Editing Immortality (section)

====Evolution of aging====
{{Main|Evolution of aging}}<!--that's how they spell aging in England ... and in that article-->
As the existence of biologically immortal species demonstrates, there is no [[second law of thermodynamics|thermodynamic]] necessity for senescence: a defining feature of life is that it takes in [[Gibbs free energy|free energy]] from the environment and unloads its [[entropy]] as waste. Living systems can even build themselves up from seed, and routinely repair themselves. Aging is therefore presumed to be a byproduct of [[evolution]], but why mortality should be selected for remains a subject of research and debate. Programmed cell death and the telomere "end replication problem" are found even in the earliest and simplest of organisms.<ref>Clark, W.R. 1999. ''A Means to an End: The biological basis of aging and death.'' New York: Oxford University Press. {{cite web |url=http://wrclarkbooks.com/means_to_an_end.html |title=A Means to an End – Why do we age? Is aging inevitable? Questions and Answers are available here by downloading a sample chapter of WR Clark's book. The WRClark Site also features a chat room for you to ask your questions about aging |access-date=25 May 2008 |url-status=dead |archive-url=https://web.archive.org/web/20080511212036/http://www.wrclarkbooks.com/means_to_an_end.html |archive-date=11 May 2008 }} About telomeres and programmed cell death.</ref> This may be a tradeoff between selecting for cancer and selecting for aging.<ref>Harrison, ''Principles of Internal Medicine'', Ch. 69, "Cancer cell biology and angiogenesis", Robert G. Fenton and Dan L. Longo, p. 454.</ref>

Modern theories on the evolution of aging include the following:
* Mutation accumulation is a theory formulated by [[Peter Medawar]] in 1952 to explain how evolution would select for aging. Essentially, aging is never selected against, as organisms have offspring before the mortal mutations surface in an individual.
* [[Pleiotropy|Antagonistic pleiotropy]] is a theory proposed as an alternative by [[George C. Williams (biologist)|George C. Williams]], a critic of Medawar, in 1957. In antagonistic pleiotropy, genes carry effects that are both beneficial and detrimental. In essence this refers to genes that offer benefits early in life, but exact a cost later on, i.e. decline and death.<ref>Williams, G.C. 1957. Pleiotropy, natural selection and the evolution of senescence. ''Evolution'', '''11''':398–411. {{cite web|url=http://www.telomere.org/Downloads/Williams_searchable.pdf |title=Archived copy |access-date=23 July 2006 |url-status=dead |archive-url=https://web.archive.org/web/20060713071204/http://www.telomere.org/Downloads/Williams_searchable.pdf |archive-date=13 July 2006 }} Paper in which Williams describes his theory of antagonistic pleiotropy.</ref>
* The disposable soma theory was proposed in 1977 by [[Tom Kirkwood|Thomas Kirkwood]], which states that an individual body must allocate energy for metabolism, reproduction, and maintenance, and must compromise when there is food scarcity. Compromise in allocating energy to the repair function is what causes the body gradually to deteriorate with age, according to Kirkwood.<ref>Kirkwood, T.B.L. 1977. Evolution of aging. ''Nature'', '''270''': 301–304. [http://www.nature.com/nature/journal/v270/n5635/abs/270301a0.html] Origin of the disposable soma theory.</ref>