Heterogeneity in Individual Mortality Risk and Its Importance for Evolutionary Studies of Senescence.

Mortality was simulated under the assumption of heterogeneity in individual age-specific mortality risk. Heterogeneity was modeled by assigning each individual its own Gompertz mortality function. Means and variances of the Gompertz intercept and slope parameters were based on published data for Drosophila melanogaster. Simulations of large cohorts reproduced mortality plateaus similar to those observed for actual cohorts of flies. Catastrophic late-age mortality was not observed except when heterogeneity was very low and rates of senescence were very high. A second set of simulations was designed to mimic experiments that have investigated age-specific patterns of genetic variance in mortality rates. Within-genotype heterogeneity in mortality risk resulted in a decline in genetic variance of mortality rates at old ages. That result suggests that the decline in genetic variance at old ages that has been observed in some experiments is an artifact of heterogeneity. Mortality rate plateaus, decrease in genetic variance of mortality rates at old ages, and absence of catastrophic late-age mortality all appear to contradict predictions of the evolutionary theory of senescence. Heterogeneity in mortality risk may explain those contradictions.