The Routledge Handbook of Evolution and Philosophy

Richard Joyce

• Publisher: Routledge
• Year: 2017
• ISBN:
  9781317655565
• Edition: 1
• Topic: Philosophy
• Subtopic: Philosophy History & Theory

In recent years, the relation between contemporary academic philosophy and evolutionary theory has become ever more active, multifaceted, and productive. The connection is a bustling two-way street. In one direction, philosophers of biology make significant contributions to theoretical discussions about the nature of evolution (such as "What is a species?"; "What is reproductive fitness?"; "Does selection operate primarily on genes?"; and "What is an evolutionary function?"). In the other direction, a broader group of philosophers appeal to Darwinian selection in an attempt to illuminate traditional philosophical puzzles (such as "How could a brain-state have representational content?"; "Are moral judgments justified?"; "Why do we enjoy fiction?"; and "Are humans invariably selfish?"). In grappling with these questions, this interdisciplinary collection includes cutting-edge examples from both directions of traffic. The thirty contributions, written exclusively for this volume, are divided into six sections: The Nature of Selection; Evolution and Information; Human Nature; Evolution and Mind; Evolution and Ethics; and Evolution, Aesthetics, and Art. Many of the contributing philosophers and psychologists are international leaders in their fields.

I
The Nature of Selection

1
The Nature of Selection: An Overview

Tim Lewens

"The Single Best Idea that Anyone has Ever Had"

Near the beginning of his book Darwin’s Dangerous Idea, Daniel Dennett tells us that “If I were to give an award for the single best idea anyone has ever had, I’d give it to Darwin, ahead of Newton and Einstein and everyone else” (1995: 21). Of course, the idea Dennett had in mind was natural selection. But what, exactly, is that idea? Should we even think of it as a “single” idea? I suggest we should think of natural selection as a family of related modes of explanation, which have changed gradually over the years as the theories in which they are embedded have been reformulated in order to address different problems. In other words, our understanding of natural selection itself has been subject to a process of “descent with modification.”

This chapter uses some of these transformations in our thinking to reflect on conceptual puzzles about what natural selection is, and how it works. In particular, I focus on a series of contentious questions. Does natural selection entail “gradualism”? In other words, is Jerry Fodor right when he asserts that “Darwinism can work only if . . . there is some organic parameter the small, incremental variation of which produces correspondingly small, incremental variations of fitness” (2001: 89)? Is sexual selection a different process to natural selection, or just a type of natural selection? In what sense does natural selection involve a “struggle for existence”? Can natural selection work with any form of inheritance, or must inheritance be “particulate”? How does our verdict on these questions affect the prospects of efforts to apply natural selection to cultural change, rather than to organic change? And what, finally, does all of this tell us about how natural selection explains the phenomena that were of central interest to Darwin—namely the emergence and perfection of structures and habits that adapt organisms so well to their conditions of life?

Darwin's Question

In the introduction to the Origin of Species, Darwin (1859) pointed out how much strong evidence there is in favor of “transformism.” This is the view, espoused by others before him such as the then-anonymous author of the 1844 work Vestiges of the Natural History of Creation, that the species we see around us are modified descendants of earlier ancestors they hold in common. Transformism can be supported, for example, by pointing to facts about the anatomical similarities of distinct species, their distribution around the globe, and so forth. But this sort of transformism faces a significant problem. How on earth, Darwin asks, have “the innumerable species inhabiting this world . . . been modified, so as to acquire that perfection of structure and coadaptation which most justly excites our admiration” (1859: 3)? By itself the hypothesis of common ancestry contains nothing that might explain, to use just one of Darwin’s examples, “the structure . . . of the woodpecker, with its feet, tail, beak, and tongue so admirably adapted to catch insects under the bark of trees” (1859: 3).

