Mortality plateaus and the evolution of senescence: Why are old-age mortality rates so low?

Age-specific mortality rates level off far below 100% at advanced ages in experimental populations of Drosophila melanogaster and other organisms. This observation is inconsistent with the equilibrium predictions of both the antagonistic pleiotropy and mutation accumulation models of senescence, which, under a wide variety of assumptions, predict a 'wall' of mortality rates near 100% at postreproductive ages. Previous models of age-specific mortality patterns are discussed in light of recent demographic data concerning late-age mortality deceleration and age-specific properties of new mutations. The most recent theory (Mueller and Rose 1996) argues that existing evolutionary models can easily and robustly explain the demographic data. Here we discuss the sensitivity of that analysis to different types of mutational effects, and demonstrate that its conclusion is very sensitive to assumptions about mutations. A legitimate resolution of evolutionary theory and demographic data will require experimental observations on the age-specificity of mutational effects for new mutations and the degree to which mortality rates in adjacent ages are constrained to be similar (positive pleiotropy), as well as consideration of redundancy and heterogeneity models from demographic theory.