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Life at the Edge of Chaos?

Darwinism Evolving

by David J. Depew, by Bruce H. Weber
MIT Press, 588 pp., $49.95

1.

Darwinism Evolving is a history of ideas about biological diversity and evolution, from Aristotle to the present day. The last part of the book is an account of some recent developments, and an attempt to forecast the future. Most of this review will be concerned with the final section, which seems to me mistaken. I must therefore start by saying that I found the historical part well informed, and full of valuable insights. The ideas discussed are fundamental, not only to biology, but also to our view of our relationship to the rest of the natural world. The last twenty years have seen an explosion of scholarship centered on Darwin, by historians and philosophers. The book is an admirable summary of, and addition to, that scholarship.

The historical thesis can be summarized by saying that Darwinism has a central core, the idea of natural selection, but that it has undergone three stages of evolution, according to the dynamic theories used to formulate it: first, the deterministic dynamics of Newton, then the probabilistic dynamics of Clerk Maxwell and Ludwig Boltzmann, and finally the dynamics of complex systems now being developed. Darwin himself, the authors suggest, formulated his theory in Newtonian terms. When I first met this claim, I was doubtful. Of course Darwin, like many of his contemporaries, wished to be seen as following the Newtonian method: Who would not claim to be following such a successful example. But how can one have “dynamics” without any mathematical equations? After reading their account, however, I was persuaded that the claim is reasonable, provided that one interprets dynamics in what Darwin himself might have called a loose and metaphorical sense.

The essential difference between Aristotle and Newton is that Aristotle thought that bodies move as they do because it is natural for them to do so, whereas Newton explained the elliptical orbits of planets as caused by an external force, gravity. A similar contrast exists between Lamarck and Darwin. Lamarck held that organisms evolve because they have an inherent tendency to become more complex. It was this idea that Darwin was rejecting when he said his theory had nothing in common with Lamarck’s. Instead of explaining evolution by an inherent tendency, Darwin thought that change was directed by an external force, natural selection.

Thus I think the authors make a good case for the claim that Newton is to Aristotle as Darwin is to Lamarck. Later in the nineteenth century, Newton’s deterministic dynamics was replaced in some fields of physics by the “stochastic” (i.e., probabilistic) dynamics of Maxwell and Boltzmann. They showed that the behavior of large aggregates of things (initially gases, which are aggregates of molecules) could be predicted by a dynamics which ignored the precise behavior of individuals, and took into account only the average behavior. Boltzmann once wrote that the nineteenth century should be seen as the century of Darwin, because Darwin explained evolution according to the chances of death or survival of millions of individuals. Boltzmann was giving Darwin the credit for inventing stochastic dynamics. Depew and Weber think that he was being overgenerous: the century, in their view, should be seen as Boltzmann’s. I cannot decide whether they are right. However that may be, Darwinism today is fully stochastic, but it did not become so until the 1920s with the invention of population genetics by R.A. Fisher, J.B.S. Haldane, and Sewall Wright.

It is a curious twist of history that when Mendel’s laws were rediscovered in 1901, Mendelism was at first seen as an alternative to Darwinism in explaining evolution. Darwinism held that change occurred by the natural selection of many minute variations, Mendelism that novelty arose suddenly, by mutation. The population geneticists showed that the two theories were complementary, not contradictory. They did so by developing a theory of the changes in the frequencies of genes, which is indeed analogous to the Maxwell-Boltzmann dynamics.

The authors rightly point out that the history of ideas about evolution is not simply a series of modulations on Darwin’s idea of natural selection. There was an alternative account, which saw the evolution of species and individual development as parallel processes, causally connected in ways that I find difficult to understand. Ernst Haeckel’s theory of recapitulation, according to which each individual during its development recapitulates the forms of its ancestors, is an example of such a theory. Although Haeckel’s ideas are hard to follow, it is clear that if a developmental change in one generation affects the nature of the next, then theories of evolution and development cannot be separated. Twentieth-century Darwinism was liberated from this difficulty by August Weismann. If, as he claimed, acquired characters are not inherited, then the processes by which organisms develop can be treated as if they were in a black box. The box will be opened one day, but in the meanwhile we can get on with genetics and evolution theory. I will return to the relation between development and evolution.

