The Nature of Selection

Evolutionary Theory in Philosophical Focus

Elliott Sober

• Publisher: University of Chicago Press
• Year: 2014
• Print ISBN:
  9780226767482
• eBook ISBN:
  9780226308883
• Topic: Biological Sciences
• Subtopic: Philosophy History & Theory
• Index: Biological Sciences

The Nature of Selection is a straightforward, self-contained introduction to philosophical and biological problems in evolutionary theory. It presents a powerful analysis of the evolutionary concepts of natural selection, fitness, and adaptation and clarifies controversial issues concerning altruism, group selection, and the idea that organisms are survival machines built for the good of the genes that inhabit them.

"Sober's is the answering philosophical voice, the voice of a first-rate philosopher and a knowledgeable student of contemporary evolutionary theory. His book merits broad attention among both communities. It should also inspire others to continue the conversation."-Philip Kitcher, Nature

"Elliott Sober has made extraordinarily important contributions to our understanding of biological problems in evolutionary biology and causality. The Nature of Selection is a major contribution to understanding epistemological problems in evolutionary theory. I predict that it will have a long lasting place in the literature."-Richard C. Lewontin

PART I

Fitness, Selection, Adaptation

Chapter 1

Evolutionary Theory as a Theory of Forces

What makes mirages and other illusions possible? The answer lies in the concept of causality. A single visual impression can be produced in many ways. The impression of an oasis might be caused by a real oasis on the horizon, or by something else. In choosing to think the former, we select a causal hypothesis that is possible but happens not to be true. When we are taken in by an illusion, we err in an inference from effect to cause.

If scientists merely chronicled sequences of events, without trying to say what caused them, mistaken inferences from effects to causes might not occur. But science is more ambitious, and therefore runs additional risks.

Evolutionary theory aims not only to describe the patterns that may be found in the history of life on earth but also to characterize the processes that produced them. Its strategy for identifying the actual causes that have shaped the tree of life is the usual scientific one: scientists try to characterize a range of possible causes of evolution, and then to determine which of these possibilities actually obtained. The actual is understood by first embedding it in the possible.

When R. A. Fisher expressed this opinion in the preface to The Genetical Theory of Natural Selection (1930, pp. viii–ix), he quoted the physicist Arthur Stanley Eddington in support. In The Nature of the Physical World, Eddington remarked that

We need scarcely add that the contemplation in natural science of a wider domain than the actual leads to a far better understanding of the actual (Eddington 1928).

It may tell us something about the state of theoretical biology at the time Fisher wrote that he felt obliged to argue that the same idea applies in evolutionary theory:

For a mathematician this statement is almost a truism. For a biologist, speaking of his own subject, it would suggest an extraordinarily wide outlook. 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 goes on to remark that even a full characterization of the range of possibilities will at times be too restrictive. Imaginary solutions, he says, also have their place in scientific explanation.

Setting to one side the remark about the usefulness of contemplating impossibilities, we can recognize that contemporary evolutionary theory has followed Fisher’s advice to the hilt. The theory has assembled a variety of models that describe possible mechanisms that can produce evolution. In assessing the significance of these models, the evolutionist asks which of these possibilities has been actualized and, in particular, which has been at work, not just occasionally, but frequently. Possibilities are articulated not for their own sake but in order to obtain a satisfying theoretical picture of the actual.

Natural selection is one kind of cause. Selection can produce evolution, but there are other mechanisms that are capable of doing the same thing. Furthermore, within the category of selection, we can distinguish a variety of different sorts of selection process. Group selection, individual selection, and genic selection are alternative possibilities; each is a species of the same genus. In this chapter, I will begin the project of characterizing the genus. What is natural selection? How does it differ from other possible causes of evolution? And how are the different possible causes of evolution brought together in a unified theoretical treatment?

In Section 1.1, I provide a bit of history. Darwin’s idea of natural selection had a number of anticipations. One useful technique for isolating the biological notion is to locate it in a field of kindred but different ideas. In conceptual analysis as well as in biology, it can help to embed the actual in a range of possibilities.

