It may well be that after, say, the
invention
of multicellularity, or the invention of segmentation, evolution was never the same again.
Richard-Dawkins-The-Devil-s-Chaplain
He'll find even
better examples if he looks at the comparative psychology literature,
which is awash with snobbish and downright silly phrases like
'subhuman primates', 'subprimate mammals' and 'submammalian
vertebrates', implying an unquestioned ladder of life denned so as to
perch us smugly on the top rung. Uncritical authors regularly move 'up'
or 'down' the 'evolutionary scale' (bear in mind that they are in fact
moving sideways among modern animals, contemporary twigs dotted
all around the tree of life). Students of comparative mentality
unabashedly and ludicrously ask, 'How far down the animal kingdom
does learning extend? ' Volume 1 of Hyman's celebrated treatise on the
invertebrates is entitled 'Protozoa through Ctenophora' (my emphasis) -
as if the phyla exist along an ordinal scale such that everybody knows
which groups sit 'between' Protozoa and Ctenophora. Unfortunately,
all zoology students do know - we've all been taught the same
121 groundless myth.
This is bad stuff, and Gould could afford to attack it even more severely than he attacks his normal targets. Whereas I would do so on logical grounds, Gould prefers an empirical assault. He looks at the actual course of evolution and argues that such apparent progress as can in general be detected is artefactual (like the baseball statistic). Cope's
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rule of increased body size, for example, follows from a simple 'drunkard's walk' model. The distribution of possible sizes is confined by a left wall, a minimal size. A random walk from a beginning near the left wall has nowhere to go but up the size distribution. The mean size has pretty well got to increase, and it doesn't imply a driven evolutionary trend towards larger size.
As Gould convincingly argues, the effect is compounded by a human tendency to give undue weight to new arrivals on the geological scene. Textbook biological histories emphasize a progression of grades of organization. As each new grade arrives, there is temptation to forget that the previous grades haven't gone away. Illustrators abet the fallacy when they draw, as representative of each era, only the newcomers. Before a certain date there were no eucaryotes. The arrival of eucaryotes looks more progressive than it really was because of the failure to depict the persisting hordes of procaryotes. The same false impression is conveyed with each new arrival on the stage: vertebrates, large-brained animals, and so on. An era may be described as the 'Age of Xs' - as though the denizens of the previous 'Age' had been replaced rather than merely supplemented.
Gould drives his point home with an admirable section on bacteria. For most of history, he reminds us, our ancestors have been bacteria. Most organisms still are bacteria, and a case can be made that most contemporary biomass is bacterial. We eucaryotes, we large animals, we brainy animals, are a recent wart on the face of a biosphere which is still fundamentally, and predominantly, procaryotic. To the extent that average size/complexity/cell number/brain size has increased since the 'age of bacteria', this could be simply because the wall of possibilities constrains the drunkard from moving in any other direction. John Maynard Smith recognized this possibility but doubted it when he
122 considered the matter in 1970.
The obvious and uninteresting explanation of the evolution of increasing complexity is that the first organisms were necessarily simple . . . And if the first organisms were simple, evolutionary change could only be in the direction of complexity.
Maynard Smith suspected that there was more to be said than this 'obvious and uninteresting explanation', but he didn't go into detail. Perhaps he was thinking of what he later came to term The Major Transitions in Evolution, or what I called 'The Evolution of Evolvability' (see below).
123
Gould's empirical treatment follows McShea , whose definition of
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124 125 complexity is reminiscent of J. W. S. Pringle's ; also of Julian Huxley's
definition of 'individuality' as 'heterogeneity of parts'. Pringle called complexity an epistemological concept, meaning a measure applied to our description of something rather than to that something itself. A crab is morphologically more complex than a millipede because, if you wrote a pair of books describing each animal down to the same level of detail, the crab book would have a higher word-count than the milli- pede book. The millipede book would describe a typical segment then simply add that, with listed exceptions, the other segments are the same. The crab book would require a separate chapter for each segment and would therefore have a higher information content. * McShea applied a similar notion to the vertebral column, expressing complexity in terms of heterogeneity among vertebrae.
With his measure of complexity in place, McShea sought statistical evidence for any general tendency for it to increase in fossil lineages. He made a distinction between passive trends (Gould's statistical artefacts) and driven trends (a true bias towards increased complexity, pre- sumably driven by natural selection). By Gould's enthusiastic account, he concluded that there is no general evidence that a statistical majority of evolutionary lineages show driven trends in the direction of increased complexity. Gould goes further, pointing out that since so many species are parasites and parasite lineages commonly favour decreased complexity, there may even be a statistical trend in the opposite direction to the one hypothesized.
Gould is sailing dangerously close to the windmill-tilting that he has previously made his personal art form. Why should any thoughtful Darwinian have expected a majority of lineages to increase in anatomical complexity? Certainly it is not clear that anybody inspired by adaptationist philosophy would. Admittedly people inspired by human vanity might (and historically Gould is right that many have fallen for this vice). Our human line happens to have specialized in complexity, especially of the nervous system, so it is only human that we should define progress as an increase in complexity or in braininess.
126 Other species will see it differently, as Julian Huxley
piece of verse entitled Progress:
The Crab to Cancer junior gave advice:
'Know what you want, my son, and then proceed Directly sideways. God has thus decreed - Progress is lateral; let that suffice'.
*See also 'The 'Information Challenge" (pp. 100-01). 210
pointed out in a
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Darwinian Tapeworms on the other hand Agree that Progress is a loss of brain,
And all that makes it hard for worms to attain The true Nirvana - peptic, pure and grand.
Man too enjoys to omphaloscopize. Himself as Navel of the Universe . . .
The poetry is not great (I couldn't bear to copy out the ending), and
there is a confusion of timescales between the crab verse (behavioural
time) and the tapeworm verse (evolutionary time), but an important
point lurks here. Gould uses a human-chauvinistic definition of progress,
measuring it in terms of complexity. This was why he was able to use
parasites as ammunition against progress. Huxley's tapeworms, using a
parasite-centred definition of progress, see the point with opposite sign.
A statistically minded swift would search in vain for evidence that a
majority of evolutionary lineages show trends towards improved flying
performance. Learned elephants, to borrow a pleasantry from Steven
127
Pinker , would ruefully fail to uphold the comforting notion that
progress, defined as a driven elongation of the nose, is manifested by a statistical majority of animal lineages.
This may seem a facetious point but that is far from my intention. On the contrary, it goes to the heart of my adaptationist definition of progress. This, to repeat, takes progress to mean an increase, not in complexity, intelligence or some other anthropocentric value, but in the accumulating number of features contributing towards whatever adaptation the lineage in question exemplifies. By this definition, adaptive evolution is not just incidentally progressive, it is deeply, dyed-in-the-wool, indispensably progressive. It is fundamentally necessary that it should be progressive if Darwinian natural selection is to perform the explanatory role in our world view that we require of it, and that it alone can perform. Here's why.
Creationists love Sir Fred Hoyle's vivid metaphor for his own mis- understanding of natural selection. It is as if a hurricane, blowing through a junkyard, had the good fortune to assemble a Boeing 747. Hoyle's point is about statistical improbability. Our answer, yours and mine and Stephen Gould's, is that natural selection is cumulative. There is a ratchet, such that small gains are saved. The hurricane doesn't spontaneously assemble the airliner in one go. Small improvements are added bit by bit. To change the metaphor, however daunting the sheer cliffs that the adaptive mountain first presents, graded ramps can be found on the other side and the peak eventually
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scaled. * Adaptive evolution must be gradual and cumulative, not because the evidence supports it (though it does) but because nothing except gradual accumulation could, in principle, do the job of solving the 747 riddle. Even divine creation wouldn't help. Quite the contrary, since any entity complicated and intelligent enough to perform the creative role would itself be the ultimate 747. And for exactly the same reason the evolution of complex, many-parted adaptations must be progressive. Later descendants will have accumulated a larger number of components towards the adaptive combination than earlier ancestors.
The evolution of the vertebrate eye must have been progressive. Ancient ancestors had a very simple eye, containing only a few features good for seeing. We don't need evidence for this (although it is nice that it is there). It has to be true because the alternative - an initially complex eye, well-endowed with features good for seeing - pitches us right back to Hoyle country and the sheer cliff of improbability. There must be a ramp of step-by-step progress towards the modern, multifeatured descendant of that optical prototype. Of course, in this case, modern analogues of every step up the ramp can be found, working serviceably in dozens of eyes dotted independently around the animal kingdom. But even without these examples, we could be confident that there must have been a gradual, progressive increase in the number of features which an engineer would recognize as contributing towards optical quality. Without stirring from our armchair, we can see that it must be so.
