Demographic constraints in evolution: Towards unifying the evolutionary theories of senescence and niche conservatism

I suggest that there may be a fundamental conceptual unity between two seemingly disparate phenomena: (1) senescence (the progressive deterioration in physiological function and, thus, individual fitness with age) and (2) niche conservatism (the observation that species often seem rather fixed over evolutionary time in their basic niche properties). I argue that both phenomena arise from demographic asymmetries. The evolutionary theory of ageing rests on the observation that the force of selection declines with age, reflecting the basic demographic facts that in persistent populations there are always fewer individuals in old than in young age classes and these individuals tend to have lower reproductive value. A similar demographic asymmetry arises when populations inhabit environments with source habitats (i.e. where conditions are within the species' niche) and sink habitats (where conditions lie outside the niche): there tend to be more individuals in sources than in sinks and individuals in sources have relatively higher reproductive values. These demographic asymmetries should often imply that the force of selection is greater in sources than in sinks, leading automatically towards niche conservatism. I suggest that niche evolution is most Likely in circumstances where these demographic asymmetries in space weaken or reverse.