental expenditure:
Let us now take the case of a species producing, from the unknown causes just alluded to, an excess of one sex - we will say of males - these being superfluous and useless, or nearly useless. Could the sexes be equalized through natural selection? We may feel sure, from all characters being variable, that certain pairs would produce a somewhat less excess of males over females than other pairs. The former, supposing the actual number of the off- spring to remain constant, would necessarily produce more females, and would therefore be more productive. On the doctrine of chances a greater number of the offspring of the more productive pairs would survive; and these would inherit a tendency to procreate fewer males and more females. Thus a tendency toward equalization of the sexes would be brought about.
Sadly, Darwin deleted this remarkable passage when he came to prepare the Second Edition, preferring the more cautious paragraph later to be quoted by Fisher. Darwin's partial anticipation of Fisher in the First Edition of Descent is all the more impressive because, as Alan Grafen points out to me, Fisher's argument depends crucially on a fact which was not available to Darwin, namely that the two parents make an equal genetic contribution to every offspring. Indeed, in historical times, different schools of thought (the spermists and the ovists respectively)
70
? had held that the male, or the female, sex had a monopoly on heredity. The whole question of Fisher's sources for the sex ratio theory has
been meticulously sleuthed by Professor A. W. F. Edwards of Cambridge
46
University, himself one of Fisher's most distinguished pupils. Edwards
not only notes Darwin's priority over the essential argument and the odd fact that he deleted it from the Second Edition. He also shows how Darwin's argument was taken up and developed by a series of other workers whose writings were probably known to Fisher. First Carl Diising of Jena, in 1884, reiterated and clarified Darwin's argument. Next, in 1908 the Italian statistician Corrado Gini discussed the argument more critically. Finally in 1914, the eugenicist J. A. Cobb gave a form of the argument which seems to have all the refinements of Fisher's own of 1930, including the economic idea of parental expenditure. Cobb seems to have been unaware of Darwin's priority, but Edwards is persuasive that Fisher was aware of Cobb's. Edwards remarks that:
commentators have assumed, and most have firmly stated, that the argument was original to Fisher, though he did not claim it to be, nor did he refer to it either before or after 1930 in any of his other publications. Indeed, there is no evidence that he saw it as particularly novel, remarkable, or likely to lead to major developments in evolutionary biology . . . he may well have regarded it as public property by 1930.
Edwards himself is one of those (I am another) who once overlooked the crucial difference between the First and Second Editions of The Descent.
Fisher's economic view of sex was developed further by Robert L. Trivers, writing in a volume published to commemorate the centenary of The Descent of Man. " Trivers's subtle application of the theory of parental investment (his name for what Fisher had called parental expenditure) to male and female roles in sexual selection greatly illuminates the facts collected by Darwin in the middle chapters of Descent. Trivers defines parental investment (PI) as (what economists would call) an opportunity cost. The cost to a parent of investing in a particular child is measured in correspondingly lost opportunity to invest in others, present or future. Sexual inequality is fundamentally economic. The mother typically invests more in any individual offspring than the father does, and this inequality has far-reaching consequences, which reach even further in a kind of self-feeding process. A member of the low-investing sex (usually male) who persuades a member of the high-investing sex (usually female) to mate with him has gained an economic prize worth fighting (or otherwise competing) for. This is why males typically devote more effort to competing with other males, while females typically shunt their
LIGHT WILL BE THROWN
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effort away from competing with other females and into investing in offspring. It is why, when one sex is more brightly coloured than the other, it is typically the male. It is why, when one sex is more choosy in selecting a mate, it is typically the female. And it is why variance in reproductive success is typically higher among males than among females: the most successful male may have many times more descendants than the least successful male, where the most successful female is only somewhat more successful than the least successful female. The Fisher/ Trivers economic inequalities between the sexes should be kept in mind while reading Darwin's enthralling review of sexual selection through the animal kingdom. It is a most striking example of a single idea uniting and explaining, at one blow, a multitude of seemingly disparate facts.