Darwin himself understood this kind of argument clearly, which is why he was such a staunch gradualist. Incidentally, it is also why Gould is unjust when he implies, not in this book but in many other places, that Darwin was against the spirit of punctuationism. The theory of punctuated equilibrium itself is gradualist (by Gad it had better be) in the sense in which Darwin was a gradualist - the sense in which all sane evolutionists must be gradualists, at least where complex adaptations are concerned. It is just that, if punctuationism is right, the progressive, gradualistic steps are compressed into a timeframe which the fossil record does not resolve. Gould admits this when pressed, but he isn't pressed often enough.
Mark Ridley quotes Darwin on orchids, in a letter to Asa Gray: 'It is
impossible to imagine so many co-adaptations being formed all by a
128
chance blow. ' As Ridley goes on, 'The evolution of complex organs
had to be gradual because all the correct changes would not occur in a single large mutation. ' And gradual, in this context, needs to mean
*This rather coy allusion to Climbing Mount Improbable seemed appropriate because, as explained in the Preamble to this section, the Editor of Evolution had simultaneously commissioned a review of that book from Dr Gould.
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progressive in my 'adaptationist' sense. The evolution of anything as complex as an advanced orchid was progressive. So was the evolution of echolocation in bats and river dolphins - progressive over many, many steps. So was the evolution of electrolocation in fish, and of skull dislocation in snakes for swallowing large prey. So was the evolution of the complex of adaptations that equips cheetahs to kill, and the corresponding complex that equips gazelles to escape.
Indeed, as Darwin again realized, although he did not use the phrase, one of the main driving forces of progressive evolution is the co- evolutionary arms race, such as that between predators and their prey. Adaptation to the weather, to the inanimate vicissitudes of ice ages and droughts, may well not be progressive: just an aimless tracking of un- progressively meandering climatic variables. But adaptation to the biotic environment is likely to be progressive because enemies, unlike the weather, themselves evolve. The resulting positive feedback loop is a good explanation for driven progressive evolution, and the drive may be sustained for many successive generations. The participants in the race do not necessarily survive more successfully as time goes by - their 'partners' in the revolutionary spiral see to that (the familiar Red Queen Effect). But the equipment for survival, on both sides, is improving as judged by engineering criteria. In hard-fought examples we may notice a progressive shift in resources from other parts of the animal's economy
129
to service the arms race. And in any case the improvement in equip-
ment will normally be progressive. Another kind of positive feedback in evolution, if R. A. Fisher and his followers are right, results from sexual selection. Once again, progressive evolution is the expected consequence.
Progressive increase in morphological complexity is to be expected only in taxa whose way of life benefits from morphological complexity. Progressive increase in brain size is to be expected only in animals where braininess is an advantage. This may, for all I know, constitute a minority of lineages. But what I do insist is that in a majority of evolutionary lineages there will be progressive evolution towards some- thing. It won't, however, be the same thing in different lineages (this was the point about swifts and elephants). And there is no general reason to expect a majority of lineages to progress in the directions pioneered by our human line.
But have I now defined progress so generally as to make it a blandly useless word? I don't think so. To say that the evolution of the verte- brate eye was progressive is to say something quite strong and quite important. If you could lay out all the intermediate ancestors in chronological order you'd find that, first, for a majority of dimensions
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of measurement, the changes would be transitive over the whole sequence. That is, if A is ancestral to B which is ancestral to C, the direction of change from A to B is likely to be the same as the direction of change from B to C. Second, the number of successive steps over which progress is seen is likely to be large: the transitive series extends beyond A, B and C, far down the alphabet. Third, an engineer would judge the performance to have improved over the sequence. Fourth, the number of separate features combining and conspiring to improve performance would increase. Finally, this kind of progress really matters because it is the key to answering the Hoyle challenge. There will be exceptional reversals, for instance in the evolution of blind cave fish, where eyes degenerate because they are not used and are costly to make. And there will doubtless be periods of stasis where there is no evolution at all, progressive or otherwise.
To conclude this point, Gould is wrong to say that the appearance of progress in evolution is a statistical illusion. It does not result just from a change in variance as a baseball-style artefact. To be sure, complexity, braininess and other particular qualities dear to the human ego should not necessarily be expected to increase progressively in a majority of lineages - though it would be interesting if they did: the investigations
130
of McShea, Jerison and others are not a waste of time. But if you
define progress less chauvinistically - if you let the animals bring their own definition - you will find progress, in a genuinely interesting sense of the word, nearly everywhere.
Now it is important to stress that, on this adaptationist view (unlike the 'evolution of evolvability' view to be discussed shortly), progressive evolution is to be expected only on the short to medium term. Coevolutionary arms races may last for millions of years, but probably not hundreds of millions. Over the very long timescale, asteroids and other catastrophes bring evolution to a dead stop, major taxa and entire radiations go extinct. Ecological vacuums are created, to befilledby new adaptive radiations driven by new ranges of arms races. The several arms races between carnivorous dinosaurs and their prey were later mirrored by a succession of analogous arms races between carnivorous mammals and their prey. Each of these successive and separate arms races powered sequences of evolution which were progressive in my sense. But there was no global progress over the hundreds of millions of years, only a sawtooth succession of small progresses terminated by extinctions. Nonetheless, the ramp phase of each sawtooth was properly and significantly progressive.
Ironically for such an eloquent foe of progress, Gould flirts with the idea that evolution itself changes over the long haul, but he puts it in a topsy-turvy way which has undoubtedly been widely misleading. It is
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more fully expounded in Wonderful Life but reprised in the present book. For Gould, evolution in the Cambrian was a different kind of process from evolution today. The Cambrian was a period of evolution- ary 'experiment', evolutionary 'trial and error', evolutionary 'false starts'. It was a period of 'explosive' invention, before evolution stabilized into the humdrum process we see today. It was the fertile time when all the great 'fundamental body plans' were invented. Nowadays, evolution just tinkers with old body plans. Back in the Cambrian, new phyla and new classes arose. Nowadays we only get new species!
This may be a slight caricature of Gould's own considered position,
but there is no doubt that the many American nonspecialists who
131
unfortunately, as Maynard Smith wickedly observes, get their
evolutionary knowledge almost entirely from Gould, have been deeply misled. Admittedly, what follows is an extreme example, but Daniel Dennett has recounted a conversation with a philosopher colleague who read Wonderful Life as arguing that the Cambrian phyla did not have a common ancestor - that they had sprung up as independently initiated life forms! When Dennett assured him that this was not Gould's claim, his colleague's response was, 'Well then, what is all the fuss about? '
Even some professional evolutionists have been inspired by Gould's rhetoric into committing some pretty remarkable solecisms. Leakey and
132
Lewin's The Sixth Extinction is an excellent book except for its Chapter
3, 'The Mainspring of Evolution', which is avowedly heavily influenced by Gould. The following quotations from that chapter could hardly be more embarrassingly explicit:
Why haven't new animal body plans continued to crawl out of the evolutionary cauldron during the past hundreds of millions of years?
In early Cambrian times, innovations at the phylum level survived because they faced little competition.
Below the level of the family, the Cambrian explosion produced relatively few species, whereas in the post-Permian a tremendous species diversity burgeoned. Above family level however, the post-Permian radiation faltered, with few new classes and no new phyla being generated. Evidently, the mainspring of evolution operated in both periods, but it propelled greater extreme experimentation in the Cambrian than in the post-Permian, and greater variations on existing themes in the post-Permian.
Hence, evolution in Cambrian organisms could take bigger leaps, including phylum-level leaps, while later on it would be more constrained, making only modest jumps, up to the class level.
It is as though a gardener looked at an old oak tree and remarked,
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wonderingly: 'Isn't it strange that no major new boughs have appeared on this tree recently. These days, all the new growth appears to be at the twig level! '
As it happens, molecular clock evidence indicates that the 'Cambrian Explosion' may never have happened. Far from the major phyla diverging from a point at the beginning of the Cambrian, Wray,
133
Levinton and Shapiro present evidence that the common ancestors of
the major phyla are staggered through hundreds of millions of years back in the Precambrian. But never mind that. That is not the point I want to make. Even if there really was a Cambrian explosion such that all the major phyla diverged during a ten million-year period, this is no reason to think that Cambrian evolution was a qualitatively special kind of super-jumpy process. Bauplane don't drop out of a clear Platonic sky, they evolve step by step from predecessors, and they do so (I bet, and so would Gould if explicitly challenged) under approximately the same Darwinian rules as we see today.
'Phylum-level leaps' and 'modest jumps, up to the class level' are the sheerest nonsense. Jumps above the species level don't happen, and nobody who thinks about it for two minutes claims that they do. Even the great phyla, when they originally bifurcated one from another, were just pairs of new species, members of the same genus. Classes are species that diverged a very long time ago, and phyla are species that diverged an even longer time ago. Indeed it is a moot - and rather empty - question precisely when in the course of the step-by-step, gradual mutual divergence of, say, mollusc ancestors and annelid ancestors after the time when they were congeneric species, we should wish to say that the divergence had reached 'Bauplan' status. A good case could be made that The Bauplan is a myth, probably as pernicious as any of the myths that Stephen Gould has so ably combatted, but this one, in its modern form, is largely perpetuated by him.
