Author Professor Sober is the 2008 winner of the Prometheus Prize. This biennial award, established in 2006 through the American Philosophical Association, is designed "to honor a distinguished philosopher in recognition of his or her lifetime contribution to expanding the frontiers of research in philosophy and science." This insightful collection of essays will be of interest to philosophers, biologists, and laypersons seeking a deeper understanding of one of the most influential scientific theories ever propounded.
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Did Darwin write the Origin Backwards?Philosophical Essays on Darwin's Theory
By ELLIOTT SOBER
Prometheus BooksCopyright © 2011 Elliott Sober
All right reserved.
Chapter OneDID DARWIN WRITE THE ORIGIN BACKWARDS?
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 his point in terms of a beautiful analogy:
Let an architect be compelled to build an edifice with uncut stones, fallen from a precipice. The shape of each fragment may be called accidental; yet the shape of each has been determined by the force of gravity, the nature of the rock, and the slope of the precipice,—events and circumstances all of which depend on natural laws; but there is no relation between these laws and the purpose for which each fragment is used by the builder. In the same manner the variations of each creature are determined by fixed and immutable laws; but these bear no relation to the living structure which is slowly built up through the power of natural selection, whether this be natural or artificial selection. (Darwin 1868, 236)
The idea of "random" mutation will be further explained in §4.3
A fourth important feature concerns the "level" at which Darwin took natural selection to act. In almost all the examples that Darwin discusses, traits are said to be selected because they help the individual organisms that possess them to survive and reproduce. Tigers have sharp teeth because tigers with sharp teeth do better than tigers with dull teeth. The reason the trait evolved is not that sharp teeth help the species avoid extinction or somehow keep the ecosystem in balance. In examples of this sort, Darwin invokes what biologists now call individual selection. However, there are a few exceptions in which Darwin says that traits evolve because they are good for the group though they are bad for the individuals that have them. Darwin uses the hypothesis of group selection to explain traits that biologists now call altruistic. His examples include the barbed stinger of the honeybee (which eviscerates the bee when she pulls away after stinging) and the Golden Rule embraced by human morality. These and other examples will be examined in chapter 2.
The fifth feature of Darwin's thinking about natural selection that I want to mention is his gradualism. A novel feature first arises in an organism by mutation, and then the novel mutation gradually increases in frequency. The trait starts rare, and then, if things go well, it becomes common. If selection causes trait T to evolve, the first question to ask is: what were the other organisms like in the population in which T initially appeared? When Darwin considers the evolution of a novel feature, he invariably thinks of the novelty as first arising in a population of other organisms that are very similar to the novel mutant. Eye evolution began with a piece of light-sensitive tissue that was able to distinguish light from dark; the other organisms in the population had no eyes at all. Only much later did more complex eyes arise in populations in which the other organisms had more rudimentary eyes (Darwin 1859, 187). Darwin thought of selection as a stepwise process in which the steps are numerous and small. One impetus that drove Darwin to biological gradualism was the geology of Charles Lyell, who theorized that great geological changes were the result of many small changes adding up. Another inspiration came from the practices of plant and animal breeders. When breeders seek to create a new trait in a population, they do not wait for the trait they seek to suddenly appear as a finished package. Rather, they build the new trait one small step at a time. They set to work on the range of variation they observe, gradually modifying the population by deciding which organisms will be the parents of the next generation. Breeders need to run their breeding programs over many generations to achieve their goals. Natural selection must do the same thing, though of course there is no "goal" that natural selection has in mind, since it has no mind.
The last facet of Darwin's concept of natural selection that I want to mention concerns his comment in the Origin (Darwin 1859, 6) that selection is "the main but not the exclusive cause" of evolution. One part of this pronouncement is clearer than the other. Darwin (1859, 134–39) allows for evolution to have other causes. For example, he endorses the Lamarckian mechanism of use and disuse, the inheritance by offspring of traits (phenotypes, in modern parlance) because they were acquired by their parents. A standard example is the blacksmith's growing big muscles because of his work and then fathering children who develop those muscles without needing to do what their father did to get them. Darwin also had the idea that descendants retain the traits that their ancestors had, sometimes in spite of the fact that these traits are no longer favored by selection. This is the idea of ancestral influence (aka phylogenetic inertia); it explains many rudimentary features (Darwin 1859, 199, 416, 450–56) as vestiges of a bygone age. For example, this is why human beings have tail bones and why human fetuses have gill slits (Darwin 1859, 191). Darwin also discusses correlation of characters as a cause of evolution. If a trait favored by selection is correlated with a trait that is neutral or even deleterious, the latter may evolve by piggybacking on the former (Darwin 1859, 143–47). To use a modern example, our blood is red, not because the color promotes survival and reproduction, but because hemoglobin is red, and hemoglobin was selected for its ability to transport oxygen. Darwin discusses other nonselective causes of evolution, but the point is clear—he denied that selection is the only cause of evolution.
Unfortunately, Darwin does not explain what he means by saying that selection is the "main cause." This might mean that selection is the most frequent cause of trait evolution—that selection is implicated in the evolution of more traits in more populations than any other cause. Or it could mean that selection is more powerful than the other causes that affect the evolution of a given trait (where the trait in question might be one of the big changes that took place in life's history). To make sense of this latter idea, we would have to consider the other causes that influence a trait's evolution in a population and then imagine how the outcome would have been different if selection had been absent and those other causes present, and how the outcome would have been different if selection had been present and one of the other causes absent (and do this for each of the other causes). Important causes are big difference makers; causes of modest importance make only a small difference in the outcome. Applying this format for separating more important causes from less to Darwin's theory (and to the evolutionary theory of the present) is an interesting exercise, but it cannot be pursued here.
