344 THE VIEWPOINT OF A PHYSIOLOGIST 



and orienting behavior, particularly in selection of an optimum 

 region in a gradient, for example of humidity or temperature, 

 may be of use in distinguishing races. Food selection, based on 

 taste "preferences," is partly established habit by "imprinting" 

 and partly based on genetic difference. 



In general, the best examples of physiological differences from 

 population to population come from genera which have a wide 

 ecological distribution and which live in transitional environ- 

 ments. Care must be taken in selecting appropriate clines or 

 circles and in separating ecotypes from sibling species. Also there 

 may be interaction among several environmental factors, and it 

 must be remembered that animals react to a total environment 

 although certain factors may be more important than others for 

 a species. 



Allopatric Speciation 



The commonly assumed sequence of allopatric speciation of 

 sexually reproducing animals may be summarized as follows: 



1. Phenotypic variation, both morphological and physiological, 

 occurs within populations of genetically similar individuals. Part 

 of this variation follows the normal distribution curve of a popu- 

 lation in an environment; part of the phenotypic variation is the 

 result of acclimation. 



2. Primary adaptive variation becomes genetically fixed by 

 natural selection of randomly occurring genetic changes. Muta- 

 tions or chromosomal rearrangements are carried in an inter- 

 breeding population and genetic strains become established. 



3. In a restricted environment or clinal situation ecotypes or 

 races become established. There may be gene flow between 

 races, but in one ecotype a given gene frequency may exceed 

 that in another ecotype. 



4. Sufficient isolation, restrictive with respect to gene flow, 

 provides for the fixation of a selected genotype; such isolation 

 may occur between ends ol a eline or in insular populations, i.e., 

 isolated by water or by unfavorable terrain surrounding breeding 

 sites. During isolation other differences become established, dif- 

 ferences whieh result in reproductive separation of the popula- 



