The Structure of Evolutionary Theory
When Stephen Jay Gould died in 2002, science lost one of its most colorful and provocative thinkers and the reading public lost an entertaining popular essayist and speaker. Gould was a paleontologist at Harvard University but was most widely known for vigorously championing Darwin’s theory of evolution by natural selection. His understanding of its history and mechanics was unsurpassed. The Structure of Evolutionary Theory is the product of his twenty-year project to collect his research and set forth his own contributions to the theory in a single, extended, integrated argument.
Gould wrote the book primarily for other scholars and scientists—evolutionary biologists, paleontologists, historians of science—who share his interest in evolution. It is a complex, technical book. Accordingly, Gould begins with a ninety-page introduction that lays out the plan of the book and comments upon his personal motives for writing it. Part 1, about one third of the text, opens the primary argument with a detailed history of the idea of evolution and a close analysis of On the Origin of Species (1859), in which Charles Darwin (1809- 1882) explained his radically new approach to it. Here the reader learns that Darwin is Gould’s intellectual hero, whom he repeatedly extols as “dogged,” “fiercely honest,” and “valiant”; Gould is devoted to Darwin’s scientific heritage, but he is not uncritical. Part 2 contains even more detailed discussion of modifications to Darwinism that Gould and others found necessary in order to incorporate modern discoveries into the theory. Whereas part 1 advances interpretations that some science historians may disagree with, part 2 introduces ideas that have been controversial since Gould and his colleagues began publishing them in the 1970’s. “Controversial” is an understatement, in fact; the disputes sometimes became so bitter and vituperative that they burst out of academia and became sensations in the public media (quite in addition to the long- standing battle between scientists and some Christians over evolution and creationism). For this reason, Gould defends the methodology of his approach to evolution as well as his alterations to it: most important, broadly interdisciplinary sources of evidence, the role of contingency as well as laws in the development of life, and the historical continuity of Darwin’s brand of evolution.
Darwin’s theory, according to Gould, rests upon three overarching principles. First, and the most radical break by Darwin from earlier evolutionists, is the agency of evolution: descent with modification. Evolution acts through the struggles of each individual organism in a species to thrive in its environment and, through some advantageous traits, to be more successful in reproducing and passing the favorable traits to the next generation. All higher-order taxonomic modifications—from species to families—emerge from the differential reproductive success among competing individuals, the essence of natural selection. Furthermore, no larger purpose or goal other than adaptation to the immediate environment directs the course of evolution.
Second is the efficacy of natural selection. Although undirected, it works as a positive mechanism for evolutionary novelty, not merely as a culling force. The accumulation of favorable traits through innumerable generations gradually improves a species. If a group of individuals within a species remains too much like its parent generation while other groups improve in adapting to the environment, then the old-fashioned group is out-competed and disappears. In this way a species slowly transforms into one or more new species.
The great scope of evolution is the third principle and, given the short-term focus of human experience, is difficult to appreciate. Although Darwin himself recognized that other mechanisms of change might supplement it, natural selection alone is sufficient to generate the entire diversity of life because it operates continuously through the vastness of geological time: billions of years. Darwin also recognized that records of ancient organisms, for the most part in the form of fossils, did not adequately support this gradualism, but he proposed that the fossil record as yet uncovered represents only a small fragment of the organisms that once lived. A complete record would reveal not only known species but also the intermediate forms between ancient and modern species.
In a studiously evenhanded critique, Gould recounts various challenges to Darwinism from other scientists—including...
(The entire section is 1875 words.)