BALANCE IN IIOMOZYCOTES AND IIETEROZ Y GOTES 



members of the population. Such a steady decline is to be expected 

 from the increasing homozygosity for numerous polygenic 

 combinations within each individual. The averaging process is 

 within the individual rather than within the population. 



Inbreeding an outbreeder consistently leads to inbreeding 

 depression: complete homozygotes are poor. Outbreeding an 

 inbreeder has no such effect. The heterozygotes are seldom very 

 different in vigour and fertility from their homozygous parents. 

 Indeed, crossing different lines has not uncommonly given 

 heterozygotes of somewhat greater vigour than either parent, for 

 example, in the diploid cultivated tomato, and in the tetraploid 

 wild Galeopsis tetrahit. 



The reason for this difference is apparent immediately we consider 

 the flow of variability. In an outbreeder this flow is strong and, 

 even were a heterozygote to be made from combinations balanced 

 in the homozygous condition, these would soon be destroyed 

 by the constant crossing, segregation and recombination. In an 

 inbreeder, on the other hand, the flow of variability is weak or 

 non-existent. Combinations, once established, remain in being, 

 apart, of course, from mutation. If, when they were established, 

 these combinations gave an adequately balanced heterozygote, as 

 well as adequately balanced homozygotes, they would retain this 

 property. 



Now the tomato is known to be derived from outbreeding 

 ancestors in Peru. Inbreeding is similarly a recent imiovation in 

 Galeopsis tetrahit, whose diploid ancestors were outbreeders. It is 

 therefore to be expected that the combinations, now homozygous 

 in the tomato and in G. tetrahit, will have retained the capacity for 

 givmg balanced heterozygotes. There is a reduction of advantage 

 in vigour and fertility of heterozygote over homozygote. But this 

 is due to the grading up of the homozygotes, by the selection ot 

 balanced homozygous combinations, not to the grading down of 

 heterozygotes, by loss of balance through shelter from selection. 

 Heterozygous or relational balance can only be produced by the 

 action of selection in an outbreeder. It is nevertheless retained in the 

 absence of selection when inbreeding freezes, as we may say, the 

 flow of variability. Balance presumably can be lost in such a case 

 through mutation; but loss by nmtation must be a slow process 



293 



