The Nature and Criteria of Senescence 



senescence is to be regarded not as the positively beneficial 

 character which Weismann believed it to be, but as a potenti- 

 ality lying outside the part of the life cycle which is relevant to 

 evolution. It has certainly been 'evolved', in that the living 

 system which senesces has evolved, but it has not evolved as a 

 physiological mechanism. The line of argument which appears 

 most plausible is that suggested by Medawar (1945, 1952). It 

 seems probable, for a number of reasons, that except in certain 

 social animals there can be little effective selection pressure 

 against senescence as such. Normal population structure in wild 

 communities of animals, even in the absence of senescence, leads 

 to a continual preponderance of young reproducing over old 

 reproducing individuals, sufficient to override the advantage in 

 number of progeny which arises from a longer reproductive life. 

 Death from senescence is itself in many species so rare an event 

 in the wild state that failure to senesce early, or at all, has little 

 value from the point of view of survival. In many forms the 

 cessation or reduction of breeding capacity precedes senescence 

 proper — with certain exceptions in social animals, events occur- 

 ring in the post-reproductive period are theoretically outside 

 the reach of selection, and irrelevant to it. A consequence even 

 more important than the mere failure of evolutionary processes 

 to operate in favour of the postponement of senescence follows 

 from the same facts. In view of the constant reproductive pre- 

 ponderance of young individuals, the postponement of the 

 action of a harmful genetic effect until late in the reproductive 

 life is almost equivalent, in selective value, to its complete 

 elimination: the longer the postponement, the closer the equiva- 

 lence. The evolutionary 'demon' is concerned only to clear the 

 part of the life-span in which he works, not the parts which 

 might be reached if the environment were artificially made 

 more favourable. This mechanism, by acting to move all ad- 

 verse genetic effects which are capable of postponement and all 

 the consequences of divergent but temporarily beneficial systems 

 into the late reproductive or post-reproductive life, may itself 

 provide a partial explanation of the evolution of senescence, as 

 Haldane (1941) has already suggested. The selective equilibrium 

 reached in man would be expected in this case to be such 

 that the force of mortality is lowest when reproductive 



39 



