AN UNSOLVED PROBLEM OF BIOLOGY 



not happened already, and, if it has not, what is the evidence 

 that it is happening now? The first question amounts to asking 

 why Huntington's chorea is not already one of the diseases of 

 the post-reproductive period, since selection of the sort I have 

 outlined must be pretty vigorous and has presumably had tens 

 of thousands of years at its disposal. My answer to this is based 

 on an aside of Professor Haldane''s. It is only in the last century 

 or so that selection has had a real chance to get a grip on it, 

 for it is only within this period that the average expectation of 

 life at birth has come to equal the average age of onset of the 

 disease.* Even so, there is indirect evidence of a postponement 

 of its age of onset. Since the male reproductive span is longer 

 than the female''s, the force of selection on men must be less 

 quickly attenuated with increasing age; postponement should 

 therefore have gone farther in men than in women — and this, as 

 I have already said, is indeed the case. Ultimately, no doubt, the 

 age of onset will come to a standstill in both men and women 

 at the end of their respective reproductive periods. I gratefully 

 acknowledge the origin of this train of thought in Professor 

 Penrose''s writings on mental disease and natural selection. 



With Huntington''s chorea as a lucky concrete example, I 

 can now propound the following general theorem. If hereditary 

 factors achieve their overt expression at some intermediate age 

 of life; if the age of overt expression is variable; and if these 

 variations are themselves inheritable; then natural selection 

 will so act as to enforce the postponement of the age of the 

 expression of those factors that are unfavourable, and, corre- 

 spondingly, to expedite the effects of those that are favourable 

 — a recession and a precession, respectively, of the variable 

 age-effects of genes. This is what I mean by saying that 



* [This is not quite fair. It is not the mean expectation of life at birth 

 that is important, but the mean expectation of further life at an age when 

 reproduction has just begun. This too has increased, but not nearly so 

 dramatically, over the past hundred years.] 



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