Optimality, mutation and the evolution of ageing.

Evolutionary explanations of ageing fall into two classes. Organisms might have evolved the optimal life history, in which survival and fertility late in life are sacrificed for the sake of early reproduction and survival. Alternatively, the life history might be depressed below this optimal compromise by deleterious mutations: because selection against late-acting mutations is weaker, these will impose a greater load on late life. Evidence for the importance of both is emerging, and unravelling their relative importance presents experimentalists with a major challenge.