Soma and senescence

Classical life history theory provides a contingent case where 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). When a soma is present, there is a decline in the force of selection with age, and this selects for early reproduction and survival at the cost of maintenance against a somatic deterioration.

It is usually assumed that the decline in the force of selection with age is absent in asomatic organisms because the framework of age-structured demography should not apply to organisms with binary fission where it is impossible to distinguish the “mother” from the “daughter” (Rose, 1991). This is based on the assumption that we need to distinguish the “mother” from the “daughter”; yet, this is not necessary because the fitness of the two individuals are statistically the same (Witting, 2008). This suggests that it is neither the soma, and nor the decline in selection with age, that are the essential factors that determine that some organisms have evolved senescence, while others have not.

Nothing that the soma may likely be the trait through which senescence is expressed; Malthusian Relativity offers the alternative explanation that a deteriorating soma can evolve as a competitive trait when energy that can be used for tissue maintenance is allocated into competitive interactions instead. This makes senescence a function of the level of interference competition, with selection for the absence of soma and senescence in self-replicating cells with a mass bias exponent below one, and selection for soma and senescence in multicellular animals with a mass bias exponent of unity or more (Witting, 1997, 2008).

Alternatively there may always be selection for senescence and a soma. But self-replicating cells may be so simple that they cannot evolve a soma and they are thus unable to express senescence.

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References

• Rose, M.R. 1991. Evolutionary biology of aging. Oxford University Press, New York.
• Williams, G.C. 1957. Pleiotropy, natural selection and the evolution of senescence. Evolution 11:398--411.
• Witting, L. 1997. A general theory of evolution. By means of selection by density dependent competitive interactions. Peregrine Publisher, Århus, 330 pp, URL https://mrLife.org.
• Witting, L. 2008. Inevitable evolution: back to The Origin and beyond the 20th Century paradigm of contingent evolution by historical natural selection. Biological Reviews 83:259--294, https://doi.org/10.1111/j.1469--185X.2008.00043.x.