THli BREAKDOWN OF CONTINUITY 



Restriction and Flexibility 



Recombination is important in discovering combinations of genes 

 which remove the need for further recombination under existing 

 conditions. The restriction of recombination is important, therefore, 

 in stabihzing present fitness. Yet in the same paradoxical way, these 

 restrictions reduce the recombination which is necessary for pros- 

 pective readjustment when conditions change, as sooner or later 

 they presumably must. Devices restricting recombination must there- 

 fore be viewed not merely in the light of the immediate fitness 

 which they will preserve. They must also be examined for their 

 effects on genetic flexibility, for on these effects will depend the 

 long-range success of the stocks or races which carry them; and 

 simultaneously, of course, the success of the devices themselves in 

 evolutionary liistory. 



Inbreeding and apomixis restrict recombination by the abolition 

 of heterozygosity and of the sexual cycle respectively. They abolish 

 the very means on which genetic adjustment depends, and as we 

 have seen they are dead ends in evolution. 



The isolation of races, whether by geographical or genetic means, 

 restricts recombination only between those genes which are of 

 immediate importance in giving good local adaptation. Cross- 

 breeding still occurs within each isolated group and the use of 

 any residual heterogeneity, or any new heterogeneity arising by 

 mutation, for adaptive adjustment is not precluded. 



The same principle of checking recombination while leaving the 

 way open for later expansion applies with inversion, as we have 

 seen, and also with interchange, provided that the system has not, 

 as in Oenothera, become adapted to inbreeding. When an Oenothera 

 stock has reached the top of the ladder with its ring of 14 chromo- 

 somes it can go no further; but this need not be true of the floating 

 interchange system of Campanula which has not become tied down 

 by inbreeding. 



Polyploidy is in a similar case. If it has arisen as a short cut to 

 a rigid inbreeding system, the polyploid must be doomed by this 

 rigidity. Probably most polyploids have a relatively short life for 

 this very reason. But, in so far as any crossbreeding has been 

 retained, and in so far as there occurs mutation or recombination 



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