Classical life-history theory suggests that senescence evolves from a somatic tissue from which no part is passed on in either sexual or asexual reproduction (Williams, 1957; reviewed by Rose, 1991). This hypothesis coincides with the presence of senescence in the large-bodied eukaryotes that have a soma, and its absence in the small-bodied prokaryotes that have no soma. But the hypothesis does not explain why the higher eukaryotes have a soma when prokaryotes do not.

Malthusian Relativity incorporates an alternative hypothesis where the soma is the trait through which senescence is expressed. When this hypothesis is combined with selection by density dependent competitive interactions it predicts that senescence and soma are unlikely to evolve in small-bodied organisms like prokaryotes, while senescence and soma is expected to evolve in large-bodied organisms like the higher eukaryotes.

References

• Rose, M. R. (1991). Evolutionary biology of aging. New York: Oxford University Press.
• Williams, G. C. (1957). Pleiotropy, natural selection and the evolution of senescence. Evolution 11, 398--411.