An Evolutionary Theory of Clonal Senescence

An evolutionary theory of senescence at the level of whole clones is formulated. The theory is based on the assumption that the Malthusian parameter measures fitness. The clone is modelled as a group of independent ramets. Survival probabilities and reproductive capacities of the ramets are allowed to vary with both ramet age and the age of the clone. An expression defining fitness as a function of the life history parameters of ramets and formulas for the parameter elasticities are derived. They are used to show that the sensitivity of fitness with respect to perturbations of the life history parameters of ramets decreases towards zero with increasing clonal age at which the perturbations take place. Thus, the force of natural selection against genes with deleterious effects on ramet survival and reproduction decreases with increasing clonal age of gene effects. Clonal senescence may therefore evolve through the accumulation of such genes and genes with antagonistic pleiotropic effects, i.e., with positive effects early in the life of the clone but deleterious effects later. Further, one special case is analyzed where the performances of ramets are, initially, independent of the age of the clone. For this case, it is shown that the decrease in sensitivity with clonal age depends on the ramets′ timing and amount of sexual reproduction relative to their total reproductive capacities. The analysis shows that the differentiation of soma and germ is not required for the evolution of senescence. Rather, it is suggested that, besides the occurrence of genes coding for senescence, the only requirement is the presence of a rejuvenating form of reproduction.