SELECTION AND VARIAIHIITY 



classes must be mainly to favour the more balanced homozygotes 

 at the expense of the less balanced. The favoured individuals wiU 

 produce offspring genetically like themselves; and in so far as the 

 environment is stable, the population must show high agreement 

 with the optimum phenotype. Should the environment change 



2 4 6 8 10 12 



SIZE OF LITTER 



Fig. 72. — The average numbers of young surviving for 3 and 6 weeks from litters of 

 various sizes at birth in pigs. The straight "birth" Hue is the line of no loss. The 

 loss becomes disproportionately greater as the size of litter increases. The maximum 

 survival to 3 or 6 v^ceks old is given by litters of interniediate size at birth. (Data 

 for 3 weeks (as dots) from Johansson, 193 1, and for 6 weeks (as crosses) from 

 Menzies-Kitchen, 1937.) 



permanently, however (as sooner or later it presumably must) the 

 population camiot change genetically in the way necessary to give 

 a new adjustment: the variability is frozen in the homozygotic 

 potential form. Such a population, therefore, shows high immediate 

 fitness but no flexibility. The inbreeding system, advantageous so 

 long as the environment is stable, becomes a handicap when the 

 environment changes. 



A population witli an outbreeding system is in the other case. 



284 



