THE PLEIOTROPY THEORY OF AGEING: CONCEPTUAL, METHODOLOGICAL AND EMPIRICAL ISSUES

This thesis discusses the pleiotropy theory of ageing in evolutionary biology in some of its most important aspects. In short, the theory says that natural selection fosters the early-life survival and reproductive performances of an organism even if this would necessarily impose a cost on later-life survivorship and reproduction thereby leading to the evolution of that peculiar pattern of age-correlated decline in biological functioning that characterizes ageing. The theory predicts that the level of external/environmental mortality should positively correlate with the rate of ageing in a population. And it is often invoked to suggest how the phenomenon of cellular senescence, which indicates the state of somatic cells unable to perform further division, may have an impact on ageing. Using the notions of the semantic view of theories it is shown that the pleiotropy theory of ageing is actually a family of related mathematical models. Among these, two main models are identified: the genetic variant, which is a population-genetic model; and the optimality variant, which represents an instance of optimality thinking in evolutionary biology. Relevant similarities and differences between the two models are discussed. It is then shown that the current ways of interpreting Gompertz’s mortality model - which is a simple empirical model to which mortality data can be fitted - in order to test the above- mentioned prediction of the theory does not make it a genuine truth-making structure for the theory thereby exposing the test of the prediction to potential unreliability. Independently from the semantic view of theories, it is then discussed how to provide new methodological foundations to the hypothesis that cellular senescence may be a case of the pleiotropy theory.