Did Darwin get it right?
John Maynard Smith
I think I can see what is breaking down in evolutionary theory – the strict construction of the modern synthesis with its belief in pervasive adaptation, gradualism and extrapolation by smooth continuity from causes of change in local populations to major trends and transitions in the history of life.
A new and general evolutionary theory will embody this notion of hierarchy and stress a variety of themes either ignored or explicitly rejected by the modern synthesis.
These quotations come from a recent paper in Palaeobiology by Stephen Jay Gould. What is the new theory? Is it indeed likely to replace the currently orthodox ‘neo-Darwinian’ view? Proponents of the new view make a minimum and a maximum claim. The minimum claim is an empirical one concerning the nature of the fossil record. It is that species, once they come into existence, persist with little or no change, often for millions of years (‘stasis’), and that evolutionary change is concentrated into relatively brief periods (‘punctuation’), these punctuational changes occurring at the moment when a single species splits into two. The maximal claim is a deduction from this, together with arguments drawn from the study of development: it is that evolutionary change, when it does occur, is not caused by natural selection operating on the genetic differences between members of populations, as Darwin argued and as most contemporary evolutionists would agree, but by some other process. I will discuss these claims in turn; as will be apparent, it would be possible to accept the first without being driven to accept the second.
The claim of stasis and punctuation will ultimately be settled by a study of the fossil record. I am not a palaeontologist, and it might therefore be wiser if I were to say merely that some palaeontologists assert that it is true, and others are vehemently denying it. There is something, however, that an outsider can say. It is that the matter can be settled only by a statistical analysis of measurements of fossil populations from different levels in the rocks, and not by an analysis of the lengths of time for which particular named species or genera persist in the fossil record. The trouble with the latter method is that one does not know whether one is studying the rates of evolution of real organisms, or merely the habits of the taxonomists who gave the names to the fossils. Suppose that in some lineage evolutionary change took place at a more or less steady rate, to such an extent that the earliest and latest forms are sufficiently different to warrant their being placed in different species. If there is at some point a gap in the record, because suitable deposits were not being laid down or have since been eroded, then there will be a gap in the sequence of forms, and taxonomists will give fossils before the gap one name and after it another. It follows that an analysis of named forms tells us little: measurements of populations, on the other hand, would reveal whether change was or was not occuring before and after the gap.
My reason for making this rather obvious point is that the only extended presentation of the punctuationist view – Stanley’s book, Macroevolution – rests almost entirely on an analysis of the durations of named species and genera. When he does present population measurements, they tend to support the view that changes are gradual rather than sudden. I think that at least some of the changes he presents as examples of sudden change will turn out on analysis to point the other way. I was unable to find any evidence in the book which supported, let alone established, the punctuationist view.
Of course, that is not to say that the punctuationist view is not correct. One study, based on a proper statistical analysis, which does support the minimal claim, but not the maximal one, is Williamson’s study of the freshwater molluscs (snails and bivalves) of the Lake Turkana region of Africa over the last five million years. Of the 21 species studied, most showed no substantial evolutionary change during the whole period: ‘stasis’ was a reality. The remaining six species were more interesting. They also showed little change for most of the period. There was, however, a time when the water table fell and the lake was isolated from the rest of the rift valley. When this occurred, these six species changed rather rapidly. Through a depth of deposit of about one metre, corresponding roughly to 50,000 years, successive populations show changes of shape great enough to justify placing the later forms in different species. Later, when the lake was again connected to the rest of the rift valley, these new forms disappear suddenly, and are replaced by the original forms, which presumably re-entered the lake from outside, where they had persisted unchanged.
This is a clear example of stasis and punctuation. However, it offers no support for the view that changes, when they do occur, are not the result of selection acting within populations. Williamson does have intermediate populations, so we know that the change did not depend on the occurrence of a ‘hopeful monster’ (see below), or on the existence of an isolated population small enough to permit random changes to outweigh natural selection. The example is also interesting in showing how we may be misled if we study the fossil record only in one place. Suppose that, when the water table rose again, the new form had replaced the original one in the rest of the rift valley, instead of the other way round. Then, if we had examined the fossil record anywhere else but in Lake Turkana, we would have concluded, wrongly, that an effectively instantaneous evolutionary change had occurred.
