The evolution of senescence through decelerating selection for system reliability

Senescence is a universal phenomenon in organisms, characterized by increasing mortality and decreasing fecundity with advancing chronological age. Most proximate agents of senescence, such as reactive oxygen species and UV radiation, are thought to operate by causing a gradual build-up of bodily damage. Yet most current evolutionary theories of senescence emphasize the deleterious effects of functioning genes in late life, leaving a gap between proximate and ultimate explanations. Here, we present an evolutionary model of senescence based on reliability theory, in which beneficial genes or gene products gradually get damaged and thereby fail, rather than actively cause harm. Specifically, the model allows organisms to evolve multiple redundant copies of a gene product (or gene) that performs a vital function, assuming that organisms can avoid condition-dependent death so long as at least one copy remains undamaged. We show that organisms with low levels of extrinsic mortality, and high levels of genetic damage, tend to evolve high levels of redundancy, and that mutation-selection balance results in a stable population distribution of the number of redundant elements. In contrast to previous evolutionary models of senescence, the mortality curves that emerge from such populations match empirical senescence patterns in three key respects: they exhibit: (1) an initially low, but rapidly increasing mortality rate at young ages, (2) a plateau in mortality at advanced ages and (3) 'mortality compensation', whereby the height of the mortality plateau is independent of the environmental conditions under which different populations evolved.