I return, finally, to the 'evolution of evolvability' and a very real sense in which evolution itself may evolve, progressively, over a longer timescale than the individual ramps of the arms race sawtooth. Notwith- standing Gould's just scepticism over the tendency to label each era by its newest arrivals, there really is a good possibility that major innovations in embryological technique open up new vistas of evolutionary possibility and that these constitute genuinely progressive improvements. * The
*This is the idea that I dubbed 'The Evolution of Evolvability' (in C. Langton (ed. ), Artificial Life (Santa Fe, Addison Wesley, 1982)) and Maynard Smith and Szathmary wrote a book about 0. Maynard Smith and E. Szathmary, The Major Transitions in Evolution (Oxford, W. H. Freeman/Spektrum, 1995)).
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origin of the chromosome, of the bounded cell, of organized meiosis, diploidy and sex, of the eucaryotic cell, of multicellularity, of gastrulation, of molluscan torsion, of segmentation - each of these may have constituted a watershed event in the history of life. Not just in the normal Darwinian sense of assisting individuals to survive and reproduce, but watershed in the sense of boosting evolution itself in ways that seem entitled to the label progressive.
It may well be that after, say, the invention of multicellularity, or the invention of segmentation, evolution was never the same again. In this sense there may be a one-way ratchet of progressive innovation in evolution.
For this reason over the long term, and because of the cumulative character of coevolutionary arms races over the shorter term, Gould's attempt to reduce all progress to a trivial, baseball-style artefact con- stitutes a surprising impoverishment, an uncharacteristic slight, an unwonted demeaning of the richness of evolutionary processes.
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Unfinished Correspondence with a Darwinian Heavyweight
The following correspondence was never completed and now, sadly, it never can be.
9 December 2001
Stephen Jay Gould Harvard
Dear Steve
Recently I received an email from Phillip Johnson, founder of the so-called 'Intelligent Design' school of creationists, crowing in triumph because one of his colleagues, Jonathan Wells, had been invited to take part in a debate at Harvard. He included the text of his email on his 'Wedge of Truth' web site, in which he announced the Wells debate under the headline 'Wells Hits a Home Run at Harvard'.
http://www. arn. org/docs/pjweekly/pj_weekly_011202. htm
The 'Home Run' turns out to be NOT a resounding success by Wells in convincing the audience, NOR any kind of besting of his opponent (Stephen Palumbi, who tells me he agreed to take part, with great reluctance, only because somebody at Harvard had ALREADY invited Wells and it was too late to do anything about that). There is no suggestion that Wells did well in the debate, nor even any obvious interest in whether he did. No, the 'Home Run' was simply and solely the matter of being invited by Harvard in the first place. These people have no hope of convincing reputable scientists by their ridiculous arguments. Instead, what they seek is the oxygen of respectability. We give them this oxygen by the mere act of ENGAGING with them at all. They don't mind being beaten in argument. What matters is that we give them recognition by bothering to argue with them in public.
You convinced me of this years ago when I phoned you up (you have probably
forgotten this) to ask your advice when I was invited to debate Duane P Gish. 218
? Ever since that phone call, I have repeatedly cited you and refused to debate these people, not because I am afraid of 'losing' the debate, but because, as you said, just to appear on a platform with them is to lend them the respectability they crave. Whatever might be the outcome of the debate, the mere fact that it is staged at all suggests to ignorant bystanders that there must be something worth debating, on something like equal terms.
First, I am interested to know whether you still hold to this view, as I do. Second, I am proposing that you might consider uniting with me (no need to involve others) in signing a short letter, say to the New York Review of Books, explaining publicly why we do not debate creationists (including the 'Intelligent Design' euphemism for creationists) and encouraging other evolutionary biologists to follow suit.
Such a letter would have great impact precisely because there have been widely publicised differences, and even animosities, between us (differences which creationists, with extreme intellectual dishonesty, have not hesitated to exploit). And I would not suggest writing a long disquisition on the technical differences which remain between us. That would only confuse the issue, make it harder to agree on a final draft, and lessen the impact. I wouldn't even mention our differences. I suggest a brief letter to the editor, explaining why we do not engage with 'intelligent design' or any other species of creationists, and offering our letter as a model for others to cite in refusing such invitations in the future. We both have better things to do with our time than give it over to such nonsense. Having just reached my sixtieth birthday (we are almost exactly the same age) I feel this keenly.
Steve replied on 11th December 2001, a warm and friendly email enthusiastically agreeing that a joint letter was an excellent idea, and saying that he'd be delighted to join me as the sole other signatory. He agreed that the New York Review of Books might well be the best place, and proposed that I should write a first draft. I reproduce that here, exactly as I sent it to him for his approval.
14 December 2001
Dear Editor
Like any flourishing science, the study of evolution has its internal controversies, as we both know. But no qualified scientist doubts that evolution is a fact, in the ordinarily accepted sense in which it is a fact that the Earth orbits the Sun. It is a fact that human beings are cousins
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to monkeys, kangaroos, jellyfish and bacteria. No reputable biologist doubts this. Nor do reputable theologians, from the Pope on. Unfortunately, many lay Americans do, including some frighteningly influential, powerful and, above all, well-financed ones.
We are continually invited to engage in public debates against creationists, including latter-day creationists disguised under the euphemism 'Intelligent Design Theorists'. We always refuse, for one overriding reason. If we may be allowed to spell this reason out publicly, we hope our letter may be helpful to other evolutionary scientists plagued by similar invitations.
The question of who would 'win' such a debate is not at issue. Winning is not what these people realistically aspire to. The coup they seek is simply the recognition of being allowed to share a platform with a real scientist in thefirstplace. This will suggest to innocent bystanders that there must be material here that is genuinely worth debating, on something like equal terms.
At the moment of writing, the leading 'Intelligent Design' website
reports a debate at Harvard under the banner headline 'Wells Hits a
134
Home Run at Harvard'. Jonathan Wells is a creationist, incidentally a
long-time devotee of the Unification Church (the 'Moonies'). * He had a debate last month against Stephen Palumbi, Professor of Biology at Harvard University. 'Home Run' might seem to suggest that Reverends (sic) Wells scored some kind of victory over Professor Palumbi. Or at least that he made powerful points and his speech was well received. No such claim is made. It doesn't even seem to be of interest.
The 'Home Run' turns out to be simply the public demonstration at Harvard that, in the words of the website's author, Phillip Johnson, 'This is the sort of debate that is now occurring in universities. ' There was a victory, but it occurred long before the debate itself. The creationist scored his home run at the moment the invitation from Harvard landed on his doormat. It came, by the way, not from any biological, or indeed scientific, department, but from the Institute of Politics.
Phillip Johnson himself, founding father of the 'Intelligent Design'
''Darwinism: Why I went for a Second Ph. D. ' is Jonathan Wells' own testimony on the turning point of his life: 'Father's words, my studies, and my prayers convinced me that I should devote my life to destroying Darwinism, just as many of my fellow Unificationists had already devoted their lives to destroying Marxism. When Father chose me (along with about a dozen other seminary graduates) to enter a Ph. D. program in 1978, I welcomed the opportunity to prepare myself for battle. ' ('Father', of course, is the Moonies' name for Rev Moon himself). http://www. tparents. org/LibraryAJnincation/Talks/Wells/DARWIN. htm.
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? movement (not a biologist, nor a scientist of any kind, but a lawyer who became a mid-life born-again Christian), wrote, in a letter of 6 April 2001, which he copied to one of us:
It isn't worth my while to debate every ambitious Darwinist who wants to try his hand at ridiculing the opposition, so my general policy is that Darwinists have to put a significant figure at risk before I will agree to a debate. That means specifically Dawkins or Gould, or someone of like stature and public visibility.
Well, we can condescend too, and we have the advantage that evolutionary scientists don't need the publicity such debates can bring. In the unlikely event that a significant argument should ever emerge from the ranks of creationism/'intelligent design', we will be happy to debate it. Meanwhile, we shall cultivate our evolutionary gardens, occasionally engaging in the more exacting and worthwhile task of debating each other. What we shall not do is abet creationists in their disreputable quest for free publicity and unearned academic respectability.
In all humility, we offer these thoughts to our colleagues who receive similar invitations to debate.
Unfortunately, Steve never got around to revising the letter, which therefore lacks the stylish panache which his dexterous touch would have lent it. I received one further email, apologizing for the delay and hoping to deal with the matter soon. The subsequent silence, I now realize, coincided with his last illness. I therefore offer my draft, imperfect as it is, in the hope that it may go some way towards conveying the message which I originally learned from him many years ago. It is my sincere hope that he would have approved the content of the letter, but of course I cannot be sure.
To close this section on a note of such concord may seem puzzling.
