Editing Senescence (section)

=== Evolutionary aging theories ===
{{Main|Evolution of ageing}}

====Antagonistic pleiotropy====
{{Main|Antagonistic pleiotropy hypothesis}}
One theory was proposed by [[George C. Williams (biologist)|George C. Williams]]<ref name = "Williams_1957" /> and involves [[antagonistic pleiotropy]]. A single gene may affect multiple traits. Some traits that increase fitness early in life may also have negative effects later in life. But, because many more individuals are alive at young ages than at old ages, even small positive effects early can be strongly selected for, and large negative effects later may be very weakly selected against. Williams suggested the following example: Perhaps a gene codes for calcium deposition in bones, which promotes juvenile survival and will therefore be favored by natural selection; however, this same gene promotes calcium deposition in the arteries, causing negative atherosclerotic effects in old age. Thus, harmful biological changes in old age may result from selection for [[pleiotropy|pleiotropic]] genes that are beneficial early in life but harmful later on. In this case, selection pressure is relatively high when [[Fisher's reproductive value]] is high and relatively low when Fisher's reproductive value is low.

====Cancer versus cellular senescence tradeoff theory of aging====
{{Main|Immunosenescence}}
Senescent cells within a [[multicellular organism]] can be purged by competition between cells, but this increases the risk of cancer. This leads to an inescapable dilemma between two possibilities—the accumulation of physiologically useless senescent cells, and cancer—both of which lead to increasing rates of mortality with age.<ref name="nelson_2017" />

==== Disposable soma ====
{{Main|Disposable soma theory of aging}}
The disposable soma theory of aging was proposed by [[Tom Kirkwood|Thomas Kirkwood]] in 1977.<ref name=":0" /><ref>{{Cite book|first=Tom|last=Kirkwood|url=http://worldcat.org/oclc/437175125|title=Time of Our Lives : the Science of Human Aging.|date=2006|publisher=Oxford University Press|isbn=978-0-19-802939-7|oclc=437175125|access-date=31 January 2022|archive-date=15 November 2023|archive-url=https://web.archive.org/web/20231115062955/https://search.worldcat.org/title/437175125|url-status=live}}</ref> The theory suggests that aging occurs due to a strategy in which an individual only invests in maintenance of the soma for as long as it has a realistic chance of survival.<ref>{{cite journal | vauthors = Hammers M, Richardson DS, Burke T, Komdeur J | title = The impact of reproductive investment and early-life environmental conditions on senescence: support for the disposable soma hypothesis | journal = Journal of Evolutionary Biology | volume = 26 | issue = 9 | pages = 1999–2007 | date = September 2013 | pmid = 23961923 | doi = 10.1111/jeb.12204 | hdl-access = free | s2cid = 46466320 | hdl = 11370/9cc6749c-f67d-40ab-a253-a06650c32102 }}</ref> A species that uses resources more efficiently will live longer, and therefore be able to pass on genetic information to the next generation. The demands of reproduction are high, so less effort is invested in repair and maintenance of somatic cells, compared to [[germline cell]]s, in order to focus on reproduction and species survival.<ref>{{cite journal | vauthors = Kirkwood TB, Rose MR | title = Evolution of senescence: late survival sacrificed for reproduction | journal = Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences | volume = 332 | issue = 1262 | pages = 15–24 | date = April 1991 | pmid = 1677205 | doi = 10.1098/rstb.1991.0028 | bibcode = 1991RSPTB.332...15K }}</ref>