16 INTROGRESSIVE HYBRIDIZATION 



new recombinations, if the pairs of genes that fit 2 differ- 

 entiated species each to its own distinctive habitats are no 

 more than 20, the F2 of this species cross w^ill require over 

 1,000,000 kinds of habitat. With no more gene differences 

 than 10 or 20, surely a conservative figure, they therefore 

 require an impossibly large number of adjacent habitats if 

 the recombinations are to be as well fitted to their situations 

 as the parental species were to their 2. 



As a crude example, let us consider the adjacent habitats 

 in which one finds Tradescantia subaspera and Tradescantia 

 canaliculata at home in the Ozark Plateau. The former 

 grows in deep, rich woods at the foot of bluffs; the latter 

 grows up above in full sun at the edge of the cliffs. We can 

 list 3 of the outstanding differences between these 2 habitats 

 as follows : 



rich loam rocky soil 



deep shade full sun 



leaf-mold cover no leaf-mold cover 



Tradescantia canaliculata and Tradescantia subaspera are 

 well-differentiated species; each is more closely related to 

 several different species than to the other. Still, experiment 

 has shown not only that they can be crossed readily by 

 artificial means but also that they do cross abundantly when 

 left to themselves in an experimental garden. Yet very few 

 of the first-generation hybrids have been found in nature. 

 The habitats of the 2 species are strikingly different in the 

 Ozarks. There one seldom finds the intermediate habitat 

 in which the hybrid is able to germinate and survive : This is 

 a gravelly soil, partial shade with some bright sunlight, and a 

 light covering of leaf mold. Imagine, however, the habitat 

 that must be pro\dded if we are to find in nature the second- 

 generation recombinations which we obtain in the breeding 

 plot. Making the example fantastically simpler than it 

 really is and assuming that the 3 differences noted above are 

 due to only 3 single-factor differences, we would find that 

 our recombinations would even then require the following 6 



