In 1977, a statistician named Thomas Kirkwood (now a biologist and professor of medicine at the University of Newcastle) published his disposable soma theory of aging. Kirkwood’s idea was that organisms only have a limited amount of energy that has to be divided between reproductive activities and the maintenance of the non-reproductive aspects of the organism (soma). Aging is the result of natural degrading processes that result in accumulation of damage but the damage can be repaired by the organism at the expense of reproductive effort. Because of the declining evolutionary impact of adverse events on older animals (Williams' hypothesis), a tradeoff exists in which it does not make sense for an organism to invest effort (food energy resources) in maintenance (at the expense of reproductive activity) to result in living much beyond the initial breeding years. Kirkwood agreed with Williams' earlier position (antagonistic pleiotropy theory) that the adverse effects of aging could not be considered negligible in evolutionary terms and that therefore aging could not be explained by Medawar's mutation accumulation theory (1952), which considered the evolutionary effect of aging to be negligible.
This theory also, in effect, combines the apparent declining force of natural selection after breeding age is reached with accumulation of damage, and suggests a relationship between reproduction and life span while avoiding conflict with traditional evolutionary mechanics theory.
The disposable soma theory is one of those based on the idea that the evolutionary value of additional life declines (but not to zero) following the age at which an organism achieves reproductive maturity. This concept in turn suggests that aging might be the result of a tradeoff between some beneficial quality (in this case reproduction) that happens to be rigidly linked to aging. Rigid linkage means that the evolution process cannot work out a way to achieve the benefit without incurring the adverse side-effect (aging). See Evolutionary Value of Life.
Problems with the Disposable Soma Theory
The disposable soma theory seems to make sense on a philosophical level but has many severe problems if one looks at it in any detail. One obvious issue is that it has not been demonstrated that “maintenance and repair” actually takes a significant amount of effort or resources when compared to the energy and resources required by the day-to-day existence of an organism much less the initial growth of the organism. Why would replacing a few cells take comparable effort or resources than producing trillions of cells in the first place or providing for their routine operation?
Another problem with the disposable soma theory is that it is not obvious why effort or energy spent in reproduction in an animal’s early years would necessarily decrease the energy available in later years for “repair.” One would think a post-menopausal woman would cease aging or even become stronger as a result of the absence of the resource drain caused by reproductive effort. Instead, damage appears to increase exponentially. Further, experimental evidence generally suggests that individual animals that live longer also tend to be more successful reproductively.
Another obvious problem is that by essentially any method of accounting females use more resources and energy in reproduction than males. Why don’t males therefore live longer than females? A fan of the disposable soma theory once suggested that males, by amazing coincidence, spend as much extra energy and resources protecting their mates and young as females spend in reproduction and therefore have approximately the same life span. An even cursory examination of typical animals (e.g. lions, turtles) suggests that this idea is implausible to the point of ridiculous.
A fundamental difficulty is that animals obtain the resources and energy they need from eating. A pregnant animal needs more resources and therefore eats more. An animal that is growing needs more resources and therefore eats more. If indeed an animal needed significant resources to perform maintenance and repair functions why would it not simply eat more to provide those resources? Animals are in competition for food but why would the resources needed for maintenance fall into any different category than those needed for any other function? Wouldn't an animal that fought to get the resources necessary for maintenance and thereby lived and reproduced longer have an evolutionary advantage over those that did not according to traditional mechanics theory? Wouldn't an organism design that provided for living longer and reproducing longer still provide an individual advantage for those organisms possessing the design? Of course, one answer to that question is that the population might be better off by using those resources for reproduction rather than maintenance of older individuals given limited resources. The population benefit might offset the individual disadvantage. However, a population benefit that results in an individual disadvantage is incompatible with traditional evolutionary mechanics. Indeed, the idea that a population could benefit from deaths of older members was suggested much earlier by Weismann in 1882. Even earlier, Darwin suggested that aging and otherwise limited life span must convey some unknown benefit that compensated for its adverse nature.
