senescence and the enduring impact of this paper to modern science

Peter Brian Medawar was a British biologist born in Brazil, whose work on graft rejection and the discovery of acquired immune tolerance was fundamental to the practice of tissue and organ transplants. He was awarded the 1960 Nobel Prize in Physiology or Medicine with Sir Frank Macfarlane Burnet, UNESCO Kalinga Prize for the Popularization of Science in 1985, …

 

Outcome of research

Medawar was awarded his Nobel Prize in 1960 with Burnet for their work in tissue grafting which is the basis of organ transplants, and their discovery of acquired immunological tolerance. This work was used in dealing with skin grafts required after burns. Medawar’s work resulted in a shift of emphasis in the science of immunology from one that attempts to deal with the fully developed immunity mechanism to one that attempts to alter the immunity mechanism itself, as in the attempt to suppress the body’s rejection of organ transplants.

Theory of senescence

Medawar’s 1951 lecture An unsolved problem of biology (published 1952)) addressed ageing and senescence, and he begins by defining both terms as follows:

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 iterrelated insights. First, there is an inexorable decline in probability of an organism’s existence, and, therefore, in what he terms “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, 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 senescence.

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