Now, to the descent of man itself. Darwin's guess that our species arose in Africa was typically ahead of its 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 denned 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 book are concerned to narrow the perceived gap between ourselves and the quadrumana, a gap which Darwin's target audience would have seen as yawning between the top rung of a ladder and the next rung down. Today we would not (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 ofSpecies. Humanity is just one little twig, nestling among many others somewhere in the middle of a thicket of African apes.
Two vital techniques which were unavailable to Darwin are radio- active dating of rocks, and molecular evidence including the 'molecular clock'. Where Darwin, in his quest to demonstrate the similarity between ourselves and the quadrumana, could point to comparative anatomy supplemented by charming anecdotes of psychological and emotional resemblance (arguments extended in The Expression of the Emotions), we are privileged to know the exact letter-by-letter sequence of massive DNA texts. It is claimed that more than 98 per cent of the human genome, when measured in this way, is identical with chimpanzees'. Darwin would have been spellbound. Such closeness of resemblance, and such precision in measuring it, would have delighted him beyond dreams.
Nevertheless, we must beware of being carried away by the euphoria of it all. That 98 per cent doesn't mean we are 98 per cent chimpanzees. And
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? it really matters which unit you choose to make your comparison. If you count the number of whole genes that are identical, the figure for humans and chimpanzees would be close to zero. This is not a paradox. Think of the human genome and the chimpanzee genome as two editions of the same book, say the first and second editions of The Descent ofMan. If you count the number of letters that are identical to their opposite numbers in the other edition, it is probably well over 90 per cent. But if you count the number of chapters that are identical, it may well be zero. This is because it takes only one letter to be different, anywhere in a chapter, for the whole chapter to be judged different between the two editions. When you are measuring the percentage similarity between two texts, whether two editions of a book or two editions of an African ape, the unit of comparison you choose (letter or chapter, DNA base pair or gene) makes
a huge difference to the final percentage similarity.
The point is that we should use such percentages not for their
absolute value but in comparisons between animals. The 98 per cent figure for humans and chimpanzees starts to make sense when we compare it with the 96 per cent resemblance between humans and orang utans (it is the same 96 per cent between chimpanzees and orang utans, and the same between gorillas and orang utans, because all the African apes are connected to the Asian orang utans via a shared African ancestor). For the same kind of reason, all the great apes share 95 per cent of their genomes with the gibbons and siamangs. And all the apes share 92 per cent of their genomes with all Old World monkeys.
The hypothesis of a molecular clock allows us to use such percentage figures to put a date on each of the splits in our family tree. It assumes that evolutionary change, at the molecular genetic level, proceeds at an approximately fixed rate for each gene. This is in accordance with the widely accepted neutral theory of the Japanese geneticist Motoo Kimura. Kimura's neutral theory is sometimes seen as anti-Darwinian but it is not. It is neutral with respect to Darwinian selection. A neutral mutation is one that makes no difference to the functioning of the protein produced. The post-mutation version is no better and no worse than the pre-mutation version, where both may be vital to the life of the organism.
From a Darwinian point of view, neutral mutations are not mutations at all. But from a molecular point of view they are extremely useful mutations because their fixed rate makes the clock reliable. The only point of controversy introduced by Kimura is how many mutations are neutral. Kimura thought it was the great majority which, if true, is very nice for the molecular clock. Darwinian selection remains the only explanation for adaptive evolution and it is arguable (I would argue)
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that most if not all of the evolutionary changes we actually see in the macroscopic world (as opposed to those concealed among the molecules) are adaptive and Darwinian.
