DNA sequences among individuals of the same species may vary by as much as 1 percent, but since the late 1960s many scientists have explained away most of the differences as meaningless. The pervasive view that the vast majority of these genetic mutations have no functional nor evolutionary consequence is called the "neutral theory." In the first chapter of his new book, however, UC Davis genetics professor John H. Gillespie, lists a number of doubt-raising exceptions followed by a serious statistical assault on the neutral theory. Reinstilling genetic mutations with a sense of evolutionary purpose, Gillespie's book, "The Causes of Molecular Evolution" (Oxford University Press, 1992), has raised what he says is the first challenge in many years to the established neutral theory. His main argument assails the prediction that evolution proceeds at a constant rate. Looking at the data, Gillespie says, "we see that it goes in fits and starts." The mathematical modeling of the dualing theories is a complicated approach that makes many of the details hard to follow. The result, however, may ultimately affect the research strategy for determining human disease resistance as well as the medical strategy for genetic therapy. Drawing on 20 years of work, Gillespie will review alternatives to the neutral model of molecular evolution as part of the Molecular Genetics and Evolution symposia at the annual meeting of the American Association for the Advancement of Science in Chicago next week. Embargoed until 2:30 pm, Sunday, Feb. 9.