Editing Peter Medawar (section)

=== Theory of senescence ===
Medawar's 1951 lecture "An Unsolved Problem of Biology" (published 1952<ref>Medawar, P. B. (1952). ''An Unsolved Problem of Biology''.  HK Lewis and Co.</ref>) addressed ageing and [[senescence]], and he begins by defining both terms as follows:

{{blockquote|We obviously need a word for mere ageing, and I propose to use 'ageing' itself for just that purpose. 'Ageing' hereafter stands for mere ageing, and has no other innuendo. I shall use the word 'senescence' to mean ageing accompanied by that decline of bodily faculties and sensibilities and energies which ageing colloquially entails.}}

He then tackles the question of why evolution has permitted organisms to senesce, even though (1) senescence lowers individual fitness, and (2) there is no obvious necessity for senescence. In answering this question, Medawar provides two fundamental and interrelated insights. First, there is an inexorable decline in probability of an organism's existence, and, therefore, in what he terms "[[Reproductive value (population genetics)|reproductive value]]." He suggests that it therefore follows that the force of [[natural selection]] weakens progressively with age late in life (because the [[fecundity]] of younger age-groups is overwhelmingly more significant in producing the next generation). What happens to an organism after reproduction is only weakly reflected in natural selection by the effect on its younger relatives. He pointed out that likelihood of death at various times of life, as judged by [[life tables]], was an indirect measure of [[fitness (biology)|fitness]], that is, the capacity of an organism to propagate its genes. Life tables for humans show, for example that the lowest likelihood of death in human females comes at about age 14, which in primitive societies would likely be an age of peak reproduction. This has served as the basis for all three modern theories for the [[evolution of ageing|evolution of senescence]].<ref>{{cite journal|last1=Ljubuncic|first1=Predrag|last2=Reznick|first2= Abraham Z.|title=The Evolutionary Theories of Aging Revisited &ndash; A Mini-Review|journal=Gerontology|year=2009|volume=55|issue=2|pages=205–216|doi=10.1159/000200772|pmid=19202326|doi-access=free}}</ref><ref>{{cite journal|last=Charlesworth|first=Brian|author-link=Brian Charlesworth|title=Fisher, Medawar, Hamilton and the evolution of aging.|journal=Genetics|year=2000|volume=156|issue=3|pages=927–31|doi=10.1093/genetics/156.3.927|pmid=11063673| pmc=1461325 }}</ref><ref>{{cite journal|last1=Promislow|first1=Daniel E. L.|last2=Pletcher|first2= Scott D.|title=Advice to an aging scientist|journal=Mechanisms of Ageing and Development|year=2002|volume=123|issue=8|pages=841–850|doi=10.1016/S0047-6374(02)00021-0|pmid=12044932|s2cid=30949013}}</ref>