Williamson’s study suggests an easy resolution of the debate. Both sides are right, and the disagreement is purely semantic. A change taking 50,000 years is sudden to a palaeontologist but gradual to a population geneticist. My own guess is that there is not much more to the argument than that. However, the debate shows no signs of going away.
One question that arises is how far the new ideas are actually new. Much less so, I think, than their proponents would have us believe. They speak and write as if the orthodox view is that evolution occurs at a rate which is not only ‘gradual’ but uniform. Yet George Gaylord Simpson, one of the main architects of the ‘modern synthesis’ now under attack, wrote a book, Tempo and Mode in Evolution, devoted to emphasising the great variability of evolutionary rates. It has never been part of the modern synthesis that evolutionary rates are uniform.
Yet there is a real point at issue. If it turns out to be the case that all, or most, evolutionary change is concentrated into brief periods, and associated with the splitting of lineages, that would require some serious rethinking. Oddly enough, it is not so much the sudden changes which would raise difficulties, but the intervening stasis. Why should a species remain unchanged for millions of years? The explanation favoured by most punctuationists is that there are ‘developmental constraints’ which must be overcome before a species can change. The suggestion is that the members of a given species share a developmental pathway which can be modified so as to produce some kinds of change in adult structure rather easily, and other kinds of change only with great difficulty, or not at all. I do not doubt that this is true: indeed, in my book The Theory of Evolution, published in 1958 and intended as a popular account of the modern synthesis, I spent some time emphasising that ‘the pattern of development of a given species is such that there are only a limited number of ways in which it can be altered without causing complete breakdown.’ Neo-Darwinists have never supposed that genetic mutation is equally likely to produce changes in adult structure in any direction: all that is assumed is that mutations do not, as a general rule, adapt organisms to withstand the agents which caused them. What is at issue, then, is not whether there are developmental constraints, because clearly there are, but whether such constraints can account for stasis in evolution.
I find it hard to accept such an explanation for stasis, for two reasons. The first is that artificial selection can and does produce dramatic morphological change: one has only to look at the breeds of dogs to appreciate that. The second is that species are not uniform in space. Most species with a wide geographical range show differences between regions. Often these differences are so great that one does not know whether the extreme forms would behave as a single species if they met. Occasionally we know that they would not. This requires that a ring of forms should arise, with the terminal links overlapping. The Herring Gull and Lesser Black-Backed Gull afford a familiar example. In Britain and Scandinavia they behave as distinct species, without hybridising, but they are linked by a series of forms encircling the Arctic.
Stasis in time is, therefore, a puzzle, since it seems not to occur in space. The simplest explanation is that species remain constant in time if their environments remain constant. It is also worth remembering that the hard parts of marine invertebrates, on which most arguments for stasis are based, tell us relatively little about the animals within. There are on our beaches two species of periwinkle whose shells are indistinguishable, but which do not interbreed and of which one lays eggs and the other bears live young.
The question of stasis and punctuation will be settled by a statistical analysis of the fossil record. But what of the wider issues? Is mutation plus natural selection within populations sufficient to explain evolution on a large scale, or must new mechanisms be proposed?
It is helpful to start by asking why Darwin himself was a believer in gradual change. The reason lies, I believe, in the nature of the problem he was trying to solve. For Darwin, the outstanding characteristic of living organisms which called for an explanation was the detailed way in which they are adapted to their forms of life. He knew that ‘sports’ – structural novelties of large extent – did arise from time to time, but felt that fine adaptation could not be explained by large changes of this kind: it would be like trying to perform a surgical operation with a mechanically-controlled scalpel which could only be moved a foot at a time. Gruber has suggested that Darwin’s equating of gradual with natural and of sudden with supernatural was a permanent feature of his thinking, which predated his evolutionary views and his loss of religious faith. It may have originated with Archbishop Sumner’s argument (on which Darwin made notes when a student at Cambridge) that Christ must have been a divine rather than a human teacher because of the suddenness with which his teachings were accepted. Darwin seems to have retained the conviction that sudden changes are supernatural long after he had rejected Sumner’s application of the idea.
