Externally Regulated Programmed Aging and Effects of Population Stress on Mammal Lifespan.

Programmed (adaptive) aging refers to the idea that mammals, including humans and other complex organisms, have evolved mechanisms that purposely cause or allow senescence or otherwise internally limit their lifespans in order to obtain an evolutionary advantage. Until recently, programmed aging had been thought to be theoretically impossible because of the mechanics of the evolution process. However, there is now substantial theoretical and empirical support for the existence of programmed aging in mammals. Therefore, a comprehensive approach to medical research on aging and age-related diseases must consider programmed aging mechanisms and the detailed nature of such mechanisms is of major importance. Theories of externally regulated programmed aging suggest that in mammals and other complex organisms, genetically specified senescence mechanisms detect local or temporary external conditions that affect the optimal lifespan for a species population and can adjust the lifespans of individual members in response. This article describes why lifespan regulation in response to external conditions adds to the evolutionary advantage produced by programmed aging and why a specific externally regulated programmed aging mechanism provides the best match to empirical evidence on mammal senescence.