Über die Autorin bzw. den Autor

Elliott Sober is Hans Reichenbach Professor and William F. Vilas Research Professor in the Philosophy Department at the University of Wisconsin, Madison. He is the author of nine other books, including Evidence and Evolution: The Logic Behind the Science and Reconstructing the Past: Parsimony, Evolution, and Inference, which won the prestigious Lakatos Prize in the Philosophy of Science.

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Did Darwin write the Origin Backwards?

Prometheus Books

Contents

PREFACE.................................................................9ACKNOWLEDGMENTS.........................................................132. DARWIN AND GROUP SELECTION...........................................453. SEX RATIO THEORY—DARWIN, BEFORE, AND AFTER.....................874. DARWIN AND NATURALISM................................................1215. POSTSCRIPT...........................................................153NOTES...................................................................189REFERENCES..............................................................211INDEX...................................................................225

Chapter One

Natural selection and common ancestry are the two central concepts in Darwin's theory. I begin this chapter by clarifying some basic features of how Darwin understood each of them. I then describe how common ancestry and natural selection are related in his theory. They are, of course, logically distinct, in the sense that neither entails the other (Mayr 1985; Sober and Orzack 2003). Imagine an array of species linked to each other via common ancestry; it does not follow that they evolve via natural selection. And now consider the converse. Imagine an array of species that evolves by natural selection; it does not follow that they have common ancestors. This logical point, I hope, is obvious, but it belies a fundamental fact about Darwin's theory—common ancestry and natural selection are entangled. To understand the theory, one must understand how these two parts of the theory are related to a third—the observational evidence that Darwin considers. How is common ancestry relevant to interpreting evidence concerning natural selection? How is natural selection relevant to interpreting evidence concerning common ancestry? Common ancestry and natural selection are logically separate, but they are evidentially connected. The details of how Darwin understood these relationships will frame the answer I will give to the puzzling question that provides the title of this chapter.

1.1. WHAT IS DARWIN'S THEORY?

To characterize Darwin's theory, what could be more natural than to cite the title that Darwin gave to his own book? How could we go wrong by saying that his is the theory of the origin of species by means of natural selection? In fact, there is trouble here, and it is of Darwin's own making. Although Darwin (1859, 1) describes the origin of species as the "mystery of mysteries" that he proposes to solve, his solution of the problem is in some ways a dissolution. I say this because Darwin had doubts about the species category; he regarded the difference between species and varieties as arbitrary. When two populations split from a common ancestor and diverge from each other under the influence of different selection pressures, they begin as two populations from the same variety, then they become two varieties of the same species, and finally they reach the point where they count as different species. It is convenience, not fact, that leads us to classify different degrees of divergence in different ways (Darwin 1859, 48–52).

The vagueness of the boundary between variety and species is no reason to deny the existence of individual species, nor did Darwin do so (Sloan 2009; Ereshefsky 2009). This is the lesson we learn from other vague concepts—from rich and poor, hairy and bald, tall and short; a vague boundary does not mean that no one is rich, or hairy, or tall. Even so, species is not the central concept in Darwin's theory. True, the process he describes produces species, but it produces traits and taxa at all levels of organization. For these reasons, Darwin's theory is better described as "the origin of diversity by means of natural selection."

Darwin's concept of natural selection has several noteworthy features. The first is that natural selection does not involve conscious choosing. When cold climate causes polar bears to evolve longer fur, the weather kills some bears while allowing others to survive, but the weather does not have a mind. In the Origin, Darwin introduces the idea of natural selection by first describing artificial selection, by which he means the activities of plant and animal breeders who consciously choose the organisms from one generation who will be the parents of the next. Natural selection is just like this, save for the fact that artificial selection involves agents with minds to do the work while natural selection does not. Darwin was so concerned to emphasize this point that he followed Alfred Russel Wallace's advice in the fifth edition of the Origin and used Herbert Spencer's phrase "the survival of the fittest" to characterize his theory (Darwin 1959, 164). Darwin hoped this new label would make it harder for readers to misunderstand his theory. Another important fact about Darwin's concept is that the direction in which selection causes populations to evolve depends on accidents of the environment. There is no inherent tendency for life to grow bigger or faster or harder or slimier or smarter. Everything depends on which traits do a better job of allowing organisms to survive and reproduce in their environments. This is a vital contrast that separates Darwin from Lamarck, who saw evolution as leading lineages to move through a preprogrammed sequence of steps, from simple to complex. Of course, if life starts simple, evolution by natural selection will lead the average complexity of the biota to increase. But that is not because the "laws of motion" of natural selection inherently favor complexity. Parasites evolve from free-living ancestors, and the effect is often a move toward greater simplicity, with parasites losing organs and abilities possessed by their ancestors (Darwin 1859, 148). Complexity increases from life's beginning because of the initial conditions, not the laws. This is analogous to the random walk depicted in figure 1.1. A marker on a line changes position because of a coin toss. If the coin lands heads, you move the marker one space to the right; if the coin lands tails, you move the marker one space to the left. These are the rules of change unless the marker happens to be at the left-most or the right-most points. If the coin lands tails when the marker is at the extreme left, you simply toss again. Suppose the game begins with the marker placed at the left-most point on the line. Where will the marker be after five or fifty or five hundred coin tosses? Probably not at square one. Even if selection is indifferent to simplicity versus complexity, evolution by natural selection can still be expected to manifest a net increase in complexity (Sober 1994).

A third characteristic of Darwin's concept is that selection acts on "random" variation. This is a loaded word, apt to mislead. Darwin says in the Origin (Darwin 1859, 131) that "random" just means that the cause of a new variant's appearance in a population is unknown. But "random" for Darwin was more than a confession of ignorance. He meant that variations do not occur because they would be useful to the organisms in which they occur. In his book Variation of Plants and Animals Under Domestication, Darwin explains...