Senescence, usually defined as progressive loss of fertility and increasing likelihood of death with increasing age (Kirkwood and Austad 233), is clearly a harmful process for an individual – and, at first glance, difficult to reconcile with the process of natural selection, which would work to ensure optimal survival and increase fitness (reproductive success). It seems, at first glance, that it would also work to prevent the aging process because aging leads to increased mortality rates and decreased reproductive capacity. Historically, the first attempt to explain aging from an evolutionary perspective was proposed as “mutation accumulation.” theory, which assumes that the strength of natural selection declines with age, allowing the accumulation of deleterious genes with age-specific effects on mortality rate. In other words, natural selection is quite effective at eliminating alleles that have deleterious effects early in life, but later in life its strength diminishes; evolution allows for the accumulation of deleterious alleles through a combination of mutational pressure and genetic drift, unchallenged by natural selection (Rose 363-371). This idea, however summary and unfounded, was further developed into the "antagonistic pleiotropy" hypothesis, which basically holds that natural selection favors alleles that have aging as a side effect, provided they have had beneficial effects during life. youth – exhibit pleiotropic, or opposite effects at different calendar ages, and senescence is fundamentally a “maladaptive byproduct of selection for survival and reproduction during youth” (Fabian and Flatt). These theoretical insights form a basis for the evolutionary theory of senescence that explains why aging is occurring. To fur...... middle of paper ......xclusive and both can explain the occurrence of senescence. Furthermore, they are not the only ones that contribute to explaining why we age: “the disposable soma theory of aging” is another of the concepts that can contribute to the understanding of senescence, but it is a concept based on ecological factors and the first two theories provide the foundation for the evolutionary theory of aging. Works Cited: Fabian, Daniel and Flatt, Thomas. “The Evolution of Aging.” Nature Education Knowledge 3.10 (2011) Kirkwood, Thomas BL, and Steven N. Austad. "Why do we age?". Nature 408.6809 (2000): 233--238. Print.Masoro, Edward J. Challenges of Biological Aging. New York: Springer Pub. Co., 1999. Print.Rose, Michael R., Molly K. Burke, Parvin Shahrestani, and Laurence D. Mueller. "Evolution of aging since Darwin." Journal of Genetics 87.4 (2008): 363-371. Press.