Editing Senescence (section)

====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.