THE STATES OF VARIABILITY 



segregate in the second generation. The heterozygotic potential is 

 thus equally the intermediate step in the opposite process — the 

 transference of free variability to the honiozygotic store. 



Where more than two genes arc concerned the same principles 

 apply : there is merely a greater range of possible balanced homo- 

 zygotes. hi the case of four genes having equal and supplementary 



NEW 



POTENTIAL 



FREE 



FIXED 



MUTANT 



i 



HETEROZYGOTIC 



i t 



PHENOTYPIC 

 SELECTED 



HOMOZYGOTIC 



Fig. 71. — The states of polygenic variability. There is a constant flow between 

 the free and potential states, the heterozygotic being distributed by segregation to 

 the free and homozygotic states in proportions depending on the linkage relations 

 of the polygenic system, and the free and homozygotic variability passing into the 

 heterozygotic state by crossing. There is no direct flow between free and homo- 

 zygotic potential. Free variability is the raw material for selection, which is con- 

 stantly using up or fixing a portion of it. This loss will be balanced over long periods 

 of time by the rise of new variability through mutation; but the balance will not 

 generally be struck accurately over short periods of time. The thinner arrows indicate 

 that the loss by selection and gain from mutation are slower than the flow between 

 the other three states (after Mather, 1943). 



effects, for example, six homozygotes exist of the general type 

 AABBcah^, and four each of the less balanced general types 

 AABBCCdd and AAhhccdd. The more the genes, the greater the 

 variety, and hence importance, of the homozygotic potential state. 



Since the variability must all go through the heterozygotic 

 potential state before it can be redistributed between the homo- 

 zygotic states, free and potential, the proportion of heterozygotes in 

 a population will govern the rate of flow of variability from one 

 state to another (Fig. 71). Heterozygotes can, of course, arise from 

 the inbreeding of pre-existing heterozygotes, but under such a 

 system they will be present in a proportion which decreases from 



281 



