378 DIFFICULTIES AND IMPORTANCE OF THE CONCEPT 



genera where species hybridize freely with each other and 

 where introgression is a major factor. 



The role of isolating mechanisms for the species problem is 

 brought into sharp focus by a consideration of hybridization. 

 Moore has paid special attention to this side of the species prob- 

 lem in his discussion of the origin of isolating mechanisms (this 

 symposium). He concludes that they originate as a consequence 

 of the genetic differences which accumulate among isolated pop- 

 ulations during adaptation to local conditions. This thesis seems 

 well substantiated and corresponds indeed to my own analysis 

 of the situation. Yet part of his evidence may have to be inter- 

 preted in a different manner, namely, the significance of the 

 climatic races of Rana pipiens. Moore has shown that the various 

 geographic races of this species are adapted in their embryonic 

 development to prevailing local water temperatures. When indi- 

 viduals of a cold-adapted race are crossed with ones of a warm- 

 adapted race, a more or less inviable hybrid will result. Moore 

 concludes from this "that the northern and southern forms have 

 developed isolating mechanisms, in reference to each other, to 

 such a degree that they could coexist and remain distinct." I am 

 not convinced that this conclusion is warranted. It is quite pos- 

 sible, if not probable, that much of this local adaptation is purely 

 ecotypic and essentially reversible. If a warm climate race would 

 reinvade a cool climate, its developmental rates and temperature 

 tolerance would have to be modified by selection to permit sur- 

 vival in the cooler waters. If it should subsequently come into 

 contact with a local cool water race, it might hybridize with it 

 and produce harmonious viable zygotes. That tin's is not pure 

 speculation is indicated by the Rana pipiens population in the 

 mountains of Costa Rica, which in the cool waters has acquired 

 the developmental properties of Vermont frogs and when crossed 

 with them produces normal viable zygotes. Differences that are 

 purely ecotypic local adaptations arc not necessarily good isolat- 

 ing mechanisms because they tend to disappear as soon as the 

 environmental differences disappear. This does not preclude the 

 possibility of other incipient isolating mechanisms in previously 

 isolated populations. 



