Approaching Complexity: Thinking About Evolution

Evolution is a complex affair. The very diversity of definitions of biological evolution illustrates the variety of ways in which we think about the subject. Perhaps still the best general description of evolution is Darwin’s “descent with modification,” simply because it simultaneously brings to mind a pattern—a result—while hinting at an understanding of process, an underlying causative mechanism. Other definitions are less broad and thus fare less well as general descriptors of this thing we call evolution. I have characterized evolution as the proposition that all organisms are descended from a single common ancestor (e.g., Eldredge 1982c, based on a suggestion from N. I. Platnick in a personal communication). This is a serviceable (and testable) construct but one that emphasizes a systematist’s concern for pattern. The utterly different and far more popular notion that evolution is “change in gene frequencies within a population” similarly emphasizes process through focusing on a completely different sort of pattern. Yet neither definition is “wrong.” The “modern synthesis” is a body of thought that grapples with the complexities of evolution. As does any good theory, the synthesis attempts to characterize the overall phenomenon and explain it in the simplest terms that seem appropriate and effective. The bewildering array of evolutionary process theories that had accumulated by the 1920s, where each biological discipline seemed bent upon establishing the primacy of its own phenomena and its own insights into processes, amounted to a net disarray for evolutionary biology. As Simpson wrote in 1944 (p. xv): Not long ago paleontologists felt that a geneticist was a person who shut himself in a room, pulled down the shades, watched small flies disporting themselves in milk bottles, and thought that he was studying nature. A pursuit so removed from the realities of life, they said, had no significance for the true biologist. On the other hand, the geneticists said that paleontology had no further contributions to make to biology, that its only point had been the completed demonstration of the truth of evolution, and that it was a subject too purely descriptive to merit the name “science.”