FINITE POPULATIONS 59 



have an effect upon population dynamics. The overall re- 

 sult of these various external and internal barriers seems to 

 be exactly that. It permits a surprising amount of gene flow 

 between well-differentiated species and races, without on 

 the other hand allowing these species and races to lose their 

 identity. 



Among the forces producing species and races, linkage is of 

 particular importance because of its complete universality. 

 It results from the fact that all germplasms are made up of 

 long chainlike proteins. It is, therefore, an always present 

 force. When by any process, accidental or otherwise, the 

 gene differences between two strains become 3 or more in any 

 chromosome region that ordinarily has no more than 1 

 chiasma, it begins to operate. Linkage may, therefore, pro- 

 vide the necessary initial isolation that allows other internal iso- 

 lating mechanisms to accumulate under the action of natural 

 selection. 



As an example of the way in which linkage might take the 

 lead in building up specific or racial isolation, let us return to 

 our hypothetical leaf shapes and pubescences in Fig. 9, 

 where there are 4 linked genes. Had these differences arisen 

 gradually in a large population, with active cross-breeding, 

 they might have been distributed independently of each 

 other in the population so that all the combinations of leaf 

 shape and pubescence illustrated in Fig. 9 could have been 

 represented. Suppose that in some way the population was 

 decimated and that the only survivors happened to be the 

 extreme ovate-glabrous type of the lower left-hand corner 

 (0/0) and the extreme pubescent-obovate one of the upper 

 right (4/4). Linkage alone would be a strong enough force 

 so that if these two strains came together again it would be 

 difficult, even with strong artificial selection, to reconstitute 

 all the eliminated types. Exactly what would happen would 

 depend upon the relative numbers of the two surviving 

 strains, and the breeding structure of the population. With- 

 out extremely strong selection away from such a condition 

 they would tend to make a population with 2 centers of vari- 



