350 



THE CHANGING GENERATIONS 



has extended the area or areas available to a population. When such new 

 territories must be entered by way of narrow highways, or across partial 

 barriers, the pioneer invaders will carry only a limited part of the gene 

 diversity of the original population, and the barrier or restricted connec- 

 tion will tend to limit the spread of all subsequent mutations formed 

 either in the parent population or in that descended from the immigrants. 



SAN CRISTOBAL 



VELLA LAVELLA fiT ^^ 



RENDOVA 



Fig. 23.4. The distribution of the geographic races (subspecies) of the golden whistler 

 (Pachycephala pectoralis) of the Solomon Islands. The races are kept distinct principally by 

 geographic isolation, which also permitted their original differentiation. In the bird figures 

 barred is the symbol for green, gray for yellow. {Modified from Dobzhansky, by permission 

 Scientific American.) 



The changes that result from such isolation are neither necessarily 

 permanent nor irreversible. There is ample evidence that changing geo- 

 graphic barriers or highways may first isolate parts of populations and 

 later bring them once more together. Such changes may occur on both 

 major and minor scales; microgeographic changes are constantly happen- 

 ing. What kind of change or how great a change is necessary to bring 

 about isolation or reunion of populations depends on the physiological re- 

 quirements and ecological limitations of the species population concerned. 



Although geographic isolation can and does provide an efficient mecha- 

 nism for speciation in itself, it appears also to be of great importance in 



