Published online by Cambridge University Press: 06 August 2009
Humankind's existence depends on evolutionary balances in the pace of conception, gestation, growth, development, maturation, and reproductive effort. The confluence of biology, environment, culture, and contingency that sculpted human life history did not fashion senescence. Instead, these interactions set minimum limits on the life span required to progress from conception to the fledging of human offspring (the minimum necessary life span (MNLS)). Somatic systems, from cells to organs, are set to the pace at which the necessary processes of life unfold (Weismann 1889; Finch and Rose 1995; Austad 1997). Successful alleles and genomes have had to predispose for phenotypes (somas) capable of outlasting their MNLS. The existence of a MNLS sets the stage for the cumulative, progressive, irreversible, and degenerative changes that we label senescence in somas designed to complete reproductive effort within a limited span.
Species' survival and reproductive strategies are mixed together in a complex web of evolutionary tradeoffs. Multiple synergistic, counterbalancing, and random alterations in molecular organization and physiological function have occurred over evolutionary time. Natural selection has molded these such that the average individual achieves about average relative fitness in competition with its conspecifics. Natural selection did not, however, directly produce senescence. Senescence results because natural selection lacks the ability to affect changes in allele frequencies once the period of reproductive effort is complete, declining in strength as the maximum reproductive potential (MRP) of organisms falls. Natural selection is based on differential survival and fitness.