THE CONCEPT Of EVOLUTION 



701 



take place without mutation, and although natural selection does not 

 create new characteristics it plays an important part in determining 

 which of them shall survive. 



Isolation. The differentiation of a new group of organisms requires 

 that they be prevented from breeding with their relatives and in 

 this way passing to them whatever new genes have appeared. Interbreed- 

 ing must be prevented by some sort of isolation. 



Perhaps the commonest type of isolation is geographic, whereby 

 groups of related organisms become separated by some physical barrier, 

 a sea, mountain, desert, glacier or river (Fig. 34.1). In mountainous 

 regions the individual ranges provide effective barriers between the 

 valleys, and there are usually a greater number of different species in a 

 given area than in a comparable area of the plains. For example, 

 twenty-three species and subspecies of rabbits are known in the moun- 

 tains of the western United States but only eight species are found in 

 the larger plains area of the Midwest and East. Valleys only a short dis- 

 tance apart, but separated by ridges perpetually covered with snow, may 

 each have species of plants and animals not found in the other. One of 

 the most striking examples of geographic isolation is provided by the 

 area divided by the Isthmus of Panama. On either side of the Isthmus 

 the phyla and classes of marine invertebrates are made up of different 



Nissan 



Bougainville 



Malaita. 



Sa.n Cristobal 



Figure 34 1 The distribution of the subspecies o£ the golden whistler (Pachy- 

 cephala pectoralis) in the Solomon Islands. A major factor in the evolution of these 

 subspecies has been their geographic isolation on separate islands. Green-colored 

 plumage is indicated by cross-hatching; yellow by light gray tone. (Modified from 

 Dobzhansky.) 



