Abstract
Fitness in the sense of actual rate of increase of genotypes, commonly used in population genetics, is contrasted with fitness in the ordinary sense (and Darwin's) of adaptedness of organisms. Fitness as actual reproductive success is interpreted as a function of variables representing intrinsic adaptive capacities and environmental properties. Adaptive capacities causally contribute to fitness as actual reproductive success which in turn, as relative increase of genotypes, determines evolutionary change. The propensity interpretation of fitness is shown not to play a role in evolutionary explanation.