Differential survival and natural selection: their impact upon aging and cancer mortality

Evidence for natural selection is often presented from the perspective of evolution. However, evolution is not the purpose of natural selection. Natural selection might also impact upon the manifestation of senescence via differential survival. The relationship between age-specific cancer mortality rates and corresponding age group population size in the USA from 1951 to 1989 for age groups over age 45 was examined. Rising age-specific cancer mortality rates among elderly men were increasingly correlated with growing age group population size with increasing age. This relationship was not observed in women. Since an age-specific rate should be independent of age group population size, this finding suggests that successive cohorts of elderly USA men were not comparable with respect to their susceptibility to cancer mortality. Differential survival, which effects men more than women, and its selective culling effect upon the surviving gene pool in an aging population may account for some of the observed increasing cancer mortality rates among elderly men. If so, evidence of natural selection can also be presented from the perspective of manifestation rates for some disorders associated with senescence.