Sni.nCTION AND VAUIAIUIITY 



previous exposure to natural selection in the homozygous state; 

 but the wholes, made up of these parts, are unlikely to be adjusted. 

 This is the more so because even the parts themselves must tend to 

 change in the flow of variability. 



Thus, forcing an unnatural inbreeding on an outbreeding species 

 will lead to the exposure of genotypes which, through the rarity 

 of their occurrence under the original system, will not have been 

 subjected to adjustment, by natural selection, in their action on the 

 phenotype. The result is maladjustment, or inbreeding depression. 

 It is contingent on inbreeding and must vanish when outbreeding 

 is resumed. 



Even where a heterozygote was made up from gene combinations 

 which were balanced when fully homozygous, inbreeding depression 

 must soon become a property of its descendants. Segregation and 

 recombination would ensure that, of the homozygotes which could 

 be extracted from such a heterozygote, those which went to its 

 making, or indeed any balanced combinations, would form but a 

 small fraction. The greater the number oi variable genes concerned, 

 the sooner and the more completely would this scrambling of the 

 genotype lead to the loss of the property of balance in homozygous 

 derivatives. 



Heterozygosity must obviously shelter recessive genes from 

 natural selection. Not only does this lead to the lack of balance of 

 polygenic combinations, liable to segregate as homozygotes, 

 in the way we have seen; deleterious mutant allelomorphs of 

 major genes, genes of drastic effect, should also survive under this 

 shelter. Plants and animals collected in the wild have remarkably 

 often proved to carry such genes when tested in the laboratory. 

 Indeed, on the basis of tests with Drosophila pseudoobscura, Dob- 

 zhansky estimates that at least one wild fly in four carries a mutation 

 causing lethality, or near lethality, when homozygous. 



The segregation of such sheltered recessive allelomorphs of major 

 genes will always follow inbreeding, but it is not, as has often been 

 supposed, the cause of the characteristic inbreeding depression. 

 Inbreeding does not produce a rising proportion of the more 

 deleterious of two alternative phenotypes in succeeding generations, 

 so giving an average decline only when viewed over the whole 

 generation. Rather it gives a steady and progressive decline of all 



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