(From L. evolutionem: action and effect of evolving). The gradual and natural self-development of systems – social and organic – excluding abrupt or sudden transformations, especially artificial interventions, in the course of the natural process. E. comprises an accumulation of changes that proceed toward growing complexity through a process extending over a more or less prolonged period of time. In biological science the doctrine of e. attempts to explain natural phenomena as successive transformations of a single primary, material reality subjected to perpetual movement, by virtue of which it passes from simple and homogeneous to compound and heterogeneous. This presents serious theoretical problems, though, because certain important cosmologies (and their derived biological positions) have attempted to prove that from an initial state everything continues being gradually transformed until the energy and order are dissipated. In recent years, however, following the study of dissipative structures (due especially to the work of Ilya Prigogine), the concept of e. has been radically modified, altering not only the old conceptions but current ones as well still based on a simple entropic principle. In light of these conceptual changes, a fundamental revision is required, not only in the idea of e., but also, for example, in the field of the social sciences, in the idea of revolution, which implies a rupture or discontinuity in an evolutionary social process.