SELECTION AND VARIABILITY 



£xcd or Stable; but the genes are still moving and their combinations 

 are changing. 



The flow of variability can be altered by selection; that is by the 

 choice of certain types of individual as parents of the next generation. 

 For example, we (or nature) can take as parents only the hetero- 

 zygotes and one of the two types of homozygote [Aa and AA). We 

 then effectively cut off the return of free variability to the potential 

 state, by rendering impossible the cross AA X aa, through which 

 this return comes about. At the same time we do not interfere with 

 the reverse change, from potential to free, which depends on 

 segregation amongst the progeny of hetcrozygotes. The net result 

 of such a selection of parents is thus to lower the potential and to 

 raise the free variability. The proportion of heterozygotes falls in 

 the next generation; that of the homozygotes, of the two types 

 taken together, increases. The joint increase is, however, due to 

 increase of only one of these homozygous types, AA. 



In this process of selection the gene frequency has been changed. 

 One allelomorph (A) is now more, and the other {a) less, common 

 than formerly. The average phenory-pe will have moved in the 

 direction of the more common type. If the selection is continued 

 indefinitely the population will ultimately come to consist wholly 

 of the one homozygote {AA), the other allelomorph of the gene 

 having been eliminated. No variability will be left: it will have 

 been expended on the change in average phenotype of the popula- 

 tion. Thus selection of particular parents in each generation changes 

 the mean phenotype, and this change is brought about by an 

 alteration of the flow of variability in such a way that ultimately 

 it will all pass into a. fixed state. Selection must then cease to produce 

 any further change, because all the individuals have become 

 genetically alike. Any differences they may show will be non- 

 heritable. In a word, selection cannot produce changes unless free 

 variability exists in the population. At the same time selection 

 fixes, we may even in one sense say destroys, the variability on 

 which the change depended. The alteration is permanent. 



A further aspect of this relation between variability and selection 

 is seen when it is the heterozygotes which are used as the sole 

 parents of the next generation. The flow from potential to free 

 variability must go on, since (with the exception we have seen in 



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