The primary determinant of survival during aging is the energetic efficiency and metabolic stability required to counter the accumulated internal and external stresses of a lifetime. Hence, genetically stress-resistant individuals should accumulate with age; frailer, less robust, less energetically efficient and less metabolically stable individuals should succumb in parallel. This selection process implies the accumulation of energetically efficient stress-resistant individuals with age to the exclusion of all others. High additive genetic variability for survival is expected under extreme circumstances, however there is limited evidence close to the absolute extremes of life that diversity may fall. At this stage, only a few highly adaptive, oxidative-stress-resistant and presumably somewhat homozygous genotypes should remain. Therefore a fall in variability may occur in these outliers, when frailer individuals are unable to cope and are eliminated at extreme ages. This process could provide an explanation of mortality-rate declines in domesticated (laboratory) and free-living populations of the extremely old. That is, mortality-rate declines may be an expectation from a process of genetic sorting resulting from the accumulated responses to environmental stress over time. Application of an ecological stress theory of aging, which combines the external stresses to which organisms are exposed with internal stresses, appears to be the prerequisite for this conclusion.