Reports of the Death of Extrinsic Mortality Moulding Senescence Have Been Greatly Exaggerated

Abstract

The classic evolutionary theory of aging posits that senescence evolves because the weakening of selection with age allows mutations with late-acting deleterious effects to accumulate. Because extrinsic mortality is an important cause of weakening selection, the central prediction of the theory has been that higher extrinsic mortality should lead to the evolution of a higher rate of senescence. However, the validity of this prediction has been questioned, even to the extent of suggesting that it is not a prediction of the theory at all, primarily on the basis that changes in population growth rate will compensate for changes in extrinsic mortality. The implication is that empiricists have been using the wrong prediction to test the theory. This claim is misleading, however, because it does not apply on an evolutionary timescale, when population size must be roughly constant. With a constant population size, Hamilton’s fitness sensitivities show that extrinsic mortality determines the rate at which the strength of selection declines with age, and thus determines the rate of senescence. The central prediction has been confirmed in the few controlled experiments with model organisms that have been conducted, but clearly this is an area ripe for further investigation.