Given that Steve was as much a neo-Darwinist as I am, what did we dis-
agree about? The major disagreement emerges clearly out of his last big
5
book, The Structure of Evolutionary Theory,TM which I had no opportunity
to see until after his death. It is appropriate, therefore, to spell out that issue here, and it also, as it happens, forms a natural bridge to the next essay. The question under dispute is this: what is the role of genes in evolution? Is it, to use Gould's phrase, 'book-keeping or causation'?
Gould saw natural selection as operating on many levels in the hierarchy of life. Indeed it may, after a fashion, but I believe that such selection can have evolutionary consequences only when the entities selected consist of 'replicators'. A replicator is a unit of coded information, of high fidelity but occasionally mutable, with some causal power over its own fate. Genes are
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such entities. So, in principle, are memes, but they are not under discus- sion here. Biological natural selection, at whatever level we may see it, results in evolutionary effects only insofar as it gives rise to changes in gene frequencies in gene pools. Gould, however, saw genes only as 'book- keepers', passively tracking the changes going on at other levels. In my view, whatever else genes are, they must be more than book-keepers, otherwise natural selection cannot work. If a genetic change has no causal influence on bodies, or at least on something that natural selection can 'see', natural selection cannot favour or disfavour it. No evolutionary change will result.
Gould and I would agree that genes can be seen as a book in which is
written the evolutionary history of a species. In Unweaving the Rainbow I
called it 'The Genetic Book of the Dead'. But the book is written via the
natural selection of randomly varying genes, chosen by virtue of their
causal influence on bodies. Book-keeping is precisely the wrong metaphor,
because it reverses the causal arrow, almost in Lamarckian fashion, and
makes the genes passive recorders. I dealt with this in 1982 (The Extended
Phenotype) in my distinction between 'active replicators' and 'passive
replicators'. The point is also explained in David Barash's superb review of
136 Gould's book.
Book-keeping is perversely - and characteristically - a valuable meta- phor precisely because it is so diametrically back to front. Not for the first time, the characteristic vividness and clarity of a Gouldian metaphor helps us to see vividly and clearly what is wrong with the Gouldian message - and how it needs to be reversed in order to get at the truth.
I hope this brief note will not be seen as taking advantage to get the last word. The Structure of Evolutionary Theory is such a massively powerful last word, it will keep us all busy replying to it for years. What a brilliant way
for a scholar to go. I shall miss him.
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I am one of those (it includes most people who have ever spent time south of the Sahara) who think of Africa as a place of enchantment. For me it stems from faint but haunting childhood memories, coupled with the mature understanding that Africa is our ancestral home. These themes recur throughout this section, and they introduce Ecology of Genes (6. 1), my Foreword to Harvey Croze and John Reader's Pyramids of Life. This book uses Africa as an illuminating case study in the principles of ecology, and I used the opportunity of the Foreword to think about the relationship between ecology and natural selection. This could be seen as a continuation of my argument in the afterword to the previous section.
In this book, and elsewhere, I have been unkind to a view favoured by some social anthropologists, the 'cultural relativism' that acknowledges the equal status of many kinds of truth, scientific truth having no privileged rank among them. If ever I could be converted to some form of relativism, it might be after reading Elspeth Huxley's remarkable epic of Kenya, Red Strangers. Out of the Soul of Africa (6. 2) is the Foreword to the new paperback edition of her novel. I wrote an article for the Financial Times, pointing out that Red Strangers had been out of print for years and challenging any publisher to do something about it. The admirable Penguin did, and they reprinted my article as the Foreword.
I am now waiting for a scholar of literature to explain to me why Red Strangers is not rated one of the great novels of the twentieth century, the equal of a John Steinbeck except that Elspeth Huxley's imagery is Kikuyu rather than American.
Run like the eland . . . Run, warriors, with feet like arrows and the hearts of lions; the lives and wealth of your fathers are yours to save . . . Their thighs were straight as saplings, their features sharp as axes, their skins lighter than honey. His limbs began to quiver like the wings of a sunbird when its beak sucks honey . . .
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It is a virtuoso feat of identification with another culture. Not only does she succeed in getting herself inside a Kikuyu skin, she achieves the same feat for the reader. And she makes you cry.
I am slightly ashamed to admit that another book that brings me close to tears - of joy this time - is a children's book. Or is it a very grown-up book which happens to be written by children? It is hard to decide, which is part of its charm and also probably why it has been unaccountably ignored by book reviews editors - they just didn't know which shelf to put it on. The Lion Children is about a family of children who are English, but whose home is a set of tents in Botswana, where they radio-track wild lions and are schooled entirely by their mother in the bush. They have written a book about their utterly extraordinary life. Never mind whether there is a conventionally labelled shelf to put it on, just read it. I Speak of Africa and Golden Joys (6. 3), my Foreword, is reproduced here.
Last in this section is a travel piece, which again takes up the two themes of Africa as our ancestral home and Africa as my personal birthplace and weaves them together in an autobiographical story of travel and personal inspiration. The title was changed by the Sunday Times to 'All Our Yesterdays', but Macbeth's world-weariness is exactly opposite to the mood of my piece, so I am reverting to my original title, Heroes and Ancestors (6. 4). Now that I think about it, Heroes and Ancestors would have made another fine title for this whole collection.
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6 Ecology of Genes137
Foreword to Pyramids of Life
by Harvey Croze and John Reader
Africa was my personal cradle. But I left when I was seven, too young to appreciate - indeed the fact was not then known - that Africa is also humanity's cradle. The fossils of our species' formative years are all from Africa, and molecular evidence suggests that the ancestors of all today's peoples stayed there until as recently as the last hundred thousand years or so. We have Africa in our blood and Africa has our bones. We are all Africans.
This alone makes the African ecosystem an object of singular fascination. It is the community that shaped us, the commonwealth of animals and plants in which we served our ecological apprenticeship. But even if it were not our home continent Africa would captivate us, as perhaps the last great refugium of Pleistocene ecologies. If you want a late glimpse of the Garden of Eden, forget Tigris and Euphrates and the dawn of agriculture. Go instead to the Serengeti or the Kalahari. Forget the Arcadia of the Greeks and the dreamtime of the outback, they are so recent. Whatever may have come down the mountain at Olympus or Sinai, or even Ayers Rock, look instead to Kilimanjaro, or down the Rift Valley towards the High Veldt. There is where we were designed to flourish.
The 'design' of all living things and their organs is, of course, an illusion; an exceedingly powerful illusion, fabricated by a suitably power- ful process, Darwinian natural selection. There is a second illusion of design in nature, less compelling but still appealing, and it is in danger of being mistaken for the first. This is the apparent design of ecosystems. Where bodies have parts that intricately harmonize and regulate to keep them alive, ecosystems have species that appear to do something similar at a higher level. There are the primary producers that convert raw solar energy into a form that others can use. There are the herbivores that consume them to use it, and then make a tithe of it available for carnivores and so on up the food chain - pyramid, rather,
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for the laws of thermodynamics rule that only a tenth of each level's energy shall make it to the level above. Finally, there are scavengers that recycle the waste products to make them available again, and in the process clean up the world and stop it becoming a tip. Everything fits with everything else like jigsaw pieces meshing in a huge multi- dimensional puzzle, and - as the cliche goes - we meddle with the parts at the risk of destroying a priceless whole.
The temptation is to think that this second illusion is crafted by the same kind of process as the first: by a version of Darwinian selection, but at a higher level. According to this erroneous view, the ecosystems that survive are the ones whose parts - species - harmonize, just as the organisms that survive in conventional Darwinism are the ones whose parts - organs and cells - work harmoniously for their survival. I believe that this theory is false. Ecosystems, like organisms, do indeed seem harmoniously designed; and the appearance of design is indeed an illusion. But there the resemblance ends. It is a different kind of illusion, brought about by a different process. The best ecologists, such as Croze and Reader, understand this.
Darwinism enters into the process, but it does not jump levels. Genes still survive, or fail to survive, within the gene pools of species, by virtue of their effects upon the survival and reproduction of the individual organisms that contain them. The illusion of harmony at a higher level is an indirect consequence of differential individual reproduction. Within any one species of animals or plants, the individuals that survive best are the ones that can exploit the other animals and plants, bacteria and fungi that are already flourishing in the environment. As Adam Smith understood long ago, an illusion of harmony and real efficiency will emerge in an economy dominated by self-interest at a lower level. A well balanced ecosystem is an economy, not an adaptation.