Whatever the source of Darwin’s conviction, I think he was correct both in his emphasis on detailed adaptation as the phenomenon to be explained, and in his conviction that to achieve such adaptation requires large numbers of selective events. It does not, however, follow that all the steps had to be small. I have always had a soft spot for ‘hopeful monsters’: new types arising by genetic mutation, strikingly different in some respects from their parents, and taking a first step in the direction of some new adaptation, which could then be perfected by further smaller changes, We know that mutations of large effect occur: our only problem is whether they are ever incorporated during evolution, or are always eliminated by selection. I see no a priori reason why such large steps should not occasionally happen in evolution. What genetic evidence we have points the other way, however. On the relatively few occasions when related species differing in some morphological feature have been analysed genetically, it has turned out, as Darwin would have expected had he known of the possibility, that the difference is caused by a number of genes, each of small effect.
As I see it, a hopeful monster would still stand or fall by the test of natural selection. There is nothing here to call for radical rethinking. Perhaps the greatest weakness of the punctuationists is their failure to suggest a plausible alternative mechanism. The nearest they have come is the hypothesis of ‘species selection’. The idea is that when a new species arises, it differs from its ancestral species in ways which are random relative to any long-term evolutionary trends. Species will differ, however, in their likelihood of going extinct, and of splitting again to form new species. Thus selection will operate between species, favouring those characteristics which make extinction unlikely and splitting likely. In ‘species selection’, as compared to classical individual selection, the species replaces the individual organism, extinction replaces death, the splitting of species into two replaces birth, and mutation is replaced by punctuational changes at the time of splitting.
Some such process must take place. I have argued elsewhere that it may have been a relevant force in maintaining sexual reproduction in higher animals. It is, however, a weak force compared to typical Darwinian between-individual selection, basically because the origin and extinction of species are rare events compared to the birth and death of individuals. Some critics of Darwinism have argued that the perfection of adaptation is too great to be accounted for by the selection of random mutations. I think, on quantitative grounds, that they are mistaken. If, however, they were to use the same argument to refute species selection as the major cause of evolutionary trends, they might well be right. For punctuationists, one way out of the difficulty would be to argue that adaptation is in fact less precise than biologists have supposed. Gould has recently tried this road. As it happens, I think he is right to complain of some of the more fanciful adaptive explanations that have been offered, but I also think that he will find that the residue of genuine adaptive fit between structure and function is orders of magnitude too great to be explained by species selection.
One other extension of the punctuationist argument is worth discussing. As explained above, stasis has been explained by developmental constraints. This amounts to saying that the developmental processes are such that only certain kinds of animal are possible and viable. The extension is to apply the same idea to explain the existence of the major patterns of organisation, or ‘bauplans’, observable in the natural world. The existence of such bauplans is not at issue. For example, all vertebrates, whether swimming, flying, creeping or burrowing, have the same basic pattern of an internal jointed backbone with a hollow nerve cord above it and segmented body muscles either side of it, and the vast majority have two pairs of fins, or of legs which are derived from fins (although a few have lost one or both pairs of appendages). Why should this be so?
Darwin’s opinion is worth quoting. In The Origin of Species, he wrote:
It is generally acknowledged that all organic beings have been formed on two laws – Unity of Type, and the Conditions of Existence. By unity of type is meant that fundamental agreement in structure which we see in organic beings of the same class, and which is quite independent of their habits of life. On my theory, unity of type is explained by unity of descent. The expression of conditions of existence, so often insisted on by the illustrious Cuvier, is fully embraced by the principle of natural selection. For natural selection acts by either now adapting the varying parts of each being to its organic and inorganic conditions of life; or by having adapted them during the long-past periods of time ... Hence, in fact, the law of Conditions of Existence is the higher law; as it includes, through the inheritance of former adaptations, that of Unity of Type.
