The Extended Phenotype: The Long Reach of the Gene (Popular Science) (2 page)
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Authors: Richard Dawkins
I was privileged to have the first draft criticized in its entirety by John Maynard Smith, David C. Smith, John Krebs, Paul Harvey, and Ric Charnov, and the final draft owes much to all of them. In all cases I acted on their advice, even if I did not always take it. Others kindly criticized chapters in their own special fields: Michael Hansell the chapter on artefacts, Pauline Lawrence that on parasites, Egbert Leigh that on fitness, Anthony Hallam the section on punctuated equilibria, W. Ford Doolittle that on selfish DNA, and Diane De Steven botanical sections. The book was finished at Oxford, but begun during a visit to the University of Florida at Gainesville on a sabbatical leave kindly granted by the University of Oxford and the Warden and Fellows of New College. I am grateful to my many
Floridan friends for giving me such a pleasant atmosphere in which to work, especially Jane Brockmann, who also provided helpful criticism of preliminary drafts, and Donna Gillis, who also did much of the typing. I benefited, too, from a month’s exposure to tropical biology as the grateful guest of the Smithsonian Institution in Panama during the writing of the book. Finally, it is a pleasure once again to thank Michael Rodgers, formerly of Oxford University Press and now of W. H. Freeman and Company, a
‘K-
selected’ editor who really believes in his books and is their tireless advocate.
Oxford | Richard Dawkins |
June 1981 |
I suppose most scientists—most authors—have one piece of work of which they would say: It doesn’t matter if you never read anything else of mine, please at least read
this
. For me, it is
The Extended Phenotype
. In particular, the last four chapters constitute the best candidate for the title ‘innovative’ that I have to offer. The rest of the book does some necessary sorting out on the way.
Chapters 2
and
3
are replies to criticisms of the now widely accepted ‘selfish gene’ view of evolution. The middle chapters deal with the ‘units of selection’ controversy currently fashionable among philosophers of biology, taking the gene’s-eye view; perhaps the most useful contribution here is the ‘Replicators and Vehicles’ distinction. My intention was that this piece of sorting out should put paid to the whole controversy once and for all!
As for the extended phenotype proper, I have never seen any alternative to placing it at the end of the book. Nevertheless, this policy has a disadvantage. The earlier chapters inevitably draw attention to the general ‘units of selection’ issue, and away from the more novel idea of the extended phenotype itself. It is for this reason that I have dropped the original subtitle, ‘The Gene as the Unit of Selection’, from this edition. The replacement, ‘The Long Reach of the Gene’, captures the idea of the gene as the centre of a web of radiating power. The book is otherwise unchanged apart from minor corrections.
Oxford | Richard Dawkins |
May 1989 |
2 Genetic Determinism and Gene Selectionism
5 The Active Germ-Line Replicator
6 Organisms, Groups and Memes: Replicators or Vehicles?
7 Selfish Wasp or Selfish Strategy?
9 Selfish DNA, Jumping Genes, and a Lamarckian Scare
11 The Genetical Evolution of Animal Artefacts
12 Host Phenotypes of Parasite Genes
pp. 141–142 ‘The Fifth Philosopher’s Song’ from
The Collected Poetry of Aldous Huxley
edited by Donald Watt. Copyright © 1971 by Laura Huxley. Reproduced by permission of Mrs. Laura Huxley and Chatto & Windus Ltd, also by permission of Harper & Row, Publishers, Inc. (USA)
p. 16 ‘McAndrew’s Hymn’ by Richard Kipling. Extract reproduced by permission of Doubleday & Company Inc.
