C. L. PROSSER 363 



as important as the study of the relations between genes and 

 enzymes. 



Conclusions 



Examination of physiological characters, particularly by stress 

 tests, can provide much useful information about variation in 

 natural populations. When populations of animals which differ 

 markedly in their responses to a given stress are properly accli- 

 mated to each other's environment, the differences often are seen 

 to be nongenetic. In fact, the functional lability of animals is very 

 great. Part of this individual adaptability is behavioral, part oc- 

 curs at the organ system level, part of it is cellular, and part may 

 result from enzyme induction. 



Some examples of genetic fixation of adaptive variations in 

 mutant strains are known, for example, insecticide-resistant in- 

 sects. Variants may become established as ecotypes or races, par- 

 ticularly in clinal situations; as an example, animals with different 

 temperature dependence of rate functions. More commonly, 

 when adaptive physiological differences between two popula- 

 tions are genetically established, the differentiation is sufficient 

 for reproductive isolation; that is, speciation occurs. Many sibling 

 species or morphologically similar species, which differ in physio- 

 logical adaptability, are recognized, and stress tests applied to 

 populations would probably reveal numerous natural species. In 

 summary, most functional variation among animal populations 

 appears to be either nongenetic or specific; relatively little is 

 racial. 



Neutral morphological variations are sometimes genetically 

 carried with physiological characters by linkage or pleiotropism. 

 Also, neutral characters may provide the basis for distinguishing 

 typological species. Most adaptive functional characters of ani- 

 mals appear to be multifactorial; adaptation resides not in single 

 genes but in the balanced genotype; the compensatory reactions 

 to deviations from a "normal" physiological state make recogni- 

 tion of small genetic changes difficult. 



There is urgent need for two kinds of research in the area of 

 physiological variation. First, there is need for more sensitive 