Darwin considers a mystical response on behalf of the transformist: “The author of the ‘Vestiges of Creation’ would, I presume, say that, after a certain unknown number of generations, some bird had given birth to a woodpecker,” and that it had been produced “perfect as we know them” (1859: 3–4). Needless to say, Darwin immediately responds that “this assumption seems to me to be no explanation, for it leaves the case of the coadaptations of organic beings to each other and to their physical conditions of life, untouched and unexplained” (1859: 4). Darwin observes, in other words, that transformism is incomplete unless it offers some explanation for the emergence of organic structures that are brilliantly adapted to each other, and to the life of the organisms that bear them. Darwin designs natural selection in such a way that it can serve as an answer to what we might call Darwin’s question:

How have all those exquisite adaptations of one part of the organic organisation to another part, and to the conditions of life, and of one distinct being to another being, been perfected? (1859: 60)

In this chapter I will argue that by focusing on the pragmatic origins of natural selection as a response to the question of adaptation—and by focusing on exactly how Darwin himself understands that question—we can understand why Darwin tends to describe natural selection as he does, and why his descriptions oft en depart from more recent accounts of what natural selection is. In some cases these differences are superficial, and in other cases they are profound.

Darwin's Answer

Darwin does not usually define natural selection in any short, pithy way, nor does he give a set of conditions that are necessary and sufficient for natural selection to act. Instead, he tends to give far longer descriptions that illustrate, in a schematic form, how complex adaptations can come to exist. The organic world, he says, is characterized by struggle. In his Notebooks he writes of “the dreadful but quiet war of organic beings” (Barrett et al. 1987: E114), a competition so intense that “a grain of sand turns the balance” between life and death (E115c). This struggle has profound consequences, as he later explains in the Origin:

Owing to this struggle for life, any variation, however slight and from whatever cause preceding, if it be in any degree profitable to an individual of any species, in its infinitely complex relations to other organic beings and to external nature, will tend to the preservation of that individual, and will generally be inherited by its offspring. The offspring, also, will thus have a better chance of surviving, for, of the many individuals of any species which are periodically born, but a small number can survive. I have called this principle, by which each slight variation, if useful, is preserved, by the term of Natural Selection, in order to mark its relation to man’s power of selection. (1859: 61)

For Darwin, “this principle” names a wide set of processes whereby valuable variants are generated, maintained, and refined in a population of organisms.

In contrast to this, modern treatments of evolution are oft en at pains to give far more compact definitions of natural selection. We might be told, for example, that natural selection occurs whenever organisms—or indeed entities of any kind—vary in their “fitness”—roughly speaking, when they vary in their abilities to leave offspring—and whenever these abilities are passed from parents to their babies (e.g., Lewontin 1970). This very general account allows us to ask whether selection might act at several different levels of natural organization—perhaps at the level of the group, or the species, perhaps at the level of the cell or the gene—and it also allows us to ask whether selection might act on entities outside the organic realm—computer viruses, tools, scientific theories. Call this the “inherited variation in fitness” definition.

We can appreciate one limitation of this definition by imagining a population that obeys all these conditions for the action of natural selection, and which also has very few members. Maybe it is divided into slow and fast runners, babies grow to run at the same speed as their parents, and running speed assists in catching prey. Consistent with this, it might also turn out, let us suppose, that the fastest running predators in this population all happen to die young from infections. These infections are just as likely to affect slow and fast individuals: the fast ones just happen to be unlucky. The result is that the slower individuals dominate. Here, modern biologists will say that “drift ” is at work, in addition to selection.

So one drawback of our equation of natural selection with “inherited variation in fitness” is that, taken by itself, it does not help us to distinguish natural selection from drift. Modern theorists oft en move on to define selection in a way that allows us to ask which evolutionary “forces” are at work on a population, and which allows us to give a quantified description of how strong those forces are (Sober 1984). In this mode, we need to find a way of understanding what “selection” is that distinguishes it sharply from other “forces” including drift, mutation, and migration. A standard way of doing this is to propose that natural selection is a force that tends to make the fitter variant in a population increase in frequency, and whose strength depends on the fitness differences between the variants in the population. Drift, on the other hand, is then understood as a force whose strength is in an inverse relationship with population size. In small populations it can overwhelm selection. The broad issue of whether evolution should be understood in terms of interacting “forces” has been the subject of lively debate in recent years, with defences from Sober (1984), Stephens (2004), Reisman & Forber (2005), and Sober & Shapiro (2007), and dissent from Walsh et al. (2002), Matthen & Ariew (2002), and Lewens (2010a), among several others.

