A consideration of some notable aging theories.

Aging is characterized as a breakdown process and the relevant events occur after reproduction, when the force of natural selection declines. Studies on the life histories of species reveal that there is an association between resource allocation and longevity and that the aging process is retarded when animals are protected from the deleterious consequences of excess metabolic activity. Although the extent to which aging is caused by environmental or genetic factors is unresolved, our understanding of the field has been enriched by the rapid development of the tools of molecular biology. In his pioneering work, Alex Comfort has postulated a hierarchical clock system as a descriptive paradigm of the aging process, and investigations at the molecular level are bringing to light evidence of a genetic link to life span that seems consistent with Comfort's model. It appears however, that the appropriate context for these mechanistic observations of functional decline is in the postreproductive period.