56 INTROGRESSIVE HYBRIDIZATION 



of these ways, its positive pull back to the original recom- 

 bination is stronger. It is, therefore, more effective to think of 

 linkage as a factor of racial and specific cohesion rather than as 

 a barrier between species and between races. 



The continuing effect of linkage, generation after genera- 

 tion, is suggested in Fig. 13. With self-pollination there is 

 a strong tendency to return to the original parental com- 

 binations of characters. Within the recombination spindle, 

 there is in the F2 zero heterozygosity at either end, rising to 

 50 per cent in the middle. Therefore, recombinations like 

 the original parents tend to reproduce themselves, whereas 

 intermediate ones segregate. Were there no linkage this 

 segregation would radiate equally in all four directions from 

 each heterozygote. Linkage causes the segregation to be 

 much greater in the direction of the recombination spindle. 

 Figures 14 to 17 show the populations to be expected upon 

 self-fertilization of certain F2 types. In each case, it will 

 be noted, the recombinations of the F3 are oriented in the 

 general direction of the F2 recombination spindle and, like 

 it, have their greatest frequencies along the center of the 

 spindle. The combined effects of (a) restriction to the re- 

 combination spindle and (b) the comparative heterozygosity 

 of forms resembling the Fi would be to increase in subsequent 

 generations the proportions of individuals rather similar to, 

 or identical with, the original parents. Backcrossing would, 

 of course, greatly accelerate this tendency. Although these 

 calculations are based upon what would happen with self- 

 fertihzation, all other forms of inbreeding would cause the 

 same general result but at a slower rate. With continuous 

 cross-pollination, in small populations, for instance, the in- 

 breeding caused by the population size would eventually 

 have the same effect. 



We therefore conclude that the cohesive force of linkage 

 would be more apparent in the F3 and succeeding genera- 

 tions than they had been in the F2. The restriction upon 

 types of recombinations would persist and would be joined 

 by the effect of linkage upon frequencies. The combination 