That is, we have two pairs of limbs because our remote ancestors had two pairs of fins, and they had two pairs of fins because that is an efficient number for a swimming animal to have.
I fully share Darwin’s opinion. The basic vertebrate pattern arose in the first place as an adaptation for sinusoidal swimming. Early fish have two pairs of fins for the same reason that most early aeroplanes had wings and tail-plane: two pairs of fins is the smallest number that can produce an upward or downward force through any point in the body. In the same vein, insects (which are descended from animals with many legs) have six legs because that is the smallest number which permits an insect to take half its legs off the ground and not fall over.
The alternative view would be that there are (as yet unknown) laws of form or development which permit only certain kinds of organisms to exist – for example, organisms with internal skeletons, dorsal nerve cords and four legs, or with external skeletons, ventral nerve cords and six legs – and which forbid all others, in the same way that the laws of physics permit only elliptical planetary orbits, or the laws of chemistry permit only certain compounds. This view is a manifestation of the ‘physics envy’ which still infects some biologists. I believe it to be mistaken. In some cases it is demonstrably false. For example, some of the earliest vertebrates had more than two pairs of fins (just as some early aeroplanes had a noseplane as well as a tailplane). Hence there is no general law forbidding such organisms.
What I have said about bauplans does not rule out the possibility that there may be a limited number of kinds of unit developmental process which occur, and which are linked together in various ways to produce adult structures. The discovery of such processes would be of profound importance for biology, and would no doubt influence our views about evolution.
One last word needs to be said about bauplans. They may, as Darwin thought, have arisen in the first place as adaptations to particular ways of life, but, once having arisen, they have proved to be far more conservative in evolution than the way of life which gave them birth. Apparently it has been easier for organisms to adapt to new ways of life by modifying existing structures than by scrapping them and starting afresh. It is for this reason that comparative anatomy is a good guide to relationship.
Punctuationist views will, I believe, prove to be a ripple rather than a revolution in the history of ideas about evolution. Their most positive achievement may be to persuade more people to study populations of fossils with adequate statistical methods. In the mean while, those who would like to believe that Darwin is dead, whether because they are creationists, or because they dislike the apparently Thatcherite conclusions which have been drawn from his theory, or find the mathematics of population genetics too hard for them, would be well advised to be cautious: the reports of his death have been exaggerated.
Vol. 3 No. 13 · 16 July 1981
SIR: I welcome Professor Maynard Smith’s characteristically lucid and perceptive essay on the current debate on Darwinism (LRB, 18 June) as a salutary sequel to some of the recent and no doubt inevitable media oversimplifications and distortions, to say nothing of Marxist and other red herrings. In defending the orthodox neo-Darwinian position he makes some telling points against what one might call the punctuationist school. Thus ‘species selection’ does not appear to account satisfactorily for the fine adaptations ubiquitous in the animal world, and I for one, though an avowed punctuationist, will remain sceptical until a few well-documented case-histories have been provided. Yet, in his own words, the debate shows no sign of going away. I think I can suggest several reasons for this.
In his light-hearted conclusion Maynard Smith suggests that some critics find the mathematics of population genetics too difficult. Well maybe, but surely that is quite beside the point. The vast majority of working scientists can and do quite rightly treat such mathematics as a black box. What matters is how closely the resulting theoretical model corresponds with the natural world, and how successfully it serves for prediction. As Maynard Smith is honest enough to admit, population genetics models have conspicuously failed to predict the phenomenon of ‘stasis’ (long-term species stability) in the fossil record. While prepared to admit data based on thorough statistical analysis of fossil populations, which is admittedly sparse as yet, he is dismissive of the data presented in, for instance, Stanley’s book on macroevolution, by taking up a nominalist position in relation to the cited species. To suggest that the taxonomic labelling may be largely arbitrary overlooks, however, some striking correspondences recorded by Stanley and others. Thus independent estimates of the species durations of bivalve molluscs of different ages are closely in accord, and significantly greater than, for instance, ammonites and mammals, implying a much higher rate of faunal turnover with time among the latter two groups of organisms. This is unlikely to be a taxonomic artifact because the morphology of ammonites and the mammalian teeth used for fossil species discrimination is no more complex than that of bivalves.
