Wonderful Life: The Burgess Shale and the Nature of History
Bully for Brontosaurus: Reflections in Natural History
Toward a New Philosophy of Biology: Observations of an Evolutionist
Although very different in style and content, the last two books by Stephen Jay Gould—Wonderful Life and Bully for Brontosaurus—and Ernst Mayr’s Toward a New Philosophy of Biology are ultimately about the same questions. Is evolutionary biology a science? If so, what kind of a science is it? Mayr’s book is a collection of essays, published over the past thirty years, and addressed both to biologists and philosophers. His aim is to clarify the concepts that underlie evolutionary biology. His central theme is that these concepts make evolutionary biology an autonomous science, and not merely a subbranch of physics. This claim must not be misunderstood. Like all serious biologists, he believes in the unity of science: in particular, he believes that the laws of physics and chemistry are the same in living and inanimate matter. The claim for autonomy rests on the existence of concepts—for example, natural selection, genetic program, species—that are needed if we are to understand biology. These concepts are consistent with physical laws, but could not be deduced from them.
In distinguishing between physics and biology, he points to the different role of laws in the two sciences. In physics, laws are intended to be universal. Popper’s falsifiability criterion depends on this: a single case of levitation would be sufficient to falsify Newton’s law of universal gravitation. Such laws do exist in some branches of biology. For example, the “central dogma of molecular biology” that information can pass from nucleic acid to nucleic acid, and from nucleic acid to protein, but not from protein to nucleic acid, is intended to be such a law, universal as far as life on earth is concerned. As yet, there is no convincing falsifying evidence. The law is important for evolutionary biology, because it provides one explanation for the noninheritance of acquired characters. In evolution, such laws are hard to come by. Even the “law” that acquired characters are not inherited has exceptions, because not all heredity depends on the sequence of bases in nucleic acids. We do, however, have theories about evolutionary processes, although, as Mayr points out, there is hardly a theory in biology for which some exceptions are not known.
An example will help to make clear the nature of such theories about evolution. It is appropriate to choose a theory for which Mayr himself is largely responsible, that of “allopatric speciation.” It holds that when a species divides into two reproductively isolated species, a period is required during which the two populations are spatially separated. The study of the geographical distribution of animals and plants lends empirical support for this view, and there are theoretical reasons, from population genetics, why it should be true. But, as Mayr was well aware, it is not a universal truth. There is at least one process—the formation of a hybrid between two species, followed by a doubling of the number of chromosomes—whereby new species have arisen without spatial isolation. The existence …