Plants flourish for their own good, not for the good of herbivores. But because plants flourish, a niche for herbivores opens up, and they fill it. Grasses are said to benefit from being grazed. The truth is more interest- ing. No individual plant benefits from being grazed per se. But a plant that suffers only slightly when it is grazed outcompetes a rival plant that suffers more. So successful grasses have benefited indirectly from the presence of grazers. And of course grazers benefit from the presence of grasses. Grasslands therefore build up as harmonious communities of relatively compatible grasses and grazers. They seem to cooperate. In a sense they do, but it is a modest sense that must be cautiously under- stood and judiciously understated. The same is true of the other African communities expounded by Croze and Reader.
better examples if he looks at the comparative psychology literature,
which is awash with snobbish and downright silly phrases like
'subhuman primates', 'subprimate mammals' and 'submammalian
vertebrates', implying an unquestioned ladder of life denned so as to
perch us smugly on the top rung. Uncritical authors regularly move 'up'
or 'down' the 'evolutionary scale' (bear in mind that they are in fact
moving sideways among modern animals, contemporary twigs dotted
all around the tree of life). Students of comparative mentality
unabashedly and ludicrously ask, 'How far down the animal kingdom
does learning extend? ' Volume 1 of Hyman's celebrated treatise on the
invertebrates is entitled 'Protozoa through Ctenophora' (my emphasis) -
as if the phyla exist along an ordinal scale such that everybody knows
which groups sit 'between' Protozoa and Ctenophora. Unfortunately,
all zoology students do know - we've all been taught the same
121 groundless myth.
This is bad stuff, and Gould could afford to attack it even more severely than he attacks his normal targets. Whereas I would do so on logical grounds, Gould prefers an empirical assault. He looks at the actual course of evolution and argues that such apparent progress as can in general be detected is artefactual (like the baseball statistic). Cope's
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rule of increased body size, for example, follows from a simple 'drunkard's walk' model. The distribution of possible sizes is confined by a left wall, a minimal size. A random walk from a beginning near the left wall has nowhere to go but up the size distribution. The mean size has pretty well got to increase, and it doesn't imply a driven evolutionary trend towards larger size.
As Gould convincingly argues, the effect is compounded by a human tendency to give undue weight to new arrivals on the geological scene. Textbook biological histories emphasize a progression of grades of organization. As each new grade arrives, there is temptation to forget that the previous grades haven't gone away. Illustrators abet the fallacy when they draw, as representative of each era, only the newcomers. Before a certain date there were no eucaryotes. The arrival of eucaryotes looks more progressive than it really was because of the failure to depict the persisting hordes of procaryotes. The same false impression is conveyed with each new arrival on the stage: vertebrates, large-brained animals, and so on. An era may be described as the 'Age of Xs' - as though the denizens of the previous 'Age' had been replaced rather than merely supplemented.
Gould drives his point home with an admirable section on bacteria. For most of history, he reminds us, our ancestors have been bacteria. Most organisms still are bacteria, and a case can be made that most contemporary biomass is bacterial. We eucaryotes, we large animals, we brainy animals, are a recent wart on the face of a biosphere which is still fundamentally, and predominantly, procaryotic. To the extent that average size/complexity/cell number/brain size has increased since the 'age of bacteria', this could be simply because the wall of possibilities constrains the drunkard from moving in any other direction. John Maynard Smith recognized this possibility but doubted it when he
122 considered the matter in 1970.
The obvious and uninteresting explanation of the evolution of increasing complexity is that the first organisms were necessarily simple . . . And if the first organisms were simple, evolutionary change could only be in the direction of complexity.
Maynard Smith suspected that there was more to be said than this 'obvious and uninteresting explanation', but he didn't go into detail. Perhaps he was thinking of what he later came to term The Major Transitions in Evolution, or what I called 'The Evolution of Evolvability' (see below).
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Gould's empirical treatment follows McShea , whose definition of
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124 125 complexity is reminiscent of J. W. S. Pringle's ; also of Julian Huxley's
definition of 'individuality' as 'heterogeneity of parts'. Pringle called complexity an epistemological concept, meaning a measure applied to our description of something rather than to that something itself. A crab is morphologically more complex than a millipede because, if you wrote a pair of books describing each animal down to the same level of detail, the crab book would have a higher word-count than the milli- pede book. The millipede book would describe a typical segment then simply add that, with listed exceptions, the other segments are the same. The crab book would require a separate chapter for each segment and would therefore have a higher information content. * McShea applied a similar notion to the vertebral column, expressing complexity in terms of heterogeneity among vertebrae.
With his measure of complexity in place, McShea sought statistical evidence for any general tendency for it to increase in fossil lineages. He made a distinction between passive trends (Gould's statistical artefacts) and driven trends (a true bias towards increased complexity, pre- sumably driven by natural selection). By Gould's enthusiastic account, he concluded that there is no general evidence that a statistical majority of evolutionary lineages show driven trends in the direction of increased complexity. Gould goes further, pointing out that since so many species are parasites and parasite lineages commonly favour decreased complexity, there may even be a statistical trend in the opposite direction to the one hypothesized.
Gould is sailing dangerously close to the windmill-tilting that he has previously made his personal art form. Why should any thoughtful Darwinian have expected a majority of lineages to increase in anatomical complexity? Certainly it is not clear that anybody inspired by adaptationist philosophy would. Admittedly people inspired by human vanity might (and historically Gould is right that many have fallen for this vice). Our human line happens to have specialized in complexity, especially of the nervous system, so it is only human that we should define progress as an increase in complexity or in braininess.
126 Other species will see it differently, as Julian Huxley
piece of verse entitled Progress:
The Crab to Cancer junior gave advice:
'Know what you want, my son, and then proceed Directly sideways. God has thus decreed - Progress is lateral; let that suffice'.
*See also 'The 'Information Challenge" (pp. 100-01). 210
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Darwinian Tapeworms on the other hand Agree that Progress is a loss of brain,
And all that makes it hard for worms to attain The true Nirvana - peptic, pure and grand.
Man too enjoys to omphaloscopize. Himself as Navel of the Universe . . .
The poetry is not great (I couldn't bear to copy out the ending), and
there is a confusion of timescales between the crab verse (behavioural
time) and the tapeworm verse (evolutionary time), but an important
point lurks here. Gould uses a human-chauvinistic definition of progress,
measuring it in terms of complexity. This was why he was able to use
parasites as ammunition against progress. Huxley's tapeworms, using a
parasite-centred definition of progress, see the point with opposite sign.
A statistically minded swift would search in vain for evidence that a
majority of evolutionary lineages show trends towards improved flying
performance. Learned elephants, to borrow a pleasantry from Steven
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Pinker , would ruefully fail to uphold the comforting notion that
progress, defined as a driven elongation of the nose, is manifested by a statistical majority of animal lineages.
This may seem a facetious point but that is far from my intention. On the contrary, it goes to the heart of my adaptationist definition of progress. This, to repeat, takes progress to mean an increase, not in complexity, intelligence or some other anthropocentric value, but in the accumulating number of features contributing towards whatever adaptation the lineage in question exemplifies. By this definition, adaptive evolution is not just incidentally progressive, it is deeply, dyed-in-the-wool, indispensably progressive. It is fundamentally necessary that it should be progressive if Darwinian natural selection is to perform the explanatory role in our world view that we require of it, and that it alone can perform. Here's why.
Creationists love Sir Fred Hoyle's vivid metaphor for his own mis- understanding of natural selection. It is as if a hurricane, blowing through a junkyard, had the good fortune to assemble a Boeing 747. Hoyle's point is about statistical improbability. Our answer, yours and mine and Stephen Gould's, is that natural selection is cumulative. There is a ratchet, such that small gains are saved. The hurricane doesn't spontaneously assemble the airliner in one go. Small improvements are added bit by bit. To change the metaphor, however daunting the sheer cliffs that the adaptive mountain first presents, graded ramps can be found on the other side and the peak eventually
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scaled. * Adaptive evolution must be gradual and cumulative, not because the evidence supports it (though it does) but because nothing except gradual accumulation could, in principle, do the job of solving the 747 riddle. Even divine creation wouldn't help. Quite the contrary, since any entity complicated and intelligent enough to perform the creative role would itself be the ultimate 747. And for exactly the same reason the evolution of complex, many-parted adaptations must be progressive. Later descendants will have accumulated a larger number of components towards the adaptive combination than earlier ancestors.
The evolution of the vertebrate eye must have been progressive. Ancient ancestors had a very simple eye, containing only a few features good for seeing. We don't need evidence for this (although it is nice that it is there). It has to be true because the alternative - an initially complex eye, well-endowed with features good for seeing - pitches us right back to Hoyle country and the sheer cliff of improbability. There must be a ramp of step-by-step progress towards the modern, multifeatured descendant of that optical prototype. Of course, in this case, modern analogues of every step up the ramp can be found, working serviceably in dozens of eyes dotted independently around the animal kingdom. But even without these examples, we could be confident that there must have been a gradual, progressive increase in the number of features which an engineer would recognize as contributing towards optical quality. Without stirring from our armchair, we can see that it must be so.
