GENETIC BASIS 45 



cence. With such a relationship a sUght increase in color 

 will always be accompanied by a slight increase in hairiness. 



In Fig. 8 is represented the kind of result that is caused 

 by introgression. In such a population color intensity and 

 pubescence tend to go together but the relation is not ab- 

 solute. Numerous pairs of individuals could be picked out 

 in which one is very much darker than the other, but no 

 more pubescent or perhaps even a little less so. Similarly 

 one could select pairs in which the more pubescent plant was 

 no darker or possibly even a httle hghter. For the popula- 

 tion as a whole, however, there is a very clear tendency for 

 the darker plants to be the hairier, for the hairier to be the 

 darker. It is also clear that on the whole the lighter plants 

 are more glabrous and the most glabrous plants are Hghter 

 colored. 



If both characters, as in this h^i^othetical illustration, 

 are multifactorial, the only possible explanation for such a 

 population is introgression. Darkness is due to many genes ; 

 heavy pubescence is due to many genes. On the whole these 

 two sets of genes tend to occur together. If, as in Fig. 6, 

 darkness and pubescence were both highly variable but were 

 not correlated, then we could explain the high variabihty 

 as due to any one of several causes that make for genie vari- 

 ability (high mutation rates, population pattern, etc.). If, 

 however, they are both variable and both multigenic, then 

 we would have to assume that gene changes affecting pubes- 

 cence tended to be accompanied by gene changes affecting 

 color intensity. No such kind of multidirection mutation is 

 known. 



If species differed only by two such characters as these, 

 the abihty to prove introgression from population analysis 

 alone, though it would rest on a sound theoretical basis, 

 would be too tenuous to be convincing. Species, however, 

 differ in a large number of ways. In the population examples 

 of Iris diagrammed in detail in Chapter 6 there was an as- 

 sociation between redness of corolla and size of sepal which 

 indicated introgression. In these same populations, how- 



