360 THE VIEWPOINT OF A PHYSIOLOGIST 



ulations are behavioral differences, particularly the behavior asso- 

 ciated with reproduction. For example, six similar species of the 

 willistoni group of Drosophila show mating habits so characteris- 

 tic that no interspecific mating occurs (Spieth, 1947, 1949). Two 

 sibling species of solitary wasps Ammophila campestris group, are 

 distinguished mainly by their mode of opening the burrow and 

 providing prey for their larvae (Adriaanse, 1947). Eastern and 

 western American meadowlarks can be distinguished by their 

 calls,* as can European chiffchaffs, several species of crickets, and 

 also two southern frogs in regions of overlap ( Blair, 1955 ) . These 

 behavioral traits could act as primary isolating characters after 

 becoming established by genetic drift as in "island" populations, 

 or they may secondarily separate species in which the initial iso- 

 lating agents are no longer active. 



Mechanisms of Nongenetic Variation of the Phenotype 



A stress physiologist is continually impressed by the phenotypic 

 lability of individual organisms. In regulating animals, acclima- 

 tion shifts the environmental limits for a stable internal state. In 

 conforming animals, acclimation raises or lowers the tolerable 

 internal variation or introduces some compensating mechanism, 

 for example, metabolic adaptation in poikilotherms. Since pheno- 

 typic variation may be important in initial stages of speciation 

 and since numerous population differences turn out to be non- 

 genetic, it is important in a discussion of the species problem to 

 point out possible mechanisms of phenotypic adaptation. 



In describing any change induced in an individual by environ- 

 mental stress, the time during which the change occurs must be 

 precisely stated. For example, mammals exposed to cold show 

 initial constriction of surface blood vessels and piloerection. Next 

 they may shiver and then increase the activity of thyroid and 

 adrenal cortex, and they may maintain elevated metabolism lor 

 weeks. Long-range adaptation, however, consists of increased in- 



A recent paper (Lanyon, W. E., 1956. Ecological aspects of the sym- 

 patric distribution of meadowlarks in the north-central states. Ecology, 37, 

 98-108) suggests that the primary mechanism isolating the eastern and 

 western species of meadowlark is indirectly related to moisture, particu- 

 larly spring precipitation. 