Section 1.2 shifts from the history of ideas to biology itself. The point is not to relate Darwin’s concept of selection to other ideas of selection but to contrast selection as a cause of evolution with other possible causes. In addition, the idea of evolution itself is provided with a preliminary clarification.

Evolutionary theory describes the effects that various causes of evolution may have, both when they act alone and when they act in conjunction. The theory may be thought of in analogy with Newtonian mechanics. Various forces are described, but the theory has at its conceptual center a view of what will happen to the systems it describes when no forces at all impinge. Evolutionary theory’s version of a “zero-force law” is examined in Section 1.3.

In Section 1.4, I describe how natural selection is characterized in the very simplest of population genetics models. In Section 1.5, I describe the other side of our theoretical understanding of natural selection. Evolutionary theory describes selection, not just in terms of its consequences but in terms of the ecological conditions that can produce it.

1.1 The Prehistory of a Concept

Charles Darwin was not the first to assert the fact of evolution. Nor was the idea that natural selection may modify the composition of a population entirely without precedent. But Darwin was the first to think of natural selection as the driving force of evolution—as the principal cause of both the adaptedness of organisms to their environments and the existence of organic diversity.

The idea that a selection process may modify the composition of a population had been thought of before, both in and out of biology. The most famous of these anticipations is the one acknowledged by Darwin in his Autobiography. Thinking back some thirty-five years to the reflections that led him to the hypothesis of evolution by natural selection, he recalled:

Fifteen months after I had begun my systematic enquiry, I happened to read for amusement Malthus on Population, and being well prepared to appreciate the struggle for existence which everywhere goes on, from long-continued observation of the habits of animals and plants, it at once struck me that under these circumstances favorable variations would tend to be preserved, and unfavorable ones to be destroyed. The result of this would be the formation of new species. Here, then, I had at last got a theory by which to work (Darwin 1876, p. 120).

Darwin’s copious notebooks, in which he worked his way through alternative theories before he reached the idea of natural selection, have survived. Thanks to this rich documentary source, historians have been able to compare this recollection completed in 1876 with the day-to-day trials and errors that brought the Darwin of 1838 to amuse himself with Malthus. Yet, the degree to which Malthus changed the direction of Darwin’s thought remains controversial: did reading the Essay on Population merely crystallize and clarify ideas that Darwin already had at his command, or did it provide him with some fundamentally new insight? This psychological question may never receive an unequivocal answer.¹ Even so, we can recognize in Malthus at least part of the idea that Darwin was to put to such powerful use.² Malthus began with the simple idea that a population that expands beyond its own food supplies will be cut back by mortality. Although the case that most concerned him was the number of indigent poor in England, Malthus (1798, pp. 14–16) thought he had reached a principle of universal scope:

Population, when unchecked, increases in geometrical ratio. Subsistence increases only in an arithmetical ratio. . . . I see no way by which man can escape from the weight of this law which pervades all animated nature.

Death and fecundity are opposing forces that together determine population size. To diminish the former, human beings must reduce the latter. Hence Malthus’ advice to the poor that sexual abstinence is the way up from squalor.

Malthus was no evolutionist, and his “law which pervades all animated nature” concerned the regulation of population numbers, not the frequency of characteristics found in a population. In fact, Malthus thought his law prevents populations from changing much. It was his opinion that an individual deviating from the population norm would be less able to cope with the vicissitudes of life (Mayr 1977). For him, fecundity and death are forces that prevent populations from changing. Darwin radically transformed this anticipation of the idea of natural selection. What was for Malthus a process of destruction and stasis became for Darwin an instrument of improvement and change.³

Still, it is important to recognize in Malthus’ idea an important innovation that survived in Darwin’s theory. For Malthus, the natural world is a world of struggle; nature is not benignly arranged to bring into existence only that number of organisms that can be supported. Nature is inefficient and cruel, not economical and beneficent. In addition, the Malthusian paradigm pictures competition between organisms of the same species as an important force. Attention was shifted from the struggle between the lion and the lamb to that between lamb and lamb. Darwin’s teacher, Charles Lyell, had thought of competition as an interspecific phenomenon; Darwin provided the idea of competition with a Malthusian reorientation. The composition of a population may be modified by differences that obtain among members of that very population (Herbert 1971).

