The Logic of Chance: The Nature and Origin of Biological Evolution

species, but rather affects concrete groups of interbreeding individuals, termed populations. The size and structure of the evolving population largely determines the trajectory and outcome of evolution. In particular, Fisher formulated and proved the fundamental theorem of natural selection (commonly known as Fisher’s theorem), which states that the intensity of selection (and, hence, the rate of evolution due to selection) is proportional to the magnitude of the standing genetic variation in an evolving population, which, in turn, is proportional to the effective population size. Box 1-1 gives the basic definitions and equations that determine the effects of mutation and selection on the elimination or fixation of 8 the logic of chance mutant alleles, depending on the effective population size. The qualitative bottom line is that, given the same mutation rate, in a population with a large effective size, selection is intense. In this case, even mutations with a small positive selection coefficient (“slightly” beneficial mutations) quickly come to fixation. On the other hand, mutations with even a small negative selection coefficient (slightly deleterious mutations) are rapidly eliminated. This effect found its rigorous realization in Fisher’s theorem. Box 1-1: The fundamental relationships defining the roles of selection and drift in the evolution of populations Nearly neutral evolution dominated by drift