AN UNSOLVED PROBLEM OF BIOLOGY 



repetitive mutation when the mutant genes make their effects 

 apparent at ages which the great majority of the members of 

 a population do not actually reach; (2) the fact that the 

 postponement of the time of action of a deleterious gene is 

 equivalent to its elimination, and may sometimes be the only 

 way in which elimination can be achieved; and (3) the fact 

 that natural selection may actually enforce such a postpone- 

 ment, and, conversely, expedite the age of onset of the overt 

 action of favourable genes. All these theorems derive from the 

 hypothesis that the efficacy of natural selection deteriorates 

 with increasing age. 



I am inclined to think that the third factor, the enforced 

 precession and recession of the ages of the overt action of genes, 

 has the widest ambit of significance. But although I have fore- 

 sworn the introduction of too many qualifying and saving 

 clauses, one indeed is most important. Real animals, unlike 

 imaginary test-tubes, are neither born mature, nor do they 

 get on with the business of self-reproduction at once. There is 

 always a pre-reproductive period during which animals are far 

 from exempt from the hazards of mortality, and during this 

 period the average reproductive value of an individual must 

 therefore rise to a maximum, irrespective of whether or not it 

 falls later. If my reasoning is correct — there is no time to go 

 into details — the precession of the time of action of genes 

 comes to a standstill at the epoch when the reproductive value 

 is at a maximum, and it is then that senescence should be 

 expected to begin. Professor R. A. Fisher has pointed out that 

 the actuarial prime of life of human beings and the age at 

 which their reproductive value is at its maximum do in fact 

 nearly coincide. 



Even with such refinements as this, my proposals can hardly 

 be said to add up to a self-sufficient theory. If we concede that 

 the force of natural selection is rather exactly weighted by the 

 ages of the animals on which it operates, it is still far from easy 



69 