The Extended Phenotype
This is a work of unabashed advocacy. I want to argue in favour of a particular way of looking at animals and plants, and a particular way of wondering why they do the things that they do. What I am advocating is not a new theory, not a hypothesis which can be verified or falsified, not a model which can be judged by its predictions. If it were any of those things, I agree with Wilson (1975, p. 28) that the ‘advocacy method’ would be inappropriate and reprehensible. But it is not any of those things. What I am advocating is a point of view, a way of looking at familiar facts and ideas, and a way of asking new questions about them. Any reader who expects a convincing new theory in the conventional sense of the word is bound to be left, therefore, with a disappointed ‘so what?’ feeling. But I am not trying to convince anyone of the truth of any factual proposition. Rather, I am trying to show the reader a way of seeing biological facts.
There is a well-known visual illusion called the Necker Cube. It consists of a line drawing which the brain interprets as a three-dimensional cube. But there are two possible orientations of the perceived cube, and both are equally compatible with the two-dimensional image on the paper. We usually begin by seeing one of the two orientations, but if we look for several seconds the cube ‘flips over’ in the mind, and we see the other apparent orientation. After a few more seconds the mental image flips back and it continues to alternate as long as we look at the picture. The point is that neither of the two perceptions of the cube is the correct or ‘true’ one. They are equally correct. Similarly the vision of life that I advocate, and label with the name of the extended phenotype, is not provably more correct than the orthodox view. It is a different view and I suspect that, at least in some respects, it provides a deeper understanding. But I doubt that there is any experiment that could be done to prove my claim.
The phenomena that I shall consider—coevolution, arms races, manipulation of hosts by parasites, manipulation of the inanimate world by living things, economic ‘strategies’ for minimizing costs and maximizing benefits—
are all familiar enough, and are already the subject of intensive study. Why, then, should the busy reader bother to go on? It is tempting to borrow Stephen Gould’s winningly ingenuous appeal at the beginning of a more substantial volume (1977a) and simply say, ‘Please read the book’ and you will find out why it was worth bothering to do so. Unfortunately I do not have the same grounds for confidence. I can only say that, as one ordinary biologist studying animal behaviour, I have found that the viewpoint represented by the label ‘extended phenotype’ has made me see animals and their behaviour differently, and I think I understand them better for it. The extended phenotype may not constitute a testable hypothesis in itself, but it so far changes the way we see animals and plants that it may cause us to think of testable hypotheses that we would otherwise never have dreamed of.
Lorenz’s (1937) discovery that a behaviour pattern can be treated like an anatomical organ was not a discovery in the ordinary sense. No experimental results were adduced in its support. It was simply a new way of seeing facts that were already commonplace, yet it dominates modern ethology (Tinbergen 1963), and it seems to us today so obvious that it is hard to understand that it ever needed ‘discovering’. Similarly, D’Arcy Thompson’s (1917) celebrated chapter ‘On the theory of transformations …’ is widely regarded as a work of importance although it does not advance or test a hypothesis. In a sense it is obviously necessarily true that any animal form can be turned into a related form by a mathematical transformation, although it is not obvious that the transformation will be a simple one. In actually doing it for a number of specific examples, D’Arcy Thompson invited a ‘so what?’ reaction from anyone fastidious enough to insist that science proceeds only by the falsifying of specific hypotheses. If we read D’Arcy Thompson’s chapter and then ask ourselves what we now know that we did not know before, the answer may well be not much. But our imagination is fired. We go back and look at animals in a new way; and we think about theoretical problems, in this case those of embryology and phylogeny and their interrelations, in a new way. I am, of course, not so presumptuous as to compare the present modest work with the masterpiece of a great biologist. I use the example simply to demonstrate that it is
possible
for a theoretical book to be worth reading even if it does not advance testable hypotheses but seeks, instead, to change the way we see.
