The Evolution Revolution
By Mark Ridley
Published: March 17, 2002
THE STRUCTURE OF EVOLUTIONARY THEORY
By Stephen Jay Gould.
Illustrated. 1,433 pp. Cambridge, Mass.:
The Belknap Press/
Harvard University Press. $39.95.
DARWINISTS have divided opinions about the fossil record. Darwin himself, and many of his followers, were skeptical. For them, it is so fragmentary and corrupt that we cannot rely on what it seems to tell us. That view is most likely to be expressed today by a molecular biologist. For others, it is mainly trustworthy, and we should face up to what it tells us about evolution. Fossil evidence may even damage some of the deep foundations of Darwinism. The best-known champion of this view, for at least 30 years, is Stephen Jay Gould.
Gould's trust in the fossil record has been unambiguously good for science. He has inspired research and raised the status of paleontology. But his campaign to challenge the Darwinian theory of evolution has a more ambiguous status. He has certainly stimulated people to think about some big questions, and that is a plus. However, he has failed to persuade many of his colleagues, and I am among those who think that his attempts to revise Darwinism are flawed.
The centerpiece of Gould's system is the theory of punctuated equilibrium, published in 1972 by him and Niles Eldredge. In the history of life, new species often appear suddenly and then persist with little change until they go extinct. The sudden origin of species may reflect the incompleteness of the fossil record, but Gould suggests the pattern is real -- evolution is fast while new species originate, and then slows down. He may be right, and his vast new book, ''The Structure of Evolutionary Theory,'' includes a chapter on this matter that is as long as most books, and it has an extensive, if selective, review of the evidence.
The real controversy begins at the next stage, when we come to the wider implications of the theory. Gould argues that the alternating fast-slow pattern
of evolution implies that species are, in a philosophical sense, ''individuals'' rather than classes. This matters to Gould because, he argues, it means that natural selection can operate on whole species, in a process called species selection. He contrasts this with the orthodox Darwinian view, that natural selection works only on organisms. He wants to expand the theory of evolution to include the higher-level process of species selection, which drives large-scale evolutionary processes and is irreducible to natural selection on organisms.
What does it mean, to call a species an individual? In ordinary language, ''individual'' refers to an organism or a person, but Gould is using it in the sense of a particular entity rather than a class of entities. Classes can be defined by a set of defining conditions: gold is the class of atoms with atomic number 79; chairs are the class of objects shaped for one person to sit on. If an entity meets the defining conditions, it is a member of that class. Contrast this with an individual. John Smith may be recognizable by a set of attributes at one time. He may be the person at Gate 8, wearing gray trousers and carrying a blue bag. But if he changes his clothes, he'll still be John Smith. He is not defined by any observable attributes. The individual John Smith is a unique stream of cells, from conception to death.
Biological species are a test case for the distinction between individual and class. They look like classes. We can define human beings as apes with relatively big brains and relatively few hairs. However, a species can change gradually during evolution and still be the same species. Humans will be humans in the future even if they have become hairy or small-brained. In evolutionary theory, species do not have defining attributes and are not classes. They are particular individuals, forming a series of ancestor-descendant populations over time. A species exists between its origin and its extinction, in much the way a person exists between birth and death.
According to Gould, the theory of punctuated equilibrium implies that species are individuals, not classes. But I do not see the logical connection. Evolution in general, not punctuated evolution in particular, is the reason species do not form classes. If anything, the relative constancy of species after their sudden origin would make them more like a class. Individual people lack defining attributes because they change as they develop and decay. If people were born fully formed and remained identical until death, it would be easier to define them by attributes in much the way we do for chemical elements.
Then we have the theory of species selection. Some species have properties that enable them to last longer, making them less likely to go extinct. For example, species with sexual reproduction have lower extinction rates than species with asexual (or clonal) reproduction. So, over time they proliferate more than clonal species because they don't die off as fast. This species selection is analogous to natural selection between organisms. In normal natural selection, some organisms have attributes -- camouflage, for example -- that enable them to proliferate relative to other (uncamouflaged) organisms. In species selection, whole species proliferate if they have attributes that lower the extinction rate.