SELECTION AND VARIABILITY 



hence to the total variabihty, is equal to the increase by mutation, 

 an increase which is obviously independent equally of both kinds of 

 variability. When loss exceeds gain, the total variability will 

 diminish until an equilibrium is reached, and vice versa . 



Loss and gain of variability must balance in the long run, but 

 they are unlikely to do so over short periods. Mutation might be 

 expected to over-compensate for loss when this arises solely from 

 erratic changes in environment, but to fall short of doing so when 

 rapid permanent change is also causing the expenditure of variability. 

 The importance of the reservoir of potential variability lies in its 

 smoothing effect, in its ability to permit compensation of rapid 

 expenditure over short periods by a steady trickle of gains over a 

 longer time. All selective changes must ultimately depend upon 

 mutation; and the capacity for mutation, itself apparently under 

 some selective control, represents the final reserve of variability. 

 But it is a reserve which is released at a more nearly constant rate. 

 The immediate reserve of potential variability represents the storage 

 of these mutations by the genotype, in such a way as to permit 

 more rapid response to selection than mutation could give directly. 



Change of Genetic Systems: Inertia 



We can now consider the properties to be expected of genetic 

 systems in the light of these principles. The genetic system seen in 

 any stock or race of organisms today, with its special properties of 

 balance, storage and release of variability, is that which has enabled 

 the ancestral line of the organisms concerned not merely to become, 

 but to be continue to be, adequately adapted to its changing environ- 

 ment throughout the course of its history. The stock has survived, 

 and with it the genetic system upon which survival has depended. 

 Continuation of this system must depend on the extent to which the 

 j^enetic system enables its possessor to meet the exigencies of future 

 change. So long as the advantage lies with the maintenance of a 

 highly uniform and constant phenotype, the inbreeder will be well 

 endowed by its genetic system for success. But whenever the 

 advantage lies less with high uniformity than with easy change of 

 phenotype, the outbreeder will be the more successful. If the 

 environment changes in its demands, the genetic system will cease 



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