Chapter 29 



RACES AND 



THE ORIGIN OF SPECIES 



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'n cross-fertilizing species, differ- 

 ent individuals in a population are 

 .heterozygous for different genes 

 (see Chapter 28). This is true even though 

 the gene pool is at equilibrium with the fac- 

 tors which cause shifts in gene frequency — 

 namely, mutation, selection, drift, and migra- 

 tion. In other words, in reaching genetic 

 equilibrium, all the members of cross- 

 fertilizing populations do not eventually be- 

 come homozygotes, nor do they all become 

 heterozygotes. Such populations, therefore, 

 do not become genetically either pure or 

 uniform with the passage of time. 



While any given population is polymorphic 

 for some genes, this does not mean that it is 

 necessarily polymorphic with regard to a 

 particular gene. So, for example, Indians in 

 South America are all of O blood type, being 

 homozygous (/ /) in this respect, but they 

 have a polymorphic pool with respect to 

 other genes. Moreover, an allele like P, for 

 example, may be rare or absent in one pop- 

 ulation, as is true in certain North American 

 Indians, while it may be relatively frequent 

 in the gene pool of another population, as 

 in central Asia. Thus, populations located 

 in different parts of the world may differ both 

 in the types and frequencies of alleles which 

 they carry in their gene pools. For many 

 purposes it is desirable to identify a popula- 

 tion with certain gene pool characteristics as 

 a race. 



In certain studies the investigator may wish 

 to define races only in terms of the distribu- 

 252 



tion of the P gene for ABO blood type. It 

 would then be perfectly reasonable and valid 

 to define as different races, populations that 

 do or do not contain /^ in their gene pool. 

 On this classification, there would be only 

 two races of man, the South American 

 Indian (without P) and all other people 

 (with P in their gene pool). 



In another study, however, it may be de- 

 sirable to define races on the basis of the 

 prevalence of / and P in the population. 

 The distribution of these genes in the gene 

 pool has been studied extensively in popula- 

 tions all over the world. The results show 

 that in western Europe, Iceland, Ireland, and 

 parts of Spain, three fourths of the gene 

 pool is /. However, as one proceeds east- 

 ward from these regions, this frequency 

 decreases. The opposite tendency is true 

 for P. In fact, in a world map, P is most 

 frequent in central Asia and some popu- 

 lations in India, and becomes less and less 

 frequent as one proceeds away from this 

 center. Since the change in frequency of P 

 is also a gradual one, it is not possible to 

 draw lines on the map which would separate 

 people into groups with sharply different 

 gene frequencies. So, where these lines are 

 drawn, and how many are drawn, are arbi- 

 trary matters, as a consequence of which, 

 more or fewer races will be defined. 



Not only are the genes for ABO blood 

 groups useful in characterizing races, but so 

 are other blood traits whose genetic basis is 

 understood. It is actually valid, in defining 

 races, to utilize differences in any trait so 

 long as these are based upon genetic dif- 

 ferences. So, for example, one can consider 

 certain genetic differences in color of hair, 

 eyes, and skin