SELECTION AND VARIABILITY 



compared with loss by redistribution of variability, as we can sec 

 from observations such as those on the tomato and Gakopsis. 



Selection ami the Reservoir of Variability 



The sheltering effect of heterozygosity and dominance depends 

 on the fact that selection discriminates primarily between phenotypes, 

 and hence between genotypes only to tlie extent that they give 

 different phenotypes. The phcnotype is, as it were, the organ by 

 which the genotype is selected. Yet it is upon the favouring of 

 particular genotypes that response to selection, as it is expressed in 

 the phenotypes of succeeding generations, must depend. A lag of at 

 least one generation must consequently intervene between the action 

 of selection and the response which it produces. Thus, changes under 

 selection can be adjustive only to the extent that the changes in 

 environment, which produce them, are permanent. 



Not all changes in the environments to which succeeding 

 generations are subjected can be of this kind, hi an ephemeral 

 organism, such as a fruit fly or a chickweed, the different generations 

 live at different times of the year, and hence are subjected to a series 

 of environments whose main changes are cyclical rather than 

 permanent. The changes of environment in which successive 

 generations of ammal organisms find themselves are, if not cyclical, 

 at least erratic rather than permanent. Only with longer-lived 

 species will the vagaries of environmental change tend to even 

 themselves out. 



Non-permanent fluctuations of environment must always be 

 occurring, though their significance to the organism may not be 

 constant. It will depend on the relative magnitude of any permanent 

 changes which may be going on at the same time and which may 

 be more rapid at one period than another. When, for example, 

 the first birds evolved, they had a whole new field to themselves. 

 As they multiplied and filled it, competition must liave increased 

 rapidly, and with it the environment of each bird must have 

 changed in the direction o{ becoming permanently more exacting. 

 With the attainment of something approaching maximum numbers 

 the environment, as determined by this competition, must have 

 become more stable, though non-permanent changes would still 



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