As so far described, the molecular clock gives relative timings but not absolute ones. We can read off timings of evolutionary splits, but only in arbitrary units. Fortunately, in another great advance that would have entranced Darwin, various absolute clocks are available for dating fossils. These include the known rates of radioactive decay of isotopes in volcanic rocks sandwiching the sedimentary strata in which fossils are found. By taking a group of animals with a rich fossil record and dating the splits in their family tree two ways - by the molecular genetic clock and by radioactive clocks - the arbitrary units of the genetic clock can be validated, and simultaneously calibrated in real millions of years. This is how we can estimate that the split between humans and chimpanzees occurred between 5 and 8 million years ago, the split between African apes and orang utans about 14 million years ago, and the split between apes and Old World monkeys about 25 million years ago.
Fossils, all discovered after Descent was published, provide us with a sporadic picture of some possible intermediates connecting us to our common ancestor with chimpanzees. Unfortunately, there are no fossils connecting modern chimpanzees to that shared ancestor, but on our side of the split reports of new fossil finds are coming in at a rate which
I find exciting and surely Darwin would have too. Going back in steps
of roughly one million years we find: Homo erectus, Homo habilis, Australopithecus afarensis, Australopithecus anamensis, Ardipithecus, Orrorin and, a recent discovery which may date from as long ago as 7 million years, Sahelanthropus. That last find is from Chad, far to the west of the great Rift Valley which had hitherto been thought to constitute
a geographic barrier dividing our lineage from that of the chimpanzees. It is good for our orthodoxies to be upset from time to time.
We must beware of assuming that this temporal series of fossils represents an ancestor/descendant series. It is always safer to assume that fossils are cousins rather than ancestors, but we need not be shy of guessing that earlier cousins may tell us at least something about the true ancestors among their contemporaries.
What are the main changes that occurred since our split from the chimpanzees? Some, such as our loss of body hair, are interesting, but fossils can tell us nothing directly about them. The two main changes that fossils can help us with, and where we therefore have a big advantage over Darwin, are that we rose up on our hind legs, and that our brains got rather dramatically bigger. Which of these changes came
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? first, or did they happen together? All three views have been supported,
and controversy has gone back and forth over the decades. Darwin
thought the two big changes happened in concert, and he makes out a
plausible case. But this is a rare instance where Darwin's tentative guess
has turned out wrong. The fossils give a satisfyingly decisive and clear
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answer. Bipedality came first, and its evolution was more or less
complete before the brain started to swell. Three million years ago, Australopithecus was bipedal and had feet like ours, although it probably still retreated up trees. But its brain, relative to its body size, was the same size as a chimpanzee's, and presumably the same as the shared ancestor with chimpanzees. Nobody knows whether the bipedal gait set up new selection pressures that encouraged the brain to grow, but Darwin's original arguments for simultaneous evolution can be adapted to make that plausible. Perhaps the enlargement of the brain had some- thing to do with language, but here nobody knows and disagreements abound. There is evidence that particular parts of the human brain are uniquely pre-wired to handle specific universals of language, although
49 the particular language spoken is, of course, locally learned.
Another twentieth-century idea which is probably important in human evolution, and which again would have intrigued Darwin, is neoteny: evo- lutionary infantilization. The axolotl, an amphibian living in a Mexican lake, looks just like the larva of a salamander, but it can reproduce, and has chopped off the adult, salamander stage of the life history. It is a sexually mature tadpole. Such neoteny has been suggested as a way in which a lineage can suddenly initiate an entirely new direction of evolution, at a stroke. Apes don't have a discrete larval stage like a tadpole or a caterpillar, but a more gradualistic version of neoteny can be discerned in human evolution. Juvenile chimpanzees resemble humans far more than adult chimpanzees do. Human evolution can be seen as infantilism. We are apes
50 that became sexually mature while still morphologically juvenile. If
humans could live for 200 years, would we finally 'grow up', drop on all fours and develop huge prognathous chimpanzee-like jaws? The possibility has not been lost on writers of ironic fiction, notably Aldous Huxley in After Many a Summer. He presumably learned about neoteny from his elder brother Julian, who was one of the pioneers of the idea and did amazing research on axolotls, injecting hormones to make them turn into salamanders never before seen.
