486 



INTRODUCTION TO EVOLUTION 



As the subpopulations develop genetic differences contributing to repro- 

 ductive isolation, they are also developing genetic differences connected 

 with the varying living conditions which face them. That is, they are be- 

 coming adapted to somewhat different habitats and environmental condi- 

 tions. If living conditions are similar, the subpopulations may be expected 

 to develop similar adaptations even while they are accumulating differ- 



VELLA LAVELLA 



RENDOVA 



FIG. 21.1. Geographic races of the golden whistler iPachycephala pecforalis) on vari- 

 ous of the Solomon Islands. The races differ in black, white, and colored markings. 

 Dark gray areas represent green markings, light gray areas yellow markings. (Redrawn 

 from Dobzhonsky, "The genetic basis of evolution," Scientific American, Vol. 182, 

 1950, p. 41.) 



ences in, for example, plumage and in neutral or nonadaptive character- 

 istics. If living conditions are dissimilar, on the other hand, the adaptations 

 acquired by subpopulations will lead to greater differences between them. 

 What will happen when two such subgroups expand their ranges and 

 come into contact with each other? Because of the reproductive isolation 

 developed they will not interbreed, but to the extent to which they are 

 adapted to the same conditions of life they will come into competition with 

 each other. What will happen as a result of this competition? (1) One 



