Antagonistic Pleiotropy Theory
Figure 7: Unlike the mutation accumulation theory the genes suggested to be involved with the ageing phenotype in the pleiotropy theory are beneficial early in life (Kirkwood and Austad, 2000).	Description Another theory, first proposed by George Williams to explain how aging occurs, concerns genes which change their effects over time (Gavrilov and Gavrilova, 2002). The pleiotropy theory (AP) states that alleles displaying beneficial effects in youth by conferring a fitness advantage would be strongly selected for in nature even if these same alleles had a detrimental effect on survival late in life (Kirkwood, 2002; and others). This is because natural selection could fix the beneficial alleles early in life without predicting their negative consequences later. The implication of this line of reasoning is that ageing is ultimately a trade-off between fitness and survival.
Evidence The pleiotropy and mutation accumulation models are not necessarily mutually exclusive nor are they necessarily an exhaustive explanation for the trend of gradual decrease in fitness with age. Many predictions from the two models are the same. For example, ageing rates will be faster when there are greater risks of death in the adult (Partridge and Gems, 2006). Experimental evidence for pleiotropic genes are obtained from comparative studies and artificial selection experiments (Hughes et al, 2002). In one such study, artificially selected long-lived Drosophila variants increased the temporal survival of progeny while reducing early age fecundity (Partridge et al., 1999). Further evidence of genes which exhibit pleiotropic effects are observed in long-lived mutants.	A circular reasoning for why ageing exists Reasoning from AP and MA theories can explain why ageing occurs. In a theoretical non-ageing population one would still expect to see increased mortality with age due to extrinsic factors. The consequences of age related mortality are positive feedback loops leading to reduced reproduction over time. The reduced reproduction at late ages further decreases the power for selection. This lack of selection results in increasing frequencies of deleterious mutations over time which then increases mortality as well. In light of this type of reasoning, the existence of ageing is not a paradox and seems inevitable in most populations (Hughes and Reynolds, 2005). However, recent work on possible negative senescence has called the inevitability of ageing into some question (Williams et al., 2006).