326 AN EMBRYOLOGIST'S VIEW 



of genomes that best fit the isolated populations for survival in 

 their own environments. Eventually some of the isolated popula- 

 tions may diverge from others to such an extent that one would 

 call them different species. 



Mayr (1948) has expressed these notions as follows: 



Wherever extrinsic factors cause a retardation or interruption of 

 gene flow between portions of a species (geographical or spatial iso- 

 lation), these subdivisions of the species tend to drift apart geneti- 

 cally. The rate of this change is different in different species. 



If the isolation is sufficiently complete and lasts sufficiently long, it 

 will permit the evolution of isolating mechanisms, which will inhibit 

 the interbreeding of the two daughter species after the elimination 

 of the extrinsic isolating factors. 



This is, of course, the now familiar concept of allopatric spe- 

 ciation. Many have regarded it as an adequate explanation of 

 the common pattern of evolution in terrestrial vertebrates and 

 probably in many other organisms as well. It is this position that 

 I will attempt to defend. On the other hand, Dobzhansky and, 

 to a lesser extent, a few others have thought it necessary to in- 

 voke an additional phase in the formation of species from geo- 

 graphic races. They do not believe that the species level of 

 divergence can be reached while the population remains sep- 

 arated and that a culminating sympatric stage is necessary for 

 the development of full species differences. The problem for 

 them is whether it is possible to obtain fully effective interpopula- 

 tion isolating mechanisms as a consequence of evolution in 

 allopatric groups. 



Thus, Dobzhansky (1940) develops his argument after first 

 asking the question ". . . whether isolating mechanisms develop 

 as a necessary consequence of the accumulation of genetic differ- 

 ences in general, or whether they represent a separate category 

 of genetic changes which appear and become established only 

 under certain special conditions." He believes that the second 

 possibility is the correct one, namely, ". . . that the origin of 

 isolation is a process separate from that of the origin of other 

 species differences." His reasons for so believing are as follows: 



