Did Darwin get it right?
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
Bites from the Bearded Crocodile
SIR: Cabrera Infante’s article (LRB, 4 June) on the Cuban alternative literary scene was one of those rare and exquisite introductions to a world that is at once distant and comprehensible. May I endorse Salman Rushdie’s plea (later in the same issue) that people in England buy, savour and propagate. But my main reason for writing is to take up the point about the imprisoned Cuban writer Armando Valladares. His case is being pursued not only by Amnesty International but also by PEN. On a more local (but, one hopes, not ineffectual) level he has been ‘adopted’ by Chelsea Lib Party, at PEN’s suggestion.
Liberal PPC, Chelsea