J. A. MOORE 337 



one another where they are sympatric than elsewhere. In no ease 

 that I know of is there good evidence to indicate that selection 

 against the formation of ill-adapted hybrids is involved. The 

 greater differences between the species in the area of overlap 

 might be due to greater genetic divergence resulting from com- 

 petition (as has been discussed before) or to still other factors. 

 There are also some reports of overlapping and closely related 

 species showing less genetic isolation where they are sympatric 

 than elsewhere. Volpe (1955) has described a case involving 

 Bufo americanus and B. fowled, and he refers to similar cases in 

 other organisms. 



Summary 



The pattern of evolution that involves the formation of new 

 species from geographical races is discussed. The conclusion is 

 reached that the most important aspect of this process is genetic 

 divergence associated with adaptation to the local environment. 

 It is felt that this divergence will involve differences that will 

 also serve as isolating mechanisms. There is no critical evidence 

 that isolating mechanisms develop as ad hoc contrivances that 

 prevent hybridization between incipient species. Furthermore, 

 there are some theoretical considerations that make it improb- 

 able that the genetic differences that serve as isolating mecha- 

 nisms commonly arise in this manner. 



REFERENCES 



Dobzhansky, T. 1940. Speciation as a stage in evolutional) divergence. 

 Am. Naturalist, 74, 312-21. 



Koopman, K. F. 1950. Natural selection for reproductive isolation be- 

 tween Drosophila pseudoobscura and Drosophila persimilis. Evolu- 

 tion, 4, 135-48. 



Mayr, E. 1948. The bearing of the new systematies on genetical prob- 

 lems. The nature of species. Advances in Genetics, 2, 205-37. 



Moore, J. A. 1946. Incipient intraspecific isolating mechanisms in Rana 

 pipiens. Genetics, 31, 304-26. 



