MEANS AND METHODS OF EVOLUTIONARY CHANGE 335 



blood cell substances A and B are present in varying proportions in different 

 human populations and in different species of apes. Being gene determined, 

 these substances afford excellent examples of the type of diversity we are 

 discussing. We noted much the same type of diversity in our brief discus- 

 sions of human races (pp. 250-255), and of the characteristics which dis- 

 tinguish Peromyscus leucopiis from Peromyscus maniculatus (p. 316). In 

 these latter instances the genetics of the differences has not been analyzed 

 as it has in the case of the blood groups, but we may feel confident that an 

 underlying diversity of genes (and of chromosome structure) forms the 

 basis for the observed diversity of adult characteristics. Much evidence of 

 this has been accumulated in animals whose genetic constitutions have been 

 more thoroughly analyzed than have those of man, apes, and Peromyscus. 



Interbreeding of Populations 



We have seen that much diversity is possible as a result of the shuffling 

 and recombination of different genes possessed by members of one popu- 

 lation. Suppose that in addition two different populations, having somewhat 

 differing genes, come into contact so that the members interbreed. Such an 

 occurrence will greatly increase the possibilities for diversity. Examples 

 spring to mind of individuals in such crossroads of the world as Hawaii 

 who exhibit a combination of characteristics inherited from, for example, 

 Polynesian, Japanese, and Irish ancestors. Indeed, on an earlier page we 

 surmised that interbreeding of originally separate human populations has 

 occurred repeatedly during the course of human evolution, and that the 

 occurrence explains much concerning the origin of observed diversity of 

 mankind past and present (pp. 246-248). 



Significance for Evolution 



Of what significance for evolution is this recombining of genes? In Chap- 

 ter 2 we noted that the positive aspect of natural selection consists of the 

 favoring of animals possessed of hereditary characteristics that are bene- 

 ficial, either in the environment in which the animal finds itself or in some 

 other environment open to it. The cumulative result of this favoring is post- 

 adaptation in the former case, preadaptation in the latter. While our earlier 

 discussions concentrated attention on single hereditary characteristics, we 

 readily appreciate that the same principles apply to combinations of charac- 

 teristics. Animals havino beneficial combinations of characteristics will be 

 favored by natural selection over those having less favorable combinations. 



