The Evolution of Darwinism: Selection, Adaptation and Progress in Evolutionary Biology. Timothy Shanahan. Cambridge University Press, New York, 2004. 342 pp. $80.00 (ISBN 0521834139 cloth).
Like many other scientists, I feel nervous when philosophers venture near my chosen field. I have nightmare visions of a formerly clear field of enquiry suddenly swamped in obfuscation, or drained of meaning by endless rounds of hairsplitting. So I approached this book, by a professor of philosophy at Loyola Marymount University, with some trepidation.
To my great pleasure, my fears proved to be groundless (in this case, at least). In The Evolution of Darwinism: Selection, Adaptation and Progress in Evolutionary Biology, Timothy Shanahan has written a clear examination of the development of a number of central ideas in evolution. He understands the science behind these ideas, and how the ideas have evolved as our scientific knowledge has grown. The result is a well-organized book that scientists can read and benefit from.
Shanahan deals with three main ideas: selection, adaptation, and progress. In each part of the book, a careful reading of Darwin and his contemporaries is juxtaposed with careful analysis of present-day evolutionists such as Richard Dawkins and Stephen Jay Gould. All the discussions are set in the framework of the overarching neo-Darwinian synthesis that was one of the great intellectual triumphs of the 20th century.
Darwin understood the full implications of his idea of natural selection, far more than did the codiscoverer of the idea, Alfred Russel Wallace. Wallace, in his original paper that forced Darwin to publish the Origin of Species, thought in terms of selection at the level of species. In Wallace's formulation, entire species would survive or go extinct as the environment changed, and as time went on the surviving species would diverge from each other. Darwin realized that natural selection acting on individual organisms can give rise to a far richer and more complex world, and Shanahan traces the implications of Darwin's insight.
Darwin pointed out that selection acting on individuals can bring about gradually increased adaptation when the environment is unchanging. It can also bring about physical and behavioral alterations when the environment is changing. Individual natural selection is happening all the time, regardless of whether or not the environment alters. Darwin also realized that sexual selection acts on individuals and can bring about remarkable changes.
Shanahan, like most others, distinguishes sexual selection from other types of natural selection. This distinction is commonly made because sexual selection for the ability to mate can act in opposition to other types of selection for abilities that allow survival. But this is a false dichotomy, because other types of natural selection can also act in opposition to each other. In the evolution of a predator, for example, power and speed, when taken to extremes, quickly become incompatible with each other. Thus, the distinctive difference between sexual and natural selection disappears, leaving us to conclude that sexual selection is a type of natural selection.
Shanahan explores in detail the debate about individual and group selection. He provides a valuable summary of the work of V. C. Wynne-Edwards, who proposed that there must be some signal associated with crowding that leads crowded organisms to adjust their reproductive rates downward. The powerful counterargument to Wynne-Edwards's view, based on individual selection, is that a cheater who is unaffected by the signal will outreproduce other members of the group. Shanahan also examines the various types of selection for altruistic behavior, including mechanisms such as kin selection, which Darwin was the first to explore. He concludes, in agreement with current evolutionary thought, that while group selection may be important in instances such as the selection of virus populations in individual hosts, Darwin's insight that individual selection is overwhelmingly important still holds.
The second section of the book, on the changing meaning of adaptation, ventures further into the realm of the philosophical. Although Darwin fully realized that no organism can be perfectly adapted to its environment, he and his contemporaries were guilty of using the term “perfection” more often and more loosely than they should have done. It is clear, as Darwin gradually began to conclude through succeeding editions of Origin, that adaptation is not the only source of evolutionary change. Shanahan examines the tendency of evolutionists to construct just-so stories about adaptations, and the difficulties that have resulted, though he does not make the point (an important one, I think) that the proper way for science to proceed is to suggest a just-so story as a hypothesis and then to test it. The ability to test such stories, through clever field and laboratory experiments, is one of the ongoing triumphs of the neo-Darwinian synthesis.
Shanahan performs a valuable service by tracing the history of the question of what constitutes the unit of selection—the gene, the organism, or the population. He summarizes the arguments of many biologists that this, too, is an artificial division. If a chicken is an egg's way of making another egg, in Samuel Butler's memorable phrase, it is equally true that an egg is a chicken's way of making another chicken. Evolution cannot take place on genes in the absence of organisms, or vice versa, and of course changes in populations are the ultimate result of evolution. Shanahan summarizes: “Because biological entities are causally connected in complex ways, perhaps the only truly accurate account of natural selection includes biological entities and their causal interrelations at a number of different functional levels, and treats entire biological systems as subject to selective forces.”
The last part of The Evolution of Darwinism deals with evolutionary progress. Just as Darwin tended to use “perfection” in a poetical rather than a scientific way, his writings are full of the use of the term “progress.” But Shanahan, following in the footsteps of others such as Michael Ruse, shows clearly that Darwin was conflicted. He knew that simple organisms have changed very little since the beginning of life, so that any evolutionary tendency toward greater complexity has not affected them. And yet organisms such as humans are so complex, with so many new and emergent properties, that surely there must be some tendency toward the selection of such complexity under some circumstances.
The book's last section is an excellent summary of the conflicted thinking of many evolutionists about this question. But it does not quite come to a resolution. One can perhaps reach such a resolution by abandoning the term “progress” entirely. In the course of evolution, organisms simply adhere to the philosophy of Tammany boss George Washington Plunkitt, who memorably said, “I seen my opportunities, and I took 'em.” The opportunities available to complex organisms have increased during the history of life—it is unimaginable that humans could have appeared, or survived, on Precambrian Earth.
This is a thoughtful and clearly written book that serves as a fine introduction to the ways in which evolutionary thought has itself evolved since the time of Darwin. I learned a lot from it, and I feel confident that anybody who is fascinated by these centrally important ideas will also take something useful away from it.