Darwin himself understood this kind of argument clearly, which is why he was such a staunch gradualist. Incidentally, it is also why Gould is unjust when he implies, not in this book but in many other places, that Darwin was against the spirit of punctuationism. The theory of punctuated equilibrium itself is gradualist (by Gad it had better be) in the sense in which Darwin was a gradualist - the sense in which all sane evolutionists must be gradualists, at least where complex adaptations are concerned. It is just that, if punctuationism is right, the progressive, gradualistic steps are compressed into a timeframe which the fossil record does not resolve. Gould admits this when pressed, but he isn't pressed often enough.
Mark Ridley quotes Darwin on orchids, in a letter to Asa Gray: 'It is
impossible to imagine so many co-adaptations being formed all by a
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chance blow. ' As Ridley goes on, 'The evolution of complex organs
had to be gradual because all the correct changes would not occur in a single large mutation. ' And gradual, in this context, needs to mean
*This rather coy allusion to Climbing Mount Improbable seemed appropriate because, as explained in the Preamble to this section, the Editor of Evolution had simultaneously commissioned a review of that book from Dr Gould.
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progressive in my 'adaptationist' sense. The evolution of anything as complex as an advanced orchid was progressive. So was the evolution of echolocation in bats and river dolphins - progressive over many, many steps. So was the evolution of electrolocation in fish, and of skull dislocation in snakes for swallowing large prey. So was the evolution of the complex of adaptations that equips cheetahs to kill, and the corresponding complex that equips gazelles to escape.
Indeed, as Darwin again realized, although he did not use the phrase, one of the main driving forces of progressive evolution is the co- evolutionary arms race, such as that between predators and their prey. Adaptation to the weather, to the inanimate vicissitudes of ice ages and droughts, may well not be progressive: just an aimless tracking of un- progressively meandering climatic variables. But adaptation to the biotic environment is likely to be progressive because enemies, unlike the weather, themselves evolve. The resulting positive feedback loop is a good explanation for driven progressive evolution, and the drive may be sustained for many successive generations. The participants in the race do not necessarily survive more successfully as time goes by - their 'partners' in the revolutionary spiral see to that (the familiar Red Queen Effect). But the equipment for survival, on both sides, is improving as judged by engineering criteria. In hard-fought examples we may notice a progressive shift in resources from other parts of the animal's economy
129
to service the arms race. And in any case the improvement in equip-
ment will normally be progressive. Another kind of positive feedback in evolution, if R. A. Fisher and his followers are right, results from sexual selection. Once again, progressive evolution is the expected consequence.
Progressive increase in morphological complexity is to be expected only in taxa whose way of life benefits from morphological complexity. Progressive increase in brain size is to be expected only in animals where braininess is an advantage. This may, for all I know, constitute a minority of lineages. But what I do insist is that in a majority of evolutionary lineages there will be progressive evolution towards some- thing. It won't, however, be the same thing in different lineages (this was the point about swifts and elephants). And there is no general reason to expect a majority of lineages to progress in the directions pioneered by our human line.
But have I now defined progress so generally as to make it a blandly useless word? I don't think so. To say that the evolution of the verte- brate eye was progressive is to say something quite strong and quite important. If you could lay out all the intermediate ancestors in chronological order you'd find that, first, for a majority of dimensions
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of measurement, the changes would be transitive over the whole sequence. That is, if A is ancestral to B which is ancestral to C, the direction of change from A to B is likely to be the same as the direction of change from B to C. Second, the number of successive steps over which progress is seen is likely to be large: the transitive series extends beyond A, B and C, far down the alphabet. Third, an engineer would judge the performance to have improved over the sequence. Fourth, the number of separate features combining and conspiring to improve performance would increase. Finally, this kind of progress really matters because it is the key to answering the Hoyle challenge. There will be exceptional reversals, for instance in the evolution of blind cave fish, where eyes degenerate because they are not used and are costly to make. And there will doubtless be periods of stasis where there is no evolution at all, progressive or otherwise.
To conclude this point, Gould is wrong to say that the appearance of progress in evolution is a statistical illusion. It does not result just from a change in variance as a baseball-style artefact. To be sure, complexity, braininess and other particular qualities dear to the human ego should not necessarily be expected to increase progressively in a majority of lineages - though it would be interesting if they did: the investigations
130
of McShea, Jerison and others are not a waste of time. But if you
define progress less chauvinistically - if you let the animals bring their own definition - you will find progress, in a genuinely interesting sense of the word, nearly everywhere.
Now it is important to stress that, on this adaptationist view (unlike the 'evolution of evolvability' view to be discussed shortly), progressive evolution is to be expected only on the short to medium term. Coevolutionary arms races may last for millions of years, but probably not hundreds of millions. Over the very long timescale, asteroids and other catastrophes bring evolution to a dead stop, major taxa and entire radiations go extinct. Ecological vacuums are created, to befilledby new adaptive radiations driven by new ranges of arms races. The several arms races between carnivorous dinosaurs and their prey were later mirrored by a succession of analogous arms races between carnivorous mammals and their prey. Each of these successive and separate arms races powered sequences of evolution which were progressive in my sense. But there was no global progress over the hundreds of millions of years, only a sawtooth succession of small progresses terminated by extinctions. Nonetheless, the ramp phase of each sawtooth was properly and significantly progressive.
Ironically for such an eloquent foe of progress, Gould flirts with the idea that evolution itself changes over the long haul, but he puts it in a topsy-turvy way which has undoubtedly been widely misleading. It is
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more fully expounded in Wonderful Life but reprised in the present book. For Gould, evolution in the Cambrian was a different kind of process from evolution today. The Cambrian was a period of evolution- ary 'experiment', evolutionary 'trial and error', evolutionary 'false starts'. It was a period of 'explosive' invention, before evolution stabilized into the humdrum process we see today. It was the fertile time when all the great 'fundamental body plans' were invented. Nowadays, evolution just tinkers with old body plans. Back in the Cambrian, new phyla and new classes arose. Nowadays we only get new species!
This may be a slight caricature of Gould's own considered position,
but there is no doubt that the many American nonspecialists who
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unfortunately, as Maynard Smith wickedly observes, get their
evolutionary knowledge almost entirely from Gould, have been deeply misled. Admittedly, what follows is an extreme example, but Daniel Dennett has recounted a conversation with a philosopher colleague who read Wonderful Life as arguing that the Cambrian phyla did not have a common ancestor - that they had sprung up as independently initiated life forms! When Dennett assured him that this was not Gould's claim, his colleague's response was, 'Well then, what is all the fuss about? '
Even some professional evolutionists have been inspired by Gould's rhetoric into committing some pretty remarkable solecisms. Leakey and
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Lewin's The Sixth Extinction is an excellent book except for its Chapter
3, 'The Mainspring of Evolution', which is avowedly heavily influenced by Gould. The following quotations from that chapter could hardly be more embarrassingly explicit:
Why haven't new animal body plans continued to crawl out of the evolutionary cauldron during the past hundreds of millions of years?
In early Cambrian times, innovations at the phylum level survived because they faced little competition.
Below the level of the family, the Cambrian explosion produced relatively few species, whereas in the post-Permian a tremendous species diversity burgeoned. Above family level however, the post-Permian radiation faltered, with few new classes and no new phyla being generated. Evidently, the mainspring of evolution operated in both periods, but it propelled greater extreme experimentation in the Cambrian than in the post-Permian, and greater variations on existing themes in the post-Permian.
Hence, evolution in Cambrian organisms could take bigger leaps, including phylum-level leaps, while later on it would be more constrained, making only modest jumps, up to the class level.
It is as though a gardener looked at an old oak tree and remarked,
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wonderingly: 'Isn't it strange that no major new boughs have appeared on this tree recently. These days, all the new growth appears to be at the twig level! '
As it happens, molecular clock evidence indicates that the 'Cambrian Explosion' may never have happened. Far from the major phyla diverging from a point at the beginning of the Cambrian, Wray,
133
Levinton and Shapiro present evidence that the common ancestors of
the major phyla are staggered through hundreds of millions of years back in the Precambrian. But never mind that. That is not the point I want to make. Even if there really was a Cambrian explosion such that all the major phyla diverged during a ten million-year period, this is no reason to think that Cambrian evolution was a qualitatively special kind of super-jumpy process. Bauplane don't drop out of a clear Platonic sky, they evolve step by step from predecessors, and they do so (I bet, and so would Gould if explicitly challenged) under approximately the same Darwinian rules as we see today.
'Phylum-level leaps' and 'modest jumps, up to the class level' are the sheerest nonsense. Jumps above the species level don't happen, and nobody who thinks about it for two minutes claims that they do. Even the great phyla, when they originally bifurcated one from another, were just pairs of new species, members of the same genus. Classes are species that diverged a very long time ago, and phyla are species that diverged an even longer time ago. Indeed it is a moot - and rather empty - question precisely when in the course of the step-by-step, gradual mutual divergence of, say, mollusc ancestors and annelid ancestors after the time when they were congeneric species, we should wish to say that the divergence had reached 'Bauplan' status. A good case could be made that The Bauplan is a myth, probably as pernicious as any of the myths that Stephen Gould has so ably combatted, but this one, in its modern form, is largely perpetuated by him.
