Age Differences in Evolutionary Selection Benefits

Abstract

From a biological perspective, the key to understanding the evolution of the human life history is the recognition that natural selection acts with different force on different ages. Natural selection acts through the differential survival and reproduction of genotypes. Genetic factors that exert their effects early in life have the potential to affect the whole of the organism’s reproduction. Conversely, genes that exert their effects late in life have much reduced impact. This is because in the wild environment most individual organisms die young, due to extrinsic sources of mortality such as predation and starvation. For this reason, most reproduction in wild populations is due to young individuals. Although humans now live much more protected lives, the conditions under which the genetic determinants of our aging processes evolved were not very different from those that exist today for other species. The reduction in the force of natural selection with advancing age tells us that the senescent phase of the life history cannot be under direct genetic control. In other words, aging itself is not programmed. Instead, the evolutionary theories of aging suggest that senescence takes place because (1) natural selection is powerless to prevent the accumulation within the genome of genes having late deleterious effects and because (2) long-term survival requires major ongoing investment in mechanisms of cellular maintenance and repair. The second point is the basis of the disposable soma theory of aging, which attributes senescence to a gradual accumulation of damage and faults in the cells and tissues of the organism. In the context of understanding human development, the evolutionary theories of aging reinforce the recognition of older people as individuals. Biologically, each individual is likely have a different set of late-acting deleterious genes and will accumulate a unique history of somatic damage at the cellular and molecular levels.