Although I found the historical account in Darwinism Evolving illuminating, one concept which has become dominant in the twentieth century is hardly mentioned, either here or in most recent discussions of evolution by philosophers and historians. This is the concept of information. Its dominance in molecular biology is obvious from the vocabulary of the subject. Messenger, code, synonymous, transcription, translation, proofreading, editing are all technical terms: they are not metaphors but words whose precise meaning is well understood. Although the concept of information has become dominant only in the last half of this century, it is already present in August Weismann’s work at the end of the last. His great insight was to see that heredity is about the transmission, not of matter or energy, but of information. Since I will certainly be accused of an anachronism in saying this, let me quote his remark, discussing the inheritance of an acquired character, that it “is very like supposing that an English telegram to China is there received in the Chinese language.”

When we turn to the authors’ account of the present state of evolutionary biology, and its likely future development, I think they are mistaken; but their comments are well informed and thoughtful: if they are mistaken, it is not from ignorance or stupidity. And it must also be said that spotting what is important in current science, and predicting future developments, are matters of judgment; one cannot know that one is right.

The pieces of research that the authors pick out to discuss have one thing in common: they either deny natural selection or, more often, diminish its importance. I therefore start by explaining why I see natural selection as the necessary component of any satisfactory theory of evolution. Like better men before me—for example, Aristotle, Darwin, Alfred Russel Wallace, August Weismann—I start from the fact of adaptation. The most striking fact about living organisms is that their parts are adapted to ensure the survival and reproduction of the whole organism. Not only are structures like hearts, wings, eyes, and kidneys explicable only by their functions: so also are different behaviors, such as the migration of birds or the dances of bees. Adaptation is not only obvious on a large scale: the molecular structures that ensure the translation of DNA into protein are also to be understood only in functional terms. Of course not all features of organisms are adapted: no one who has suffered from myopia, appendicitis, cancer, or arthritis could think that. But myopia does not call for an explanation: it is to be expected in a universe in which entropy increases. What does need an explanation is how an eye can form an image and how a brain can interpret it.

Adaptation, then, is the phenomenon we have to explain. Natural selection is the only theory so far proposed that can do so. Lamarckism might seem to be a possible alternative. Thus if an animal acquires an adaptive characteristic during its lifetime, and passes it on to its offspring, that could lead to the evolution of adaptation. The snag is that most “acquired characters” are not adaptations: they are the results of injury, disease, and aging. For Lamarckism to work, there would have to be some process of selection whereby only those acquired characters that were adaptive were passed on. If we learn many ideas, but teach our children only the ones that worked, then our children would be better off than we are. That is what people mean when they speak of cultural inheritance being Lamarckian. But for genetically transmitted traits, there seems to be no way this can work. If we are to explain adaptation, we must choose between natural selection and creation.

Natural selection, in turn, requires that there be entities that multiply, and that have heredity. Like must beget like. In fact, more than heredity is needed. There must be entities that can exist in an indefinitely large number of forms, each of which can be accurately replicated. This is why the concept of information is central both to genetics and evolution theory.

With this as background, I turn to the particular topics that the authors pick out as significant for the future. I have space to discuss only three: the neutral theory of molecular evolution, genetic revolutions, and the dynamics of complex systems.

The neutral theory, first proposed by the Japanese geneticist Motoo Kimura, argues that most of the changes that occur in proteins and nucleic acids in the course of evolution do so because they are selectively “neutral”: they do not make their possessor either more or less likely to survive. The first thing to understand about this theory is that it does not deny either the phenomenon of adaptation or Darwin’s explanation of it. In Kimura’s own words, “the theory does not deny the role of natural selection in determining the course of adaptive evolution.” What it claims is that in addition to genetic changes caused by selection there are much more frequent changes that occur because they do not matter. I have never seen any reason why, as a naive Darwinist, I should reject this theory. It is mathematically elegant: I have even done a little of the mathematics myself, and would hate to see it wasted. For me, the only question is the empirical one: Is the theory actually true? At least as far as nucleic acids are concerned, I think that it is near enough true to be interesting and important. I was delighted when, perhaps somewhat ironically, Kimura was given the Darwin Medal of the Royal Society. But his ideas have not altered the way I think about the evolution of morphology or behavior.

The second issue is that of genetic revolutions. It is now known that changes in the genetic information—DNA—can occur not only by “point mutations” that alter a single base at a time, but by a variety of processes in which pieces of DNA are duplicated or moved to new sites in the chromosome. Perhaps inevitably, this has led to the idea that such genomic shuffling can, without the need for selection, give rise to evolutionary novelty, and in particular to new species. This is nonsense. It would take too long to clear up all the confusions involved, but I will address three points: the non-adaptive nature of mutation, the significance of gene duplication, and “transposition.”

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