Besides transforming selection from a mechanism of stasis into a mechanism of change, Darwin changed the Malthusian concept in another way. The Malthusian picture was one in which individuals in the same population compete with each other for a resource in short supply. Darwin had to broaden the idea of the “struggle for existence”:

I should premise that I use the term Struggle for Existence in a large and metaphorical sense, including dependence of one being on another, and including (which is more important) not only the life of the individual, but success in leaving progeny. Two canine animals in a time of dearth, may be truly said to struggle with each other which shall get food and live. But a plant on the edge of a desert is said to struggle for life against the drought (Darwin 1859, p. 62).

Consider the difference between tennis and golf. Both games have a winner, but the relationship of the players’ scores is different. In tennis, a point for one player necessarily means no point for the other. As each point is played, there is a benefit at stake that one and only one player can obtain. In golf, on the other hand, one player’s score does not constrain the other’s. In both games, a criterion for winning is describable in relative terms: the player with the better score at the end of a certain time is the winner. The games differ in the way that end state is produced.

Malthusian competition is more like tennis than golf. If one thought that the struggle for existence was only of this sort, one would fail to detect the existence of contests in which one player’s score fails to affect the other’s. Two plants at the edge of the desert may differ in their abilities to withstand the drought. They thereby struggle against each other, but not in the Malthusian sense. In Darwin’s “large and metaphorical sense,” all that matters is that the players end up with different scores—that they do unequally well in “the struggle for existence.” Whether this relationship takes the form of a “zero-sum game” is entirely incidental (Ghiselin 1974a, p. 51).

A second precedent for the idea of natural selection was artificial selection. Darwin read a great deal about the techniques that plant and animal breeders used to modify living forms (Ruse 1975, 1979). Just as in the case of this debt to Malthus, the precise psychological role of this research in Darwin’s development remains controversial. But we may recognize here an idea whose structure approximates the one that Darwin finally crystallized.

Certainly, attention to artificial selection alerted Darwin to the immense store of variability that natural populations contain. This was a premise of the first importance; selection is impossible unless there are variants to select among. Yet the facts of artificial selection did not point unequivocally to the hypothesis of evolution by natural selection. Just as in the case of Malthus, the idea required transformation and a new theoretical context. Three facts blocked an easy extrapolation from artificial to natural selection. One concerned the observed effects of artificial selection, the other two its causes.

Plant and animal breeders had been able to create new varieties within a species. But when Darwin wrote, artificial selection had never managed to produce a new species. Darwin’s contemporaries standardly took this to show that there are limits beyond which a species cannot be modified. A breeder may tinker and fine-tune, but nothing more. Darwin looked at the same data, harnessed them to an argument by extrapolation, and reached the opposite conclusion: If artificial selection has achieved so much in the comparatively short time of recorded human history, then natural selection can be expected to generate even more impressive results on the time scale of natural history.⁴

The second obstacle that Darwin had to overcome in using artificial selection as a conceptual precedent for his notion of natural selection was consciousness. Artificial selection is the product of intelligent manipulation. Why think that organisms could be adapted to their environments without this sort of guidance?

Here we must see that Darwin’s argument contradicted what was at the time the most influential argument for the existence of God—the argument from design. William Paley (1819) (to single out one of the many writers who formulated this line of thought in detail) observed that if we found a watch on a beach, we would unhesitatingly hypothesize that it had been created by some intelligent being. The id...

Table of contents

• Cover
• Copyright
• Title Page
• Dedication
• Contents
• Acknowledgments
• Introduction
• Part I. Fitness, Selection, Adaptation
• Part II. The Group Above and the Gene Below
• Notes
• References
• Index