Again stasis has been recorded in a wide variety of fossil animals, from molluscs to antelopes. My colleague Russell Coope has discovered many striking examples of stasis among Pleistocene beetles and his complex data indicating how they have repeatedly tracked the pronounced climatic changes corresponding with glacial advances and retreats only make sense if one assumes that physiological tolerance as well as hard-part morphology has remained stable within given species.
Now if species, as a result of punctuated stasis, are mostly discrete entities in time (only the extreme punctuationist would deny any gradualism), it is a logical corollary that they should likewise be mostly discrete entities in space, as Maynard Smith well appreciates. He points out what no reasonable person would wish to deny, that wide-ranging species may exhibit much geographic variation. The key question is: does such variation merely represent deviation about an average, with minimal morphological or behavioural overlap with neighbouring species, or is there commonly the sort of lateral gradation implied by ‘clines’ and ‘ring speciation’? Maynard Smith cites for the latter the textbook example of the gulls. Shouldn’t such phenomena be commonplace if one is to extrapolate consistently from infraspecific to interspecific relationships, and if so why have not many more well-documented examples been forthcoming from the intensive fieldwork of biologists over many decades?
Ernst Mayr cites the remarkable fact that the many species of birds distinguished by western ornithologists in the jungles of New Guinea correspond very closely to the birds given separate names by the natives. This hardly supports the nominalist view that species are artifacts of a natural continuum because people of vastly different cultures and education would be expected to impose their arbitrary boundaries in different places. Why therefore shouldn’t palaeontological taxonomists be given the same kind of credit? (The argument about discontinuities in the record of rock strata, used as a refuge by Darwin and resurrected by Maynard Smith, can be adequately taken care of in most modern stratigraphic work.) Hard-part morphology is by no means foolproof as a taxonomic criterion, of course, as evidenced by sibling species (species that are genetically different but well nigh indistinguishable morphologically). Nevertheless an adequate methodology can be devised in favourable cases. In my own limited experience with fossil bivalve molluscs, I have taken some pains to ensure that the range of variation within my selected ‘species’ corresponds closely to what malacologists who work on living bivalves would call species. Since we only have genetic information for a minute fraction of living organisms we can generally do no better than that.
Punctuationists do not deny the operation of natural selection but suspect that it may substantially be confined to adaptational ‘fine tuning’ to the environment, and believe that extrapolation to processes of species formation may not be warranted. After all, no one has actually observed a new animal species arise, so arguments for speciation are of necessity based only on circumstantial evidence. The phenomenon of stasis in the fossil record suggests that, for time periods far longer than those available for biologists’ research, the prime role of selection may be a stabilising one. A very occasional environmental crisis may be required to destabilise a ‘homeostatic’ system and hence perhaps promote speciation. This is suggested, for instance, by Williamson’s study of East African fossil molluscs, cited by Maynard Smith, who omits, however, to point out that the morphological variance at the ‘crisis’ intervals tended to increase, before a new species established itself. This is precisely the kind of change that could be predicted if speciation events involve a temporary relaxation of stabilising selection pressure.
Maynard Smith sees the punctuationist view not as a revolution but as a mere ripple on the ocean of neo-Darwinian thought. I see the debate in quite different terms. For several decades the principal drive of evolutionary research has been reductionist, from comparative anatomy to experimental embryology and genetics to molecular biology. Without denying for a moment the many scientific triumphs achieved en route, there has been a growing sense of unease among many evolution-minded palaeontologists and biologists about important problems that they consider to have been glossed over or ignored. The science of evolution is at least as much to do with understanding how the rich diversity of the organic world came about as it is to do with shifting gene balances in populations of fruit flies and land snails. We are more intrigued by the cause of the difference between lions and leopards than that between long-tailed and short-tailed lions.
