Differential survival, natural selection, and the manifestation of senescence

Evidence for natural selection is typically sought by searching for changes in organisms over generations (that is, evolution). However, evolution may represent but one biological manifestation of the force of natural selection. Although its evolutionary influence diminishes with aging, natural selection might also impact upon the manifestation of senescence via differential survival. Annual population estimates in the United States from 1951 to 1990 for age groups over 60 years old were analyzed. The rate of increase in the size of these age groups increased with increasing age. Since birth rates in developed countries have declined, this finding is a direct manifestation of differential survival over time. Correspondingly, increasing mortality rates from many disorders associated with senescence have been shown to be correlated with increasing age group population size. These observations suggest that natural selection, via differential survival, has had a demonstrable impact upon manifestation rates of the disorders of senescence.