One way out of this problem that maintains compatibility with the individual benefit requirement of traditional evolutionary mechanics theory is to assume that some unknown property that increases reproductive effectiveness somehow, for unknown reasons, decreases life span by causing deterioration in later years. This is a subset of the earlier antagonistic pleiotropy theory. Aging must be a hidden unavoidable side effect of reproduction. Of course this in turn amounts to a restatement of Darwin’s thinking about life span: There must be some hidden benefit to offset the disadvantage of aging. Some traditional biologists such as Leonid Gavrilov, of the Center on Aging at the University of Chicago, consider the disposable soma theory to be a “version of” the antagonistic pleiotropy theory and a “widowed concept.”
Some organisms apparently do not age and yet are able to continue reproducing. Some are said to actually display increased reproductive capacity with age. This observation obviously conflicts with the idea that there is some unavoidable biological tradeoff between reproduction and aging.
The energy and resources associated with organism maintenance seem to be largely short-term in nature. For example, in humans, hair, nails, epithelial cells, and red blood cells are frequently replaced (weeks). If we added up the weight of all of these items produced during an average human life span it would likely exceed the weight of the person. On the other hand, nerve and brain cells are very infrequently replaced. The total mass of material and associated energy associated with replacing these cells or other long-term maintenance is therefore negligible by comparison. Our need for sleep (widely seen as associated with maintenance) is very short-term. These observations make it extremely difficult to see how there could be a tradeoff between the major maintenance and repair effort (short-term) and aging (long-term). A young animal cannot afford to cease short-term maintenance and repair because doing so would damage its ability to reproduce. Ceasing long-term maintenance and repair (to the extent that it exists) would not save significant resources.
Despite the massive and multiple difficulties, the disposable soma theory is among the most respected theories that are compatible with traditional (~1950) evolutionary mechanics theory and is popular with people who still consider it "impossible" that traditional theory could be less than perfectly comprehensive.
Difficulties in explaining conflicts between organism observations other than aging and traditional theory led to development (post-1962) of proposed adjustments to traditional evolutionary mechanics theory, all of which allow for an expanded definition of evolutionary benefit. These adjustments, including group selection, kin selection, evolvability, and gene-centered theories support the idea that the deteriorative and life span limiting aspects of aging directly serve a necessary evolutionary purpose and that therefore no compensating individually beneficial purpose is required. As suggested above, a limited life span does convey plausible group or evolvability benefits. See Evolvability Theories of Aging and Group/Kin Selection Based Theories of Aging.
Kirkwood is of course still a proponent of non-programmed aging and his particular disposable soma theory and is still (2011) publishing papers attacking programmed aging theories and other non-programmed theories. See On the Programmed/ Non-Programmed Aging Controversy for specific counter-arguments to his recent paper. V. Skulachev also wrote an article responding to Kirkwood's article and making arguments for programmed aging.
More Issues with Disposable Soma Theory.
See also: Programmed and Non-Programmed Maintenance and Repair Theories of Aging.
1. Kirkwood TB. Evolution of ageing. 1977. Nature 170(5635) 201-4
2. Kirkwood T.B.L. & F.R.S. Holliday. The evolution of ageing and longevity. 1979. Proceedings of the Royal Society of London B 205: 531-546
3. Kirkwood T, Melov S. On the Programmed/Non-Programmed Nature of Ageing within the Life History. Current Biology 21, R701-7 2011
4. Goldsmith T. On the Programmed/ Non-Programmed Aging Controversy 2012. Biochemistry (Moscow) Phenoptosis 77-7. PubMed (A programmed aging response to 2011 Kirkwood article above.)
5. Skulachev V. Aging as a particular case of phenoptosis, the programmed death of an organism (a response to Kirkwood and Melov "On the programmed/non-programmed nature of ageing within the life history").Aging (Albany NY). 2011 Nov;3(11):1120-3. PubMed
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