In their reaction against the ultraselectionist view there is no desire to reject out of hand the findings of population genetics and, for instance, flirt with ‘macromutations’ and ‘hopeful monsters’. It is more a question of a shift of emphasis and a change of direction in attacking the problems of speciation and generation of major new morphotypes, because of a sense of some degree of impasse having been reached by the orthodox approach. Hence the more open-minded attitude towards, for example, random effects of genetic drift in small populations, chromosomal changes and the action of regulatory genes operating at an early stage in an individual’s life history. We are very much at the exploratory stage and no doubt will chase up many blind alleys. In fact, about all we have in common in our diverse and eclectic approach is some degree of dissatisfaction with the current orthodoxy. With the advent of much new geological evidence palaeontologists may before long have plausible answers to previously little considered questions such as what kinds of environmental change have promoted speciation and extinction in particular groups of organisms, and what kinds have merely provoked migration into ecological refuges.
That not all evolutionary geneticists are entirely satisfied with the status quo is indicated by a recent remark by the Harvard biologist R.C. Lewontin: ‘Systematics and palaeontology deal in phenotypes, yet they continue to use the Mendelian population genetics of single genes of fixed effect as a theoretical base for explanation, which only shows consistency and not entailment. As an evolutionary geneticist, I do not see how the origin of higher taxa are the necessary consequence of neo-Darwinism. They are sufficiently explained, but they are not the necessary consequence.’
University of Birmingham
Vol. 3 No. 14 · 6 August 1981
SIR: Dr Hallam’s comments (Letters, 16 July) on my article are a helpful contribution to the debate. I note that the disagreements between us, although important, are certainly not of a kind to suggest that he is supporting a new paradigm, incommensurable with neo-Darwinism, as suggested by Stephen Gould. His main point is that we should pay more attention to palaeontology, and to the major features of the fossil record, and perhaps somewhat less to the population genetics of flies and snails, when constructing our picture of evolution. I sympathise with his opinion. It is a great pity that, since G.G. Simpson, palaeontologists have contributed rather little to evolutionary theory, and most encouraging that there is now an active group determined to end this state of affairs.
There are, however, a few points on which I would take issue with him, or on which further clarification is needed:
1. Hallam defends Stanley’s book Macroevolution, and its reliance on data concerning species durations in the fossil record, on the grounds that such data do show that evolutionary rates have been slower in molluscs than in ammonites or mammals. I agree that species durations can show differences in evolutionary rates between major taxa, but that was not the question I was discussing. My claim was that the occurrence of stasis and punctuation could not be settled as Stanley attempted to settle it, and Hallam says nothing to alter my opinion. I should repeat that I am open-minded as to how frequent a pattern stasis plus punctuation in fact is, and that it is in any case an empirical question for palaeontologists to answer.
2. Hallam argues that if geographical variation within species commonly resulted in distant populations being so different that they should be regarded as distinct species, then more examples should be forthcoming than the old textbook example of the gulls which I quoted. Other cases of ‘ring species’ are known: for example, Clarke and Murray recently described a case in the snail, Partula. As Hallam implies, however, they are relatively rare. The reason for this is that it is rare for a species to form a ring, as the gulls do round the arctic, and not that it is rare to have terminal populations which are sufficiently distinct to raise doubts as to whether they should be classified as different species. Since Hallam quotes Mayr in a different context, let me quote from his Animal Species and Evolution: ‘We find that in every actively evolving genus there are populations that are hardly different from each other, others that are as different as subspecies, others that have almost reached species level, and finally still others that are full species.’