In contrast to these modern theorists, Darwin did not approach evolution in a way that demanded a quantified decomposition of different evolutionary “forces,” hence he was not driven to define evolutionary processes in a way that would permit sharp differentiation between selection, drift, mutation, and migration. His strong conceptual linkage between natural selection and the explanation of adaptation meant that he sometimes omitted to distinguish between what we would now think of as mutation, on the one hand, and selection, on the other. Instead, because a constant supply of novel variation is essential if complex adaptations are to be produced at all, he oft en understood the introduction of variation itself as part of the overall process of selection.

More generally, Darwin thought that a diverse variety of circumstances would tend to augment, or undermine, the production of complex adaptations, and he tended to think of these as factors “favorable” or “unfavorable” to the action of selection. The sorts of factors he mentioned include traumatic environmental shift s that can (he thought) act on reproductive organs to stimulate the production of a wide range of “profitable variations” (1859: 82); increases in population size that increase the chances of beneficial variations arising merely because the population is larger; and the geographical isolation of populations, which can allow new varieties to become established and improved in an environment that is comparatively shielded from competitive immigrants (1859: 101–109).

We should not exaggerate how significant these differences are. Darwin understood, even if he did not approach the topic in a mathematically disciplined way, that factors such as the size of a population and the rate at which variation appears within it can affect the production of complex adaptations. Similarly, even though they might isolate selection as just one evolutionary force among many, more recent theorists have oft en argued that the question of whether a population is able to produce complex adaptations will depend on many other factors, in addition to the question of whether the population is affected by selection in their own rigorously defined sense. For example, Sewall Wright (1932) is well known for his suggestion that drift can in fact facilitate the production of complex adaptation, roughly speaking because of the way it frees an evolving lineage from the demands of immediate gradual improvement, allowing it to colonize unexplored, and potentially profitable, areas of design space. (For a skeptical assessment of Wright’s ideas see, among others, Coyne et al. 1997.)

To take another example, Richard Lewontin (1978) has argued that the production of complex adaptations will be favored if the developmental organization of individual organisms is “quasi-independent.” Suppose an organism’s developmental processes are so tightly enmeshed and integrated that mutations affecting, for example, the structure of the eye end up having further knock-on effects on the heart, the ears, the brain, the kidneys, and so forth. And suppose the same is true for all traits: a mutation that alters one ends up altering all the others. Lewontin’s idea is that under these circumstances, even when a mutation arises which improves the functioning of the eye, the chances are that its overall effects on the fitness of the organism will be negative, because it will most likely damage the functioning of many of those other systems the mutation affects. The end result is that iterated sequences of adaptive improvement will be vanishingly unlikely to arise. Hence Lewontin’s notion that developmental processes themselves need to be fairly isolated from each other if complex fitness-enhancing organs like eyes are to be built over time. On this view, natural selection is an important element of the explanation for the emergence of complex adaptations, but it is not the full explanation for how these structures come to be.

To summarize the results of this section, we can say that Darwin and more modern theorists disagree in their definition of natural selection. Darwin tends to favor a conception of selection that is explanatorily expansive, in that it encompasses many processes that contribute to adaptation. The price paid is that selection, as he understands it, is resistant to quantification and comparison with alternative evolutionary “forces.” Modern theorists make the opposite choices, defining natural selection in a way that is more narrowly focused on just one aspect of the evolutionary process, but more amenable to quantification because of that. Nonetheless, all agree on the more general and pragmatic point that if we want to understand the production of complex adaptations, we cannot focus solely on the processes that cause the fittest variants to dominate in a population.

Gradualism

By considering the explanatory task that Darwin intends natural selection to discharge, we can also understand why he describes natural selection in a way that makes it an essentially gradual process. Darwin tells us, for example, that, “As natural selecti...

Table of contents

• Cover
• Title
• Copyright
• Contents
• List of Figures
• List of Tables
• List of Contributors
• Preface
• PART I: THE NATURE OF SELECTION
• PART II: EVOLUTION AND INFORMATION
• PART III: HUMAN NATURE
• PART IV: EVOLUTION AND MIND
• PART V: EVOLUTION AND ETHICS
• PART VI: EVOLUTION, AESTHETICS, AND ART
• Index