Let me end by bringing together once again the two halves of Darwin's book. He went to town on sexual selection in The Descent of Man because he thought it was important in human evolution, and especially because he thought it was the key to understanding the
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differences among human races. Race, in Victorian times, was not the political and emotional minefield it is today, when one can give offence j by so much as mentioning the word. I shall tread carefully, but I cannot I ignore the topic because it 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 than most of his contemporaries, emphasized the fundamental unity of our species. In Descent he carefully considered, and decisively rejected, the idea, rather favoured in his own time, that different human races should be regarded as separate species. Today we know that, at the genetic level, our species is more than usually uniform. It has been said that there is more genetic variation among the chimpanzees of a small region of Africa than among the entire world population of humans (suggesting that we have been through a bottleneck in the past hundred thousand years or so). Moreover, the great majority of human genetic variation is to be found within races, not between them. This means that if you were to wipe out all human races except one, the I great majority of human genetic variance would be preserved. The variance between races is just a bit extra, stuck on the top of the greater quantity of variation within all races. It is for this reason that many geneticists advocate the complete abandonment of the concept of race.
At the same time - the paradox is similar to one recognized by Darwin - the superficially conspicuous features characteristic of local populations around the world seem very different. A Martian taxonomist who didn't know that all human races happily interbreed with one another, and didn't know that most of the underlying genetic variance in our species is shared by all races, might be tempted by our regional differences in skin colour, facial features, hair, body size and proportions to split us into more than one species. What is the resolution of the paradox? And why did such pronounced superficial differences evolve in different geographical areas, while most of the less conspicuous variation is dotted around across all geographical areas? Could Darwin have been right all along? Is sexual selection the answer to the paradox? The distinguished
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biologist Jared Diamond thinks so, and I am inclined to agree.
Utilitarian answers have been suggested to the question of the evolution of racial differences, and there may well be some truth in them. Dark skin may protect against skin cancer in the tropics, light skin admit beneficial rays in sun-starved latitudes where there is a danger of Vitamin D deficiency. Small stature probably is of benefit to hunters in dense forest, such as the pygmies of central Africa, and various independently evolved hunter gatherers of Amazon and South East Asian
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1
? forests. The ability to digest milk when adult seems to have evolved in peoples who, for cultural reasons, prolong the use of this primitively juvenile food. But I am impressed by the diversity of features that are superficial and conspicuous, while deeper differences are so slight.
What sexual selection explains, better than natural selection, is diversity that seems arbitrary, even driven by aesthetic whim. Especially if the variation concerned is geographical. And also especially if some of 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 fashions like headdresses, body paint, penis sheaths, ritual mutilations or ornamental clothes. Given that cultural differences such as those of language, religion, manners and customs certainly provide resistance to interbreeding and gene flow, I think it is entirely plausible that genetic differences between peoples of different regions, at least where superficial, externally prominent features are concerned, have evolved through sexual selection. Our species really does seem to have unusually conspicuous, even ostentatious, superficial differences between local populations, coupled with unusually low levels of overall genetic variation. This double circumstance carries, to my mind, the stamp of sexual selection.
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In this respect, human races seem a lot like breeds of dog, another
favourite topic of Darwin. Superficially, the domestic breeds of dogs are astonishingly varied, even more so than human races, yet the under- lying genetic differences are slight, and they are all clearly descended
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from wolves within the past few thousand years. Reproductive
isolation is today maintained by disciplined pedigree breeders, and the shapes and colours of the dogs themselves are steered through their rapid evolution by the whim of the human eye rather than the whim of female dogs. But the essential features of the situation, as Darwin realized, are similar to those of sexual selection.