3. Hallam quotes Mayr’s observation that the natives of New Guinea recognise the same species of birds as do modern ornithologists as evidence for the discreteness and reality of species. This seems to me a misunderstanding. I do not for a moment doubt the reality of species, at least in sexually reproducing organisms, so long as one remains in the same place. The question at issue is whether this discreteness remains sharp when one travels about.
The point is important, so let m illustrate it with some personal reminiscences. I have always been fascinated by birds. When I was eight, my family moved to the country, and I learnt to recognise some dozen or so ‘kinds’ of birds which came to be fed in the winter. When, later, I was given a field guide, I was pleased to discover that my kinds corresponded to those in the book, although I had erred in thinking that the male and female blackbird were different kinds. Thus in a minor way I had confirmed the point that, for species in a given place, independent classifications come up with the same answer.
Things are quite different, however, if one travels about. On a recent visit to the US, I was much confused to discover that two easily distinguishable ‘Species’ of warblers listed in my field guide have been lumped into one, because they have been found to hybridise. In fact, ornithologists are regularly changing their minds about the status – good species or merely subspecies – of birds on the east and west coasts.
4. Hallam suggests that the sudden ‘punctuational’ change in the molluscs of Lake Turkana described by Williamson may have been caused by a breakdown of stabilising selection, and reminds me that there was an increase in variability of the populations during the change. I entirely agree. To me, the significant thing to emerge from Williamson’s work is that there is no reason to invoke any mechanism other than a change in selection pressure (from stabilising to directional) to account for an observed punctuational change.
More generally, I would certainly not rule out the possibility that genetic drift, chromosomal change and changes in regulatory genes acting early in development may have been important in evolution. Further, I do not regard population genetics as the only theoretical input needed for an understanding of evolution, although it is a necessary input. All that population genetics can do is to predict that if certain selection pressures operate on populations of particular kinds, then certain results will follow. Since it cannot in general predict what selection pressures will in fact operate, or what kinds of populations will exist, it cannot predict, or rule out, such phenomena as stasis or adaptive radiation, any more than Newtonian physics can predict the existence of meteorites or motor-cars. But meteorites and motor-cars obey Newton’s laws (so long as they do not go too fast), and species, whether in stasis or change, will, I believe, be found to obey the laws of population genetics. However, the theory of evolution will be richer if it incorporates theories of ecology and of development. Part of our present trouble arises because such theories, particularly of development, are sadly inadequate.
John Maynard Smith
University of Sussex
Vol. 3 No. 15 · 20 August 1981
SIR: In his lucid article ‘Did Darwin get it right?’ (LRB, 18 June), my colleague John Maynard Smith states that he fully shares Darwin’s views on how we are to understand the conservation of major patterns of morphological organisation in the living world. It is worth commenting on these views since they are significant in assessing the supposed status of Darwinian theory as the unifying theory in biology.
Maynard Smith quotes from the conclusions to Chapter Six of the Origin of Species where Darwin states: ‘On my theory, unity of type is explained by unity of descent.’ A few pages earlier in the same chapter Darwin asserts that ‘the chief part of the organisation of every being is simply due to inheritance’ of former adaptations from a ‘common progenitor’. These remarks are elaborated elsewhere in the book, especially in Chapter 13.
The explanatory power of Darwin’s remarks on conservation or invariance, insofar as they have any, depends upon the truth of an implicit theory of inheritance, which in turn is based on a specific assumption about the nature of biological patterns. The theory of inheritance can be reconstructed and asserts that the individual elements of a pattern (say, the bones of a limb) are effectively capable of self-reproduction. Thus, if all the elements reproduce exactly, both number and size will be conserved; if reproduction is inexact, then the size of an element may change and, if this mode of reproduction continues over a number of generations, the progressive reduction in size of an element may lead to its disappearance, and, hence, a change in number. Apart from the fact that it does not account for the conservation of spatial relations, the theory is simple and neat. Unfortunately, it is also false – the elements of biological patterns are not self-reproducing. Moreover, the assumption on which the theory rests – namely, that biological patterns are composed of independent elements which retain their individual identities through historical change – is, at best, problematic, if not simply wrong. A similar assumption was made by some of the early geneticists, who believed that a specific self-reproducing ‘gene’ (or group of ‘genes’) stood in direct causal relation to a specific individual ‘part’ of an organism, a belief which was not supported by the discoveries of classical genetics – there seems to be no one-to-one relationship between ‘genes’ and ‘parts’. More significantly, there is a reasonable amount of evidence which suggests that when at least some biological patterns change, they do so not in a piecemeal fashion but as wholes, so that the identification of an element in the ‘new’ pattern with one in the ‘old’ is difficult, if not impossible. Despite its questionable status, this assumption about the nature of biological patterns is rarely questioned.
