Aging: the fitness-stress continuum and genetic variability.

Assuming the stress theory of aging, longevity depends upon primary selection for stress resistance and metabolic efficiency. Predominantly based upon experimental studies in the insect Drosophila melanogaster, high genetic variability for fitness, especially mortality, occurs under extreme stress. Isofemale strains derived from the progeny of recently collected single inseminated Drosophila females from the wild should provide useful biological material for extrapolating to quantitative genetic studies in man. Furthermore, environments from the benign (hormetic) to the extreme can be incorporated. Survival to old age may depend upon genes for metabolic efficiency that respond to the environmental challenges of living as limits to adaptation are approached. Under this scenario the survival of longevity mutants in man to ages analogous to the extreme life spans found in some experimental organisms under benign or protected laboratory conditions is unlikely. More future emphasis is needed on genetic variation of longevity in natural populations of experimental organisms under an array of realistically stressful environments to act as an evolutionary model for longevity in our own species.