Equilibrium Conditions in the Evolution of Senescence

Senescence evolves because natural selection is less sensitive to late life than to early life. Hamilton formalized this as the sensitivities of fitness to small additive changes to age-specific mortality or fecundity; his framework has since been extended to alternative ecological and genetic assumptions. However, such forces of selection only explicitly model evolution in the short term; in the long term, as life histories evolve, their forces of selection evolve too. This thesis investigates long-term evolution of senescence by deriving conditions a population must satisfy in order to be in evolutionary equilibrium. It considers two models: a mutation-selection balance model; and an optimality model with a same-age log mortality cost of fecundity. The derived conditions are discussed, and heuristically compared to two species: Soay sheep, which senesce, and desert tortoises, which don't. Other, intermediate theoretical results are also given, including the force of selection on proportional hazards in stationary populations.