  The notion that there is a very large family of principles of natural selection, each with a restricted range of application, will be attractive to those biologists uncomfortable with a single principle or law of natural selection, and to those philosophers of science who treat the theory of natural selection as a class of models. On the “semantic approach” to the theory of natural selection, each of the substitution instances of the schematic principle of natural selection generated by a particular specification of the propensity definition of fitness is treated as a definition of a different Darwinian system of population change over time. See Lloyd 1994, Beatty 1981, and Thompson 1989 for exposition of this position. The evolutionary biologist’s task is to identify which definition is instantiated by a population in an environment.