124 INTRODUCTION TO EVOLUTION 



some lower mammals (e.g., chimpanzee) are uniformly distributed. That 

 is, all men, and all chimpanzees, possess them. Although as noted above 

 antigens A and B seem to be distributed among apes in somewhat this 

 manner, among human beings they are so variably distributed that the 

 brothers and sisters in one family may differ in possession of them. This 

 does not mean that their distribution is a haphazard matter. Far from it. 

 Antigens A and B follow rigidly the rules of inheritance first discovered 

 by Mendel in his experiments with garden peas and subsequently named 

 "mendelian" inheritance. In other words, these antigens depend upon the 

 genes. As mentioned above, the genes occasionally change, undergoing 

 mutation. The distinctive properties of substances A and B probably origi- 

 nated as mutations, as did the absence of either which characterizes peo- 

 ple belonging to group O. From what we know of rates of mutation in 

 lower animals we surely need not assume that the mutation producing O, 

 for example, occurred just once in evolutionary history, and that all ani- 

 mals and human beings inherited the lack of antigens A and B from the 

 one ancestor or ancestral group. Such a view might lead to the absurd con- 

 clusion that people who belong to group O are more closely related to 

 chimpanzees than are people who belong to group B (Table 6.1)! 



What, then, is the basic similarity between man and the other mammals 

 which is expressed in common possession of the blood group antigens? It 

 is a basic similarity of germ plasm, demonstrated by the fact that genes 

 possessed by man and at least his nearest relatives are so similar that 

 when they undergo mutation the products of the latter are identical or 

 closely similar. Here is another example of that parallel evolution al- 

 ready mentioned (p. 29). Doubtless such basic similarity of germ plasm 

 underlies all the morphological and serological similarities which we have 

 called homologies and interpreted as indicative of evolution. But in the 

 case of the blood groups the relationships between genes and their prod- 

 ucts (antigens A and B) are more direct and clearly evident than are rela- 

 tionships between genes and many of the other characteristics of animals. 

 Hence antigens A and B afford more distinct evidence of fundamental 

 similarities existing between germ plasms than is supplied by characteristics 

 whose genetic basis is not so thoroughly known. 



Other Antigens in the Red Blood Cells 



Antigens A and B are not the only antigens which have been identified 

 in red blood cells. Another pair of substances have been designated M and 

 N. These are similar to A and B in many ways although they are separate 