I return, finally, to the 'evolution of evolvability' and a very real sense in which evolution itself may evolve, progressively, over a longer timescale than the individual ramps of the arms race sawtooth. Notwith- standing Gould's just scepticism over the tendency to label each era by its newest arrivals, there really is a good possibility that major innovations in embryological technique open up new vistas of evolutionary possibility and that these constitute genuinely progressive improvements. * The
*This is the idea that I dubbed 'The Evolution of Evolvability' (in C. Langton (ed. ), Artificial Life (Santa Fe, Addison Wesley, 1982)) and Maynard Smith and Szathmary wrote a book about 0. Maynard Smith and E. Szathmary, The Major Transitions in Evolution (Oxford, W. H. Freeman/Spektrum, 1995)).
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origin of the chromosome, of the bounded cell, of organized meiosis, diploidy and sex, of the eucaryotic cell, of multicellularity, of gastrulation, of molluscan torsion, of segmentation - each of these may have constituted a watershed event in the history of life. Not just in the normal Darwinian sense of assisting individuals to survive and reproduce, but watershed in the sense of boosting evolution itself in ways that seem entitled to the label progressive.
It may well be that after, say, the invention of multicellularity, or the invention of segmentation, evolution was never the same again. In this sense there may be a one-way ratchet of progressive innovation in evolution.
For this reason over the long term, and because of the cumulative character of coevolutionary arms races over the shorter term, Gould's attempt to reduce all progress to a trivial, baseball-style artefact con- stitutes a surprising impoverishment, an uncharacteristic slight, an unwonted demeaning of the richness of evolutionary processes.
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Unfinished Correspondence with a Darwinian Heavyweight
The following correspondence was never completed and now, sadly, it never can be.
9 December 2001
Stephen Jay Gould Harvard
Dear Steve
Recently I received an email from Phillip Johnson, founder of the so-called 'Intelligent Design' school of creationists, crowing in triumph because one of his colleagues, Jonathan Wells, had been invited to take part in a debate at Harvard. He included the text of his email on his 'Wedge of Truth' web site, in which he announced the Wells debate under the headline 'Wells Hits a Home Run at Harvard'.
http://www. arn. org/docs/pjweekly/pj_weekly_011202. htm
The 'Home Run' turns out to be NOT a resounding success by Wells in convincing the audience, NOR any kind of besting of his opponent (Stephen Palumbi, who tells me he agreed to take part, with great reluctance, only because somebody at Harvard had ALREADY invited Wells and it was too late to do anything about that). There is no suggestion that Wells did well in the debate, nor even any obvious interest in whether he did. No, the 'Home Run' was simply and solely the matter of being invited by Harvard in the first place. These people have no hope of convincing reputable scientists by their ridiculous arguments. Instead, what they seek is the oxygen of respectability. We give them this oxygen by the mere act of ENGAGING with them at all. They don't mind being beaten in argument. What matters is that we give them recognition by bothering to argue with them in public.
You convinced me of this years ago when I phoned you up (you have probably
forgotten this) to ask your advice when I was invited to debate Duane P Gish. 218
? Ever since that phone call, I have repeatedly cited you and refused to debate these people, not because I am afraid of 'losing' the debate, but because, as you said, just to appear on a platform with them is to lend them the respectability they crave. Whatever might be the outcome of the debate, the mere fact that it is staged at all suggests to ignorant bystanders that there must be something worth debating, on something like equal terms.
First, I am interested to know whether you still hold to this view, as I do. Second, I am proposing that you might consider uniting with me (no need to involve others) in signing a short letter, say to the New York Review of Books, explaining publicly why we do not debate creationists (including the 'Intelligent Design' euphemism for creationists) and encouraging other evolutionary biologists to follow suit.
Such a letter would have great impact precisely because there have been widely publicised differences, and even animosities, between us (differences which creationists, with extreme intellectual dishonesty, have not hesitated to exploit). And I would not suggest writing a long disquisition on the technical differences which remain between us. That would only confuse the issue, make it harder to agree on a final draft, and lessen the impact. I wouldn't even mention our differences. I suggest a brief letter to the editor, explaining why we do not engage with 'intelligent design' or any other species of creationists, and offering our letter as a model for others to cite in refusing such invitations in the future. We both have better things to do with our time than give it over to such nonsense. Having just reached my sixtieth birthday (we are almost exactly the same age) I feel this keenly.
Steve replied on 11th December 2001, a warm and friendly email enthusiastically agreeing that a joint letter was an excellent idea, and saying that he'd be delighted to join me as the sole other signatory. He agreed that the New York Review of Books might well be the best place, and proposed that I should write a first draft. I reproduce that here, exactly as I sent it to him for his approval.
14 December 2001
Dear Editor
Like any flourishing science, the study of evolution has its internal controversies, as we both know. But no qualified scientist doubts that evolution is a fact, in the ordinarily accepted sense in which it is a fact that the Earth orbits the Sun. It is a fact that human beings are cousins
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to monkeys, kangaroos, jellyfish and bacteria. No reputable biologist doubts this. Nor do reputable theologians, from the Pope on. Unfortunately, many lay Americans do, including some frighteningly influential, powerful and, above all, well-financed ones.
We are continually invited to engage in public debates against creationists, including latter-day creationists disguised under the euphemism 'Intelligent Design Theorists'. We always refuse, for one overriding reason. If we may be allowed to spell this reason out publicly, we hope our letter may be helpful to other evolutionary scientists plagued by similar invitations.
The question of who would 'win' such a debate is not at issue. Winning is not what these people realistically aspire to. The coup they seek is simply the recognition of being allowed to share a platform with a real scientist in thefirstplace. This will suggest to innocent bystanders that there must be material here that is genuinely worth debating, on something like equal terms.
At the moment of writing, the leading 'Intelligent Design' website
reports a debate at Harvard under the banner headline 'Wells Hits a
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Home Run at Harvard'. Jonathan Wells is a creationist, incidentally a
long-time devotee of the Unification Church (the 'Moonies'). * He had a debate last month against Stephen Palumbi, Professor of Biology at Harvard University. 'Home Run' might seem to suggest that Reverends (sic) Wells scored some kind of victory over Professor Palumbi. Or at least that he made powerful points and his speech was well received. No such claim is made. It doesn't even seem to be of interest.
The 'Home Run' turns out to be simply the public demonstration at Harvard that, in the words of the website's author, Phillip Johnson, 'This is the sort of debate that is now occurring in universities. ' There was a victory, but it occurred long before the debate itself. The creationist scored his home run at the moment the invitation from Harvard landed on his doormat. It came, by the way, not from any biological, or indeed scientific, department, but from the Institute of Politics.
Phillip Johnson himself, founding father of the 'Intelligent Design'
''Darwinism: Why I went for a Second Ph. D. ' is Jonathan Wells' own testimony on the turning point of his life: 'Father's words, my studies, and my prayers convinced me that I should devote my life to destroying Darwinism, just as many of my fellow Unificationists had already devoted their lives to destroying Marxism. When Father chose me (along with about a dozen other seminary graduates) to enter a Ph. D. program in 1978, I welcomed the opportunity to prepare myself for battle. ' ('Father', of course, is the Moonies' name for Rev Moon himself). http://www. tparents. org/LibraryAJnincation/Talks/Wells/DARWIN. htm.
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It isn't worth my while to debate every ambitious Darwinist who wants to try his hand at ridiculing the opposition, so my general policy is that Darwinists have to put a significant figure at risk before I will agree to a debate. That means specifically Dawkins or Gould, or someone of like stature and public visibility.
Well, we can condescend too, and we have the advantage that evolutionary scientists don't need the publicity such debates can bring. In the unlikely event that a significant argument should ever emerge from the ranks of creationism/'intelligent design', we will be happy to debate it. Meanwhile, we shall cultivate our evolutionary gardens, occasionally engaging in the more exacting and worthwhile task of debating each other. What we shall not do is abet creationists in their disreputable quest for free publicity and unearned academic respectability.
In all humility, we offer these thoughts to our colleagues who receive similar invitations to debate.
Unfortunately, Steve never got around to revising the letter, which therefore lacks the stylish panache which his dexterous touch would have lent it. I received one further email, apologizing for the delay and hoping to deal with the matter soon. The subsequent silence, I now realize, coincided with his last illness. I therefore offer my draft, imperfect as it is, in the hope that it may go some way towards conveying the message which I originally learned from him many years ago. It is my sincere hope that he would have approved the content of the letter, but of course I cannot be sure.
To close this section on a note of such concord may seem puzzling.
Given that Steve was as much a neo-Darwinist as I am, what did we dis-
agree about? The major disagreement emerges clearly out of his last big
5
book, The Structure of Evolutionary Theory,TM which I had no opportunity
to see until after his death. It is appropriate, therefore, to spell out that issue here, and it also, as it happens, forms a natural bridge to the next essay. The question under dispute is this: what is the role of genes in evolution? Is it, to use Gould's phrase, 'book-keeping or causation'?
