Aging as an evolutionary trade-off

Juan Moreno Klemming

Senescence is the deterioration of cellular functions with age in certain organisms and ends with death. Demographically it is defined as the increased age-dependent mortality due exclusively to causes intrinsic to the organism. Various processes are involved in this deterioration such as the accumulation of somatic mutations, increased oxidative damage in tissues, limitation of cell division capacity, telomere loss, limited energy expended during life, etc. The existence of these processes does not explain why the maintenance of the organism allows such deterioration. The existence of organisms that do not age shows that aging is not a simple product of physical or biological laws but a result of evolution. Only those animals age, which show early germline sequestration and whose somatic cells display a limit of their proliferative capacity. The alternative to the continued proliferation and elimination of cell lineages with errors is the permanent repair of DNA and proteins. Selection for this perfect maintenance is weak due to accumulated extrinsic mortality (accumulation of unrepaired mutations) that renders late reproduction highly improbable. There could even be selection against permanent repair if early reproduction is genetically associated with later deterioration (antagonistic pleiotropy). Selection may also be contrary to permanent repair if the investment in ongoing maintenance involves low reproduction due to unavoidable trade-offs in the allocation of limited resources (disposable soma). The simile of the "worn machine" based on the idea of a "tax for living" is false. Natural selection has favored senescence in organisms faced with multiple risks during life that limit their natural longevity and renders the soma perishable in the interest of the successful propagation of the germ line.