Aging selected for its own sake

Ageing has a negative impact on individual fitness. From this, it has been inferred that ageing could not have arisen as an adaptation. Two alternative hypotheses were proposed more than 40 years ago: (1) that ageing has been selected as a side-effect of fertility maximization (‘antagonistic pleiotropy’) and (2) that ageing is a manifestation of mutational load (‘mutation accumulation’). There was good theoretical support for these hypotheses at the time. But in the intervening years, a body of experimental data has accumulated that is surprisingly distant from theoretical expectations. Indeed, some results may be interpreted as a direct refutation of each of the two theories. The evidence reviewed here is adduced in support of an adaptive theory, in which ageing has been selected for its own sake. This possibility has been dismissed historically because it requires strong group selection. In a companion paper, I intend to address this objection and describe a computational model in which ageing is affirmatively selected for its contribution to demographic homeostasis