218 I The Process of Evolution 



Latitude Locality 



46°N Quebec 



45 Vermont 



44 Wisconsin 



40 New Jersey 



30 Louisiana 



29 Ocala, Fla. 



27 EngL, Fla. 



32 Texas 



22 Mexico 



I I I I I I I I I 



5 10 15 20 25 30 35 40°C 



Embryonic temperature range 



Fig. 10.2 I Temperature tolerance ranges for normal embryological 

 development of Rana pipiens from difPerent localities. The lower limit for 

 Quebec and Wisconsin has not been determined but is believed to be 

 identical with Vermont. {From Moore, 1949, Evolution 3. ) 



Rana case, however, the distribution does not form a ring, and the 

 terminal populations of the series do not occur together in nature. 



The British satyrine butterfly Coenonympha tullia shows a pattern 

 of differentiation reminiscent of Rana. Crosses between individuals 

 from widely separated populations resulted in some broods in which 

 a number of "females" were intersexual, indicating some genetic 

 incompatibility. (This result follows Haldane's rule that inviability 

 or sterility in hybrids will most likely appear in the heterogametic 

 sex, in this case the females. ) In crosses between less distant popu- 

 lations no abnormalities were found. However, this butterfly has 

 not been as intensively studied as the gypsy moth, Lijmantria dispar, 

 for which Goldschmidt has described many degrees of intersexuality 

 in crosses between populations of various levels of differentiation. 

 For those interested in details, this work is well summarized by 

 Dobzhansky. 



The First Stages of Genetic Isolation. Populations of some ani- 

 mals that are connected by long series of intermediate forms may 

 occur together and remain distinct. For instance, an interesting 

 pattern of variation has been described in the plethodontid sala- 



