The Biology of Senescence 



activity is potentially highest, though the observed lowest level 

 falls rather earlier than this (10-12 years in males, Greville, 

 1946). 



The selectionist argument which regards senescence as the 

 decline of evolved survival-power through successive age groups 

 is most convincing when we apply it to mammals and birds: 

 among invertebrates, reservations require to be made. In those 

 which are predominantly seasonal, with a total life-span less 

 than one year, and which winter as fertilized adults, it is by no 

 means true that at all times of the year young individuals must 

 outnumber old in a free-running population. The autumn con- 

 tingent of overwintering animals will consist of 'old' individuals. 

 In such forms, the selective advantage of different genotypes 

 will vary from season to season, and there will be an ultimate 

 requirement that the adult be capable of living long enough to 

 overwinter. Forms producing two broods annually will tend to 

 select fertility in the spring brood and longevity in the autumn, 

 but with a time-lag of one generation between selection and 

 potential expression. The mechanism of selection in such a 

 system must be very complicated. 



In mammals some selective advantage would also presum- 

 ably attach to longevity where older males are polygamous and 

 younger males compete for the remaining females (deer, 

 baboons). The solipsist model of selection operating on 'the 

 individual' can obviously be upset by any selection pressures 

 introduced into the system by interaction between individuals, 

 and by community-patterns of ecological behaviour in the 

 species; the idea of an 'individual' animal unsupported by the 

 rest of the ecological community in which it lives is in fact 

 unbiological, and large unpredictable selection pressures affect- 

 ing the life-span may well arise from such hidden social 

 relationships. 



In spite of this criticism, the theory of senescence as a measure 

 of declining selection-pressure is important. The declining 

 evolutionary importance of the individual with age may be 

 expressed in another way in the 'morphogenetic' senescence 

 seen in mammals. At the point where a system of differential 

 growth ceased to be regulated by forces which arose from 

 natural selection, it would cease to be under effectively direc- 



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