RESTRICTIVE PRACTICES 



genes which are so desirable as to make further recombination 

 undesirable — at least for the time being. 



Thus, wherever there exists an advantageous combination of 

 genes, selection must favour any device which restricts the effective 

 recombination of its constituents. These devices are of various 

 kinds, and they do their work in various ways. 



Restrictive Practices 



Recombination is effective only to the extent that the individual 

 is heterozygous. The evil effects of recombination will not therefore 

 be felt if heterozygosity can be eliminated, or at least reduced so 

 that it does not endanger the basic advantageous combinations. 

 We have already seen a great deal of restriction of inbreeding : in 

 fact outbreeding itself is, in one sense, the result of restricting 

 inbreeding. We now have to meet the opposite — a restriction of 

 outbreeding. Restriction of outbreeding helps to preserve favourable 

 combinations of genes. It may arise from either environmental or 

 genetic causes, and it may be either partial or complete. 



The partial restriction of an individual's freedom of outbreeding 

 is described as isolation. Mating is unhampered within the group 

 but prevented between isolated groups. This restriction may be 

 imposed by geographical means, as in the case of the different 

 species of finch which Darwin found on each of the Galapagos 

 islands. Or it may be by genetic means, that is to say by the develop- 

 ment of gene or chromosome differences preventing mating, 

 pollination, or fertilization, such as we have already seen at work 

 in Petunia species. Geographical isolation cannot, of course, be under 

 the control of the organism, and it is therefore in a sense accidental. 

 Nevertheless such accidental isolation, particularly where it is in- 

 complete may, as we shall see, favour the rise of genetic isolation. 



In the last chapter we have seen that the balance of gene 

 combinations is brought about by the action of selection. In 

 wild populations the poor combinations are eliminated and the 

 advantageous combinations favoured. Now, in outbreeding organ- 

 isms, many combinations are possible for every one which is 

 advantageous, and only the test of natural selection can lead to the 

 differential ehmination of those that are less favourable. If they are 



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