260 



RADISH 



FIGURE 29-3 (Right). Seed pods of 

 cabbage and radish, of their hybrid 

 and aniphiploid. After G. D. Karpe- 

 chenko.) 



AMPHIPLOID 



DIPLOID 

 HYBRID 



FIGURE 29-4 {Below). 

 Distribution o/ Delphinium 

 species in California. Each 

 species has a unique habitat. 



Similarly, crosses of gypsophilum with either 

 of its parent species are not as regular or 

 fertile as are those made between it and the 

 hybrid of the parent species. This com- 

 prises good evidence that gypsophilum arose 

 as the hybrid between recurvatum and 

 hesperium. Figure 29-4 shows the distribu- 

 tion of these species in California. 



A third way in which interspecific hybrids 

 can become stabilized as new species is by 

 introgression. In this process, a new type 

 arises after the interspecific hybrid back- 

 crosses with one of the parental types. The 

 backcross recombinant types favored by 

 natural selection may contain some genetic 

 components from both species. These re- 

 combinants may be true breeding and may 

 eventually become a new species. 



^ D. GYPSOPHILUM 

 D. RECURVATUM 

 D. HESPERIUM 



SUMMARY AND CONCLUSIONS 



The races within a cross-fertilizing species are characterized by the content of their gene 

 pools. Each of the different races is adapted to the territory in which it lives. The races 

 involved may be sympatric or allopatric. Races become species by the accumulation of 

 genetic differences whose end effect is genetic discontinuity — i.e., the formation of isolated 



