Fig. 5.

Theory about how adaptive death in C. elegans could have evolved by natural selection. Speculative scheme, based on the spandrels of San Marco arguments (Gould and Lewontin, 1979), including a Bauplan-type evolutionary trade-offs model. Here, the evolution of protandry (step 1) increased overall fitness by increasing population growth rate, but reduced some individual fitness elements, particularly by causing cessation of reproduction after sperm depletion (assuming the predominance of selfing in wild populations). Thus, after ∼day 3 of adulthood, animals become futile, and unable to contribution to fitness. This removed evolutionary constraints on deleterious effects of reproduction-senescent pathology trade offs, leading to the evolution of reproductive death: suicidal investment in reproductive effort (step 2). This in turn introduced into populations genetic variation affecting timing of early death with differential effects on colony-level fitness through reproductive death, leading to selection for still earlier death. This model suggests the possible existence of trade offs between individual and colony level fitness. For example, early proteostatic collapse might lead to increased mortality during individual reproduction that reduces individual fitness, e.g. when food availability is high early in colony development, yet increase population growth rate through benefits of consumer sacrifice as food depletion approaches, which will affect a larger number of individual worms.