In this, as in so much else, I suspect that Darwin was right. Sexual
selection really is a good candidate for explaining a great deal about the
unique evolution of our species. It may also be responsible for some
unique features of our species which are shared equally by all races, for
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example our enormous brain. Geoffrey Miller, in The Mating Mind, strongly developed precisely this case, and Darwin would have loved it
no less because Miller takes a Wallacean view of sexual selection. It is starting to look as though, despite initial appearances, Darwin really was right to bring together, in one volume, Selection in Relation to Sex and The Descent ofMan.
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has
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Darwin Triumphant55 Darwinism as a Universal Truth
If we are visited by superior creatures from another star system - they will have to be superior if they are to get here at all - what common ground shall we find for discussion with them? Shall we overcome the barriers simply by learning one another's language, or will the subjects that interest our two cultures be so divergent as to preclude serious conversation? It seems unlikely that the star travellers will want to talk about many of our intellectual stocks-in-trade: about literary criticism or music, religion or politics. Shakespeare may mean nothing to those without human experiences and human emotions, and if they have a literature or an art these will probably be too alien to excite our sensi- bilities. To name two thinkers who have more than once been promoted as Darwin's equals, I rather doubt whether our visitors will have much interest in talking about Marx or Freud, other than perhaps as anthropological curiosities. We have no reason to suppose that these men's works are of more than local, parochial, human, earthly, post- Pleistocene (some would add European and male) significance.
Mathematics and physics are another matter. Our guests may find our level of sophistication quaintly low, but there will be common ground. We shall agree that certain questions about the universe are important, and we shall almost certainly agree on the answers to many of these questions. Conversation will flourish, even if most of the questions flow one way and most of the answers the other. If we discuss the histories of our respective cultures, our visitors will surely point with pride, however far back in time, to their equivalents of Einstein and Newton, of Planck and Heisenberg. But they won't point to an equivalent of Freud or Marx any more than we, visiting a hitherto undiscovered tribe in a remote forest clearing, would nominate our civilization's equiva- lent of the local rainmaker or gully-gully man. One does not have to disparage the local achievements of Freud and Marx on this planet to agree that their findings have no universality.
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? What about Darwin? Will our guests revere another Darwin as one of their greatest thinkers of all time? Shall we be able to have a serious conversation with them about evolution? I suggest that the answer is yes (unless, as a colleague suggests to me, their Darwin is on the expedi- tion and we are her Galapagos*). Darwin's achievement, like Einstein's, is universal and timeless, whereas that of Marx is parochial and ephemeral. That Darwin's question is universal, wherever there is life, is surely undeniable. The feature of living matter that most demands explanation is that it is almost unimaginably complicated in directions that convey a powerful illusion of deliberate design. Darwin's question, or rather the most fundamental and important of Darwin's many questions, is the question of how such complicated 'design' could come into being. All living creatures, everywhere in the universe and at any time in history, provoke this question. It is less obvious that Darwin's answer to the riddle - cumulative evolution by nonrandom survival of random hereditary changes - is universal. It is at first sight conceivable that Darwin's answer might be valid only parochially, only for the kind of life that happens to exist in our own little clearing in the universal
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forest. I have previously made the case that this is not so, that the
general form of Darwin's answer is not merely incidentally true of our kind of life but almost certainly true of all life, everywhere in the universe. Here, let me for the moment make the more modest claim that, at the very least, Darwin's bid for immortality is closer to the Einstein end of the spectrum than to the Marx end. Darwinism really matters in the universe.
When I was an undergraduate in the early nineteen sixties, we
were taught that although Darwin was an important figure in his own
time, modern neo-Darwinism was so much further advanced that it
hardly deserved the name Darwinism at all. My father's generation of
biologist undergraduates read, in an authoritative Short History of 57
. . . the struggle of living forms leading to natural selection by the survival of the fittest, is certainly far less emphasized by naturalists now than in the years that immediately followed the appearance of Darwin's book. At the time, however, it was an extremely stimulating suggestion.
