Late-life fecundity plateaus in Drosophila melanogaster can be explained by variation in reproductive life spans

Population trajectories of age-specific fecundity in Drosophila melanogaster typically decline with increasing age and then exhibit an upward inflection, or "plateau", at the oldest ages. This pattern has been interpreted as evidence of an evolved and physiologically distinct life history stage in late life. While low levels of fecundity are common in the last few days of life of individual flies, it is unclear that defining a single age as the beginning of a period of low fecundity for the entire cohort is useful, since reproductive life spans vary substantially from fly to fly. Here I analyze published data on survival and reproduction of individual female flies and show that non-linearities in late-life fecundity trajectories arise from a type of demographic selection that occurs when sub-groups with different reproductive life spans (RLS) are mixed. For groups of flies stratified by RLS late-life fecundity declines linearly with age. A simulation incorporating strictly linear decline of individual fecundities and realistic levels of variation in RLS produces late-life plateaus similar to those observed in experiments. Existing population heterogeneity is a sufficient explanation, and no special evolutionary argument is required. For these data survival and reproduction are governed by positive correlations.