The nature and causes of ageing.

Ageing is a process where the end result is obvious but the mechanism remains obstinately obscure. The phenomenology of senescence is rich in the abundance of model systems that it offers for the experimental study of ageing. The field is also rich in the theories to account for ageing in terms of specific changes noted or postulated to occur as organisms grow older. Since neither models nor theories are scarce, the slowness of progress to date may therefore be due at least partly to inadequate cross-referencing between the two. Both in the choice of a model organism or cell system and in the selection of a specific mechanism to study, it is important to have in mind the nature and role of ageing at the organism level. Recent evolutionary insights into ageing suggest that senescence occurs because through natural selection a strategy is favoured in which organisms invest fewer resources in the maintenance and repair of somatic cells and tissues than are necessary for indefinite survival of the individual. This 'disposable soma' theory provides a broad predictive framework within which to assess the relevance of models with which to investigate specific mechanisms of ageing.