And the generation of biologists before that could read, in the words of William Bateson, perhaps the dominant British geneticist of the time,
This is how my friend worded her suggestion. The joke was rather ruined by the political scruples of the original article's copy-editor, who changed 'her Galapagos' to 'his or her Galapagos'.
Biology , that
DARWIN TRIUMPHANT
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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 . . . 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 . . . at the want of
58 penetration displayed by the advocates of such a proposition.
And yet the editors of this volume can commission an article with the title 'Darwin Triumphant'. I do not normally like writing to titles that others have proposed, but I can accept this one without reservation. In the last quarter of the twentieth century, it seems to me that Darwin's standing among serious biologists (as opposed to nonbiologists influenced by religious preconceptions) is rightly as high as it has been at any time since his death. A similar story, of even more extreme eclipse in earlier years followed by triumphant recent rehabilitation, can be told of Darwin's 'other theory', that of sexual selection. *
It is only to be expected that, a century and a quarter on, the version
of his theory that we now have should be different from the original. Modern Darwinism is Darwinism plus Weismannism plus Fisherism plus Hamiltonism (arguably plus Kimuraism and a few other isms). But when I read Darwin himself, I am continually astonished at how modern
he sounds. Considering how utterly wrong he was on the all-important topic of genetics, he showed an uncanny gift for getting almost every- thing else right. Maybe we are neo-Darwinists today, but let us spell the neo with a very small n\ Our neo-Darwinism is very much in the spirit of Darwin himself. The changes that Darwin would see if he came back today are in most cases changes that, I venture to suggest, he would instantly approve and welcome as the elegant and obviously correct answers to riddles that troubled him in his own time. Upon learning that evolution is change in frequencies within a pool of particulate hereditary elements, he might even quote T. H. Huxley's alleged remark upon reading the Origin itself: 'How extremely stupid not to have thought of thatl't
*See 'Light Will Be Thrown' (pp. 63-77).
tOf the two stories about Huxley that have become chestnuts, I greatly prefer this to the one about his so-called 'debate' with the Bishop of Oxford, Sam Wilberforce. There is something admirably honest about Huxley's exasperation at not having thought of such a simple idea. I have long found it a complete mystery why it had to wait until the nineteenth century before anyone thought of it. Archimedes' and Newton's achievements seem, on the face of it, far more difficult. But the fact that nobody did think of natural selection before the nineteenth century clearly shows that I am wrong. As does the fact that so many people, even today, don't get it.
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? I referred to Darwin's gift for getting things right, but surely this can only mean right as we see it today. Shouldn't we be humble enough to admit that our right may be utterly wrong in the sight of future scientific generations? No, there are occasions when a generation's humility can be misplaced, not to say pedantic. We can now assert with confidence that the theory that the Earth moves round the Sun not only is right in our time but will be right in all future times even if flat- Earthism happens to become revived and universally accepted in some new dark age of human history. We cannot quite say that Darwinism is in the same unassailable class. Respectable opposition to it can still be mounted, and it can be seriously argued that the current high standing of Darwinism in educated minds may not last through all future generations. Darwin may be triumphant at the end of the twentieth century, but we must acknowledge the possibility that new facts may come to light which will force our successors of the twenty-first century to abandon Darwinism or modify it beyond recognition. But is there, perhaps, an essential core of Darwinism, a core that Darwin himself might have nominated as the irreducible heart of his theory, which we might set up as a candidate for discussion as potentially beyond the reach of factual refutation?
Core Darwinism, I shall suggest, is the minimal theory that evolution is guided in adaptively nonrandom directions by the nonrandom survival of small random hereditary changes. Note especially the words small and adaptively. Small implies that adaptive evolution is gradualistic, and we shall see why this must be so in a moment. Adaptive does not imply that all evolution is adaptive, only that core Darwinism's concern is limited to the part of evolution that is. There is no reason to assume
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that all evolutionary change is adaptive.