Another great biologist once recommended that to understand the actual we must contemplate the possible: ‘No practical biologist interested in sexual reproduction would be led to work out the detailed consequences experienced by organisms having three or more sexes; yet what else should he do if he wishes to understand why the sexes are, in fact, always two?’ (Fisher 1930a, p. ix). Williams (1975), Maynard Smith (1978a) and others have taught us that one of the commonest, most universal features of life on Earth, sexuality itself, should not be accepted without question. Indeed, its
existence turns out to be positively surprising when set against the imagined possibility of asexual reproduction. To imagine asexual reproduction as a hypothetical possibility is not difficult, since we know it is a reality in some animals and plants. But are there other cases where our imagination receives no such prompting? Are there important facts about life that we hardly notice simply because we lack the imagination to visualise alternatives which, like Fisher’s three sexes, might have existed in some possible world? I shall try to show that the answer is yes.
Playing with an imaginary world, in order to increase our understanding of the actual world, is the technique of ‘thought experiment’. It is much used by philosophers. For instance in a collection of essays on
The Philosophy of Mind
(ed. Glover 1976), various authors imagine surgical operations in which one person’s brain is transplanted into another person’s body, and they use the thought experiment to clarify the meaning of ‘personal identity’. At times philosophers’ thought experiments are purely imaginary and wildly improbable, but this doesn’t matter given the purpose for which they are made. At other times they are informed, to a greater or lesser extent, by facts from the real world, for instance the facts of split-brain experiments.
Consider another thought experiment, this time from evolutionary biology. When I was an undergraduate obliged to write speculative essays on ‘the origin of the Chordates’ and other topics of remote phylogeny, one of my tutors rightly tried to shake my faith in the value of such speculations by suggesting that anything could, in principle, evolve into anything else. Even insects could evolve into mammals, if only the right sequence of selection pressures were provided in the right order. At the time, as most zoologists would, I dismissed the idea as obvious nonsense, and I still, of course, don’t believe that the right sequence of selection pressures ever would be provided. Nor did my tutor. But as far as the principle is concerned, a simple thought experiment shows it to be nearly incontrovertible. We need only prove that there exists a continuous series of small steps leading from an insect, say a stag beetle, to a mammal, say a stag. By this I mean that, starting with the beetle, we could lay out a sequence of hypothetical animals, each one as similar to the previous member of the series as a pair of brothers might be, and the sequence would culminate in a red deer stag.
The proof is easy, provided only that we accept, as everyone does, that beetle and deer have a common ancestor, however far back. Even if there is no other sequence of steps from beetle to deer, we know that at least one sequence must be obtained by simply tracing the beetle’s ancestors back to the common ancestor, then working forwards down the other line to the deer.
We have proved that there exists a trajectory of stepwise change connecting beetle to deer and, by implication, a similar trajectory from any modern animal to any other modern animal. In principle, therefore, we may
presume that a series of selection pressures could be artificially contrived to propel a lineage along one of these trajectories. It was a quick thought experiment along these lines that enabled me to say, when discussing D’Arcy Thompson’s transformations, that ‘In a sense it is obviously necessarily true that any animal form can be turned into a related form by a mathematical transformation, although it is not obvious that the transformation will be a simple one.’ In this book I shall make frequent use of the thought-experiment technique. I warn the reader of this in advance, since scientists are sometimes annoyed by the lack of realism in such forms of reasoning. Thought experiments are not supposed to be realistic. They are supposed to clarify our thinking about reality.
One feature of life in this world which, like sex, we have taken for granted and maybe should not, is that living matter comes in discrete packages called organisms. In particular, biologists interested in functional explanation usually assume that the appropriate unit for discussion is the individual organism. To us, ‘conflict’ usually means conflict between organisms, each one striving to maximize its own individual ‘fitness’. We recognize smaller units such as cells and genes, and larger units such as populations, societies and ecosystems, but there is no doubt that the individual body, as a discrete unit of action, exerts a powerful hold over the minds of zoologists, especially those interested in the adaptive significance of animal behaviour. One of my aims in this book is to break that hold. I want to switch emphasis from the individual body as focal unit of functional discussion. At the very least I want to make us aware of how much we take for granted when we look at life as a collection of discrete individual organisms.
