A concept in population genetics describing the difference in reproductive success of an individual or genotype relative to another. It is often treated as a mixture of survival, or longevity, and annual fecundity, and regarded as the degree of adaptation of an organism to its environment. The concept also includes benefits accrued to relatives of an individual in terms of an altruistic act toward them enhancing the fitness of the individual performing the act. From Darwin culminating in W.D. Hamilton (1936-2000), there is now a testable theory of inclusive fitness.
See Adaptation, Cultural evolution and biological evolution, Genotype and phenotype, Population genetics