Given his statement that selection is the main cause of evolution, how are we to interpret the following comment in the Origin concerning the importance of ancestral influence: "The chief part of the organization of every being is simply due to inheritance; and consequently, though each being assuredly is well fitted for its place in nature, many structures now have no direct relation to the habits of life of each species" (Darwin 1859, 199)? If ancestral influence has played so large a role, how can natural selection have been the main cause? Maybe the answer is that inheritance explains why descendants retain the features of their ancestors whereas selection is the main cause of change. Even so, it is worth considering whether Darwin should simply have said that selection has been a very important cause of evolution. Was it needlessly audacious to put selection at the top of a list whose members he had no reason to think he could completely enumerate?
1.2. COMMON ANCESTRY
With these caveats about natural selection duly noted, is "evolution by natural selection" a good characterization of Darwin's theory? The answer is emphatically no, as can be seen by considering figure 1.2. Darwin's theory gives the concept of common ancestry a central place. The phrase "evolution by natural selection" does not capture this idea, nor does "descent with modification." Instead of describing Darwin's theory as evolution by natural selection, the theory is better described as common ancestry plus natural selection.
How much common ancestry did Darwin embrace? In the last paragraph of the Origin, where Darwin waxes poetic in his description of the "grandeur in this view of life" (Darwin 1859, 490), he says that, in the beginning, "life was breathed into a few forms, or into one." A few pages earlier, he is less cautious:
I believe that animals have descended from at most only four or five progenitors, and plants from an equal or lesser number. Analogy would lead me one step further, namely to the belief that all animals and plants have descended from some one prototype. But analogy may be a deceitful guide. Nevertheless all living things have much in common, in their chemical composition, their germinal vesicles, their cellular structure, and their laws of growth and reproduction. We see this even in so trifling a circumstance as that the same poison often similarly affects plants and animals; or that the poison secreted by the gall-fly produces monstrous growths on the wild rose or oak-tree. Therefore I should infer from analogy that probably all organic beings which have ever lived on this earth have descended from some one primordial form, into which life was first breathed. (Darwin 1859, 484)
Although these passages may suggest that Darwin's view was that there was one start-up of life from nonliving materials, or just a few of them, this is not what his theory really says. In the fifth edition of the Origin, he adds the following remark:
No doubt it is possible, as Mr. G. H. Lewes has urged, that at the first commencement of life many different forms were evolved; but if so, we may conclude that only a few have left modified descendants. (Darwin 1959, 753)
Excerpted from Did Darwin write the Origin Backwards? by ELLIOTT SOBER Copyright © 2011 by Elliott Sober. Excerpted by permission of Prometheus Books. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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Table of Contents
|1. DID DARWIN WRITE THE ORIGIN BACKWARDS?||15|
|1.1. What Is Darwin's Theory?||15|
|1.2. Common Ancestry||21|
|1.3. Darwin's Principle||25|
|1.4. Exceptions to Darwin's Principle||28|
|1.5. Causal and Evidential Orderings||32|
|1.6. Using Common Ancestry to Think about Natural Selection||36|
|1.7. Tree Thinking||42|
|2. DARWIN AND GROUP SELECTION||45|
|2.1. Back to the 60s||46|
|2.2. Human Morality||57|
|2.3. The Honeybee's Barbed Stinger||62|
|2.4. The Risk of Anachronism||63|
|2.5. More on Darwin on Morality||66|
|2.6. Sterile Workers in the Social Insects||69|
|2.7. Darwin's Disagreement with Wallace about Hybrid Sterility||76|
|2.8. Darwin's General View of Group Selection||82|
|3. SEX RATIO THEORYDARWIN, BEFORE, AND AFTER||87|
|3.1. Arbuthnot on 'the exact balance that is maintained between the numbers of men and women . . . that the Species may never fail, nor perish'||88|
|3.2. Bernoulli on 18/35||96|
|3.3. DeMoivre-'if we blind not ourselves with metaphysical dust'||98|
|3.4. Darwin's Argument from Monogamy, and His Retraction||101|
|3.5. Düsing's Model-Monogamy Drops Out||106|
|3.6. Fisher and Parental Expenditure||109|
|3.7. Hamilton-Group and Individual Selection||113|
|3.8. Sex Ratio as a Test Case||115|
|Appendix: An Example of Hamiltonian Sex Ratio Evolution in Group with Two Foundresses||119|
|4. DARWIN AND NATURALISM||121|
|4.1. Darwin's Discussions of God||121|
|4.2. Refining Methodological Naturalism||128|
|4.3. Why Evolutionary Theory Does Not Rule Out an Intervening God||134|
|4.4. Should Scientific Theories Talk Only about What Exists in Nature?||139|
|4.5. Are All Claims about the Supernatural Untestable?||142|
|4.6. Is Violating Methodological Naturalism a Science-Stopper?||146|
|4.7. If Numbers, Why Not God?||147|
|4.8. Concluding Comments||151|
|5.1. Second Thoughts about Cladistic Parsimony and the Test of Adaptive Hypotheses||153|
|5.2. More on Units of Selection||162|
|5.3. Evolutionary Theory and the Reality of Macroprobabilities||177|