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 orga nisms. This observation is inconsistent with the equilibrium predictio ns of both the antagonistic pleiotropy and mutation accumulation model s of senescence, which, under a wide variety of assumptions, predict a ''wall'' of mortality rates near 100% at postreproductive ages. Previ ous models of age-specific mortality patterns are discussed in light o f recent demographic data concerning late-age mortality deceleration a nd age-specific properties of new mutations. The most recent theory (M ueller and Rose 1996) argues that existing evolutionary models can eas ily and robustly explain the demographic data. Here we discuss the sen sitivity of that analysis to different types of mutational effects, an d demonstrate that its conclusion is very sensitive to assumptions abo ut mutations. A legitimate resolution of evolutionary theory and demog raphic data will require experimental observations on the age-specific ity of mutational effects for new mutations and the degree to which mo rtality rates in adjacent ages are constrained to be similar (positive pleiotropy), as well as consideration of redundancy and heterogeneity models from demographic theory.