Senescence in Animals: Why Evolutionary Theories Matter

This is a clearly deleterious process, which seems difficult to conciliate with natural selection, which predicts evolution towards increasing fitness. Historically, the first evolutionary explanation able to conciliate these two processes is known as the mutation accumulation theory (Medawar, 1952). According to this theory, in age-structured populations the force of selection decreases with increasing age, allowing the accumulation of deleterious genes with age-specific effects on mortality rate (Hamilton, 1966). Under population genetics mechanisms, senescence is not necessarily deleterious: the original Medawar’s proposition implies that the postponement of age-specific effects of harmful genes is equivalent to their elimination in such a way that they become effectively neutral. Hence, such postponement is beneficial and senescence can be regarded as a side effect of the process.