The whole issue was discussed in a masterly fashion in 1894 by William Bateson in his Materials for the Study of Variation, where he pointed out that Darwin’s view of the inheritance of patterns in organisms is based on a false analogy with the transmission of property in human society. Bateson’s remarks on historical explanations in biology, made a few years earlier in the context of a discussion of the repetitive, segmental, organisation of vertebrates, are still relevant: ‘This much alone is clear, that the meaning of cases of complex repetition will not be found in the search for an ancestral form … Such forms there may be, but in finding them the real problem is not even resolved a single stage; for from whence was their repetition derived? The answer to this question can only come in a fuller understanding of the laws of growth and variation, which are as yet merely terms.’ The point is that in each generation the organism re-produces a specific pattern, and it is in terms of this process that conservation must be explained. We do not understand how this happens, so that to talk, as Darwin does, of the invariance of biological patterns in terms of ‘descent’ and ‘inheritance’ is not to provide an explanation, as Maynard Smith seems to imply, but to pose a problem. Chomsky has made a similar point about historical explanations of the ‘formal universals’ of language.
As Maynard Smith correctly remarks, Bateson’s ‘laws of growth’ or ‘laws of form’ are still unknown. It is possible that there are no such laws, but their existence is not refuted by his observation that some early vertebrates had more than two pairs of fins, any more than the existence of laws of motion is refuted by the observation that not all moving bodies describe elliptical trajectories.
The search for ‘laws of form’ is characteristic of a coherent ‘rationalist’ tradition in biology. Initiated by Linnaeus, analysed by Kant, developed by ‘the illustrious Cuvier’ and his followers and partially eclipsed by ‘Darwinism’, it survived in the thought of relatively isolated individuals such as Bateson, Driesch, D’Arcy Thompson and Waddington among others. Reconstructed, one can see it as a tradition which seeks to understand the biological domain in logical and systematic terms and supposes that different organisms might be transformations of each other, not merely in the historical and material sense of the term, but in the more fundamental mathematical sense; just as, in a much simpler way, the different forms of motion which are possible for a body moving under central attractive forces are transformations of each other and comprise a system which is intelligible in terms of the ‘laws’ responsible for its production. Within the empirical diversity of organisms, it is supposed, might be a rational, and therefore intelligible, unity.
Is this a manifestation of ‘physics envy’, as Maynard Smith maintains? Possibly, but I suspect that simpler emotions are involved. One is plain dissatisfaction. ‘Darwinism’ has provided a population biology – the theory accounts more or less satisfactorily for the differential growth of populations of given forms as a consequence of the relative degree of functional adaptation of the individuals comprising them – but has little of interest to say about the production and reproduction of these forms. The other is simple boredom. After 120 years of talk, almost everything of interest that can be said about functional adaptation has been said.
Whether the suppositions of the ‘rationalist’ tradition are tenable remains to be seen, but it at least offers the possibility of devising new and interesting hypotheses about structure, reproduction, transformation, the nature of species and taxonomy. As the detective in Borges’s ‘Death and the Compass’ observes, ‘You’ll reply that reality hasn’t the least obligation to be interesting. And I’ll answer you that reality may avoid that obligation but that hypotheses may not.’
School of Biological Sciences, University of Sussex