346 THE VIEWPOINT OF A PHYSIOLOGIST 



to the Baldwin effect, if a genetic change occurs in the direction 

 favored by selection, e.g., from Gi toward G2 in a population 

 already living near its limit with respect to that character, as in 

 P2, it is more likely to become fixed (step 2 above) than if such 

 a change should occur in a population far from this limit, as 

 Pi.* Conversely the genotype may become more limited. For 

 example, P2 may become genetically incompetent of acclimation 

 to Pi, and thus if these two populations are isolated by some 

 means they may become true species. There are many physiolog- 

 ical examples of Pi's and P2S but few physiological examples of 

 Gi's and G2's within single species. Distinction as to whether a 

 given difference between two populations is genetic or not can be 

 made only by acclimation or breeding experiments. 



Inadequacies of the Simplified Scheme of Speciation 



When one examines with care some of the physiological 

 analyses of populations and when one attempts field studies of 

 so-called races and related species and examines data from popu- 

 lation genetics, he begins to question the simple picture of specia- 

 tion which has been outlined above. None of the following 

 considerations is in itself decisive but in the aggregate they raise 

 serious doubts. 



1. Every serious systematist is impressed more by the diversity 

 than by the uniformity of his species. Mutants of many sorts are 

 carried at certain levels in most breeding populations. We read- 

 ily recognize individual diversity in man and domestic animals 

 without knowing much about similar diversity in natural popula- 

 tions. There is pronounced heterozygosity in virile strains, and 

 balanced polymorphism permits the maintenance of several phe- 

 notypes in one population. Tt is unlikely that genetic factors 

 related to susceptibility to disease and to longevity are found 

 in man only. 



' This docs not imply any causal relation between phenotype and genetic 

 change. Rather, individuals of P, would not be likely to move to the geo- 

 graphic range limit where the genetic change would be favored; pheno- 

 typically adapted individuals of P s would tend to survive, hence reproduce 

 better in respeel to the critical stress, and if selection pressure is strong, G, 

 would permit migration to new range limits. 



