Disposable Soma Theory
	Figure 8: A representation of the disposable soma theory illustrating that effective cellular maintenance is only beneficial while there is a reasonable probability of survival (Kirkwood and Austad, 2000)	Description A theory related to the pleiotropy model was termed the disposable soma theory. It is based on the idea that maintenance and repair take up valuable metabolic energy that evolutionarily speaking is better allocated towards increasing reproduction early in life (Partridge and Gems, 2006). In particular, since survival in the wild is strongly influenced by stochastic factors leading to low probabilities of long life, natural selection could not invest in repair mechanisms that would only aid a small fraction of a population. In other words, there would not be enough individuals surviving to this age for evolution of late-acting repair mechanisms to occur. Selection will instead increase the frequency of genes which have an early age survival benefit.
		A side note about Hydra … Another important aspect of evolutionary theory is that germ lines appear to be immortal while somatic cells are not. In fact ageing is said to only occur in organisms where there is a clear distinction between the two cellular lineages (Kirkwood and Austad, 2000). In a study on Hydra, Martínez reported no sign of decline in reproductive rate or increase in mortality and concluded that this species may be intrinsically ‘immortal’. Hydra vulgaris has a simple body plan with no division between soma and germ line cells and can thus reproduce asexually from most parts of the organism (Martínez, 1998; Kirkwood and Austad, 2000). Figure 9: Evolutionary theory predicts that because ageing is a concequence between longevity and reproduction trade-offs, the age at which organisms begin to reproduce should be positively correlated with maximum longevity. Hydra live significantly longer than predicted based on the age at frist reproduction and show no sign of deterioration over a study period of four years (Martínez, 1998)
	Comparison and Predictions The disposable soma theory, like pleiotropy, suggests that ageing occurs because of trade-offs although the soma theory is concerned with higher order mechanisms rather than genes (Kirkwood and Austad, 2000). Predictions from this theory are that mechanisms, such as DNA repair and decreased generation of free radicals should be observed in organisms which live longer (Kirkwood, 2002). Oxidative stress causes cellular damage by the production of free radicals in mitochondria which harm proteins and DNA (Hughes and Reynolds, 2005). Long-lived variants would thus have to allocate resources, such as antioxidant production, for combating these sources of error. It is also suggested that experiments extending lifespan should lead to a decreased fecundity due to these trade-offs involved in longer maintenance. A mathematical model for describing optimal strategies of investment in reproduction and cellular repair has recently been reported (Drenos and Kirkwood, 2005). It can be used to model the optimum age at which a peak is obtained for somatic maintenance under different life-history characters such as increased adolescent mortality.	Experimental Evidence There are different lines of comparative evidence suggesting that the disposable soma theory is applicable to the evolution of ageing. · Long-lived rodents and Drosophila exhibit lower oxygen reactivity species than short lived variants. Drosophila specifically show that resistance or susceptibility to oxidative stress detrimental to proteins can be selected for artificially. Antioxidant genes can increase in frequency in populations with long lives (Kirkwood and Austad, 2000). · DNA repair is correlated with increased lifespan in mammals. Both suggest that longer lived organisms will have superior stress coping mechanism for better repair and maintenance in the environment (Kirkwood and Austad, 2000).
	The evolutionary theories in conclusion Sterns et al., found that a central caveat of the evolutionary theory, namely that intrinsic mortality rates evolve to an optimum based on extrinsic mortality, was true in Drosophila. When adult mortality was high flies had higher, earlier fecundity and had evolved a shorter life span over time than adult populations where mortality was lower (Sterns et al., 1998). All three classical concepts of why ageing occurs have empirical evidence supporting them so determining which is occurring in a given situation is more likely a question of the relative importance of each. Recent work has focused more on how changing environments typical of the natural settings affect the results of these theories. Recent causes for concern The historical reasons for the presence of ageing focused on a decrease in natural selection as ageing occurs but it is likely that this view is too simplistic. Recent debate has suggested that in certain cases selection might increase over a lifetime (Martin, 2006). At advanced maturity the increasing trend of mortality with age (termed the Gompertz curve) has been observed to change to a decreasing slope. It was suggested that life-history and behavioural changes that occur in the very old would lead to a reduction in activity and thus less chance of injury. Similarly, criticism also concerns the treatment of the abrupt period when reproduction is said to end and when natural selection thus loses power. Behaviours such as parental care and cultural transmission are used to describe instances where natural selection should act to reduce mortality after reproduction has ended because of the advantages to the survival of progeny. Although interesting, one has to question the relevance of these types of data in the wild where it is unlikely that ‘ancient’ phenotypes would be able to survive. It seems likely that, as with most scientific theories, the ideas presented by history are merely basic reductionistic explanations that can describe general trends. Recent criticism occurs because more precise answers require more complex descriptions than classical reasoning sometimes allows.