Gould saw natural selection as operating on many levels in the hierarchy of life. Indeed it may, after a fashion, but I believe that such selection can have evolutionary consequences only when the entities selected consist of 'replicators'. A replicator is a unit of coded information, of high fidelity but occasionally mutable, with some causal power over its own fate. Genes are
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such entities. So, in principle, are memes, but they are not under discus- sion here. Biological natural selection, at whatever level we may see it, results in evolutionary effects only insofar as it gives rise to changes in gene frequencies in gene pools. Gould, however, saw genes only as 'book- keepers', passively tracking the changes going on at other levels. In my view, whatever else genes are, they must be more than book-keepers, otherwise natural selection cannot work. If a genetic change has no causal influence on bodies, or at least on something that natural selection can 'see', natural selection cannot favour or disfavour it. No evolutionary change will result.
Gould and I would agree that genes can be seen as a book in which is
written the evolutionary history of a species. In Unweaving the Rainbow I
called it 'The Genetic Book of the Dead'. But the book is written via the
natural selection of randomly varying genes, chosen by virtue of their
causal influence on bodies. Book-keeping is precisely the wrong metaphor,
because it reverses the causal arrow, almost in Lamarckian fashion, and
makes the genes passive recorders. I dealt with this in 1982 (The Extended
Phenotype) in my distinction between 'active replicators' and 'passive
replicators'. The point is also explained in David Barash's superb review of
136 Gould's book.
Book-keeping is perversely - and characteristically - a valuable meta- phor precisely because it is so diametrically back to front. Not for the first time, the characteristic vividness and clarity of a Gouldian metaphor helps us to see vividly and clearly what is wrong with the Gouldian message - and how it needs to be reversed in order to get at the truth.
I hope this brief note will not be seen as taking advantage to get the last word. The Structure of Evolutionary Theory is such a massively powerful last word, it will keep us all busy replying to it for years. What a brilliant way
for a scholar to go. I shall miss him.
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THERE IS ALL AFRICA AND HER PRODIGIES IN US
I am one of those (it includes most people who have ever spent time south of the Sahara) who think of Africa as a place of enchantment. For me it stems from faint but haunting childhood memories, coupled with the mature understanding that Africa is our ancestral home. These themes recur throughout this section, and they introduce Ecology of Genes (6. 1), my Foreword to Harvey Croze and John Reader's Pyramids of Life. This book uses Africa as an illuminating case study in the principles of ecology, and I used the opportunity of the Foreword to think about the relationship between ecology and natural selection. This could be seen as a continuation of my argument in the afterword to the previous section.
In this book, and elsewhere, I have been unkind to a view favoured by some social anthropologists, the 'cultural relativism' that acknowledges the equal status of many kinds of truth, scientific truth having no privileged rank among them. If ever I could be converted to some form of relativism, it might be after reading Elspeth Huxley's remarkable epic of Kenya, Red Strangers. Out of the Soul of Africa (6. 2) is the Foreword to the new paperback edition of her novel. I wrote an article for the Financial Times, pointing out that Red Strangers had been out of print for years and challenging any publisher to do something about it. The admirable Penguin did, and they reprinted my article as the Foreword.
I am now waiting for a scholar of literature to explain to me why Red Strangers is not rated one of the great novels of the twentieth century, the equal of a John Steinbeck except that Elspeth Huxley's imagery is Kikuyu rather than American.
Run like the eland . . . Run, warriors, with feet like arrows and the hearts of lions; the lives and wealth of your fathers are yours to save . . . Their thighs were straight as saplings, their features sharp as axes, their skins lighter than honey. His limbs began to quiver like the wings of a sunbird when its beak sucks honey . . .
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It is a virtuoso feat of identification with another culture. Not only does she succeed in getting herself inside a Kikuyu skin, she achieves the same feat for the reader. And she makes you cry.
I am slightly ashamed to admit that another book that brings me close to tears - of joy this time - is a children's book. Or is it a very grown-up book which happens to be written by children? It is hard to decide, which is part of its charm and also probably why it has been unaccountably ignored by book reviews editors - they just didn't know which shelf to put it on. The Lion Children is about a family of children who are English, but whose home is a set of tents in Botswana, where they radio-track wild lions and are schooled entirely by their mother in the bush. They have written a book about their utterly extraordinary life. Never mind whether there is a conventionally labelled shelf to put it on, just read it. I Speak of Africa and Golden Joys (6. 3), my Foreword, is reproduced here.
Last in this section is a travel piece, which again takes up the two themes of Africa as our ancestral home and Africa as my personal birthplace and weaves them together in an autobiographical story of travel and personal inspiration. The title was changed by the Sunday Times to 'All Our Yesterdays', but Macbeth's world-weariness is exactly opposite to the mood of my piece, so I am reverting to my original title, Heroes and Ancestors (6. 4). Now that I think about it, Heroes and Ancestors would have made another fine title for this whole collection.
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6 Ecology of Genes137
Foreword to Pyramids of Life
by Harvey Croze and John Reader
Africa was my personal cradle. But I left when I was seven, too young to appreciate - indeed the fact was not then known - that Africa is also humanity's cradle. The fossils of our species' formative years are all from Africa, and molecular evidence suggests that the ancestors of all today's peoples stayed there until as recently as the last hundred thousand years or so. We have Africa in our blood and Africa has our bones. We are all Africans.
This alone makes the African ecosystem an object of singular fascination. It is the community that shaped us, the commonwealth of animals and plants in which we served our ecological apprenticeship. But even if it were not our home continent Africa would captivate us, as perhaps the last great refugium of Pleistocene ecologies. If you want a late glimpse of the Garden of Eden, forget Tigris and Euphrates and the dawn of agriculture. Go instead to the Serengeti or the Kalahari. Forget the Arcadia of the Greeks and the dreamtime of the outback, they are so recent. Whatever may have come down the mountain at Olympus or Sinai, or even Ayers Rock, look instead to Kilimanjaro, or down the Rift Valley towards the High Veldt. There is where we were designed to flourish.
The 'design' of all living things and their organs is, of course, an illusion; an exceedingly powerful illusion, fabricated by a suitably power- ful process, Darwinian natural selection. There is a second illusion of design in nature, less compelling but still appealing, and it is in danger of being mistaken for the first. This is the apparent design of ecosystems. Where bodies have parts that intricately harmonize and regulate to keep them alive, ecosystems have species that appear to do something similar at a higher level. There are the primary producers that convert raw solar energy into a form that others can use. There are the herbivores that consume them to use it, and then make a tithe of it available for carnivores and so on up the food chain - pyramid, rather,
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for the laws of thermodynamics rule that only a tenth of each level's energy shall make it to the level above. Finally, there are scavengers that recycle the waste products to make them available again, and in the process clean up the world and stop it becoming a tip. Everything fits with everything else like jigsaw pieces meshing in a huge multi- dimensional puzzle, and - as the cliche goes - we meddle with the parts at the risk of destroying a priceless whole.
The temptation is to think that this second illusion is crafted by the same kind of process as the first: by a version of Darwinian selection, but at a higher level. According to this erroneous view, the ecosystems that survive are the ones whose parts - species - harmonize, just as the organisms that survive in conventional Darwinism are the ones whose parts - organs and cells - work harmoniously for their survival. I believe that this theory is false. Ecosystems, like organisms, do indeed seem harmoniously designed; and the appearance of design is indeed an illusion. But there the resemblance ends. It is a different kind of illusion, brought about by a different process. The best ecologists, such as Croze and Reader, understand this.
Darwinism enters into the process, but it does not jump levels. Genes still survive, or fail to survive, within the gene pools of species, by virtue of their effects upon the survival and reproduction of the individual organisms that contain them. The illusion of harmony at a higher level is an indirect consequence of differential individual reproduction. Within any one species of animals or plants, the individuals that survive best are the ones that can exploit the other animals and plants, bacteria and fungi that are already flourishing in the environment. As Adam Smith understood long ago, an illusion of harmony and real efficiency will emerge in an economy dominated by self-interest at a lower level. A well balanced ecosystem is an economy, not an adaptation.
Plants flourish for their own good, not for the good of herbivores. But because plants flourish, a niche for herbivores opens up, and they fill it. Grasses are said to benefit from being grazed. The truth is more interest- ing. No individual plant benefits from being grazed per se. But a plant that suffers only slightly when it is grazed outcompetes a rival plant that suffers more. So successful grasses have benefited indirectly from the presence of grazers. And of course grazers benefit from the presence of grasses. Grasslands therefore build up as harmonious communities of relatively compatible grasses and grazers. They seem to cooperate. In a sense they do, but it is a modest sense that must be cautiously under- stood and judiciously understated. The same is true of the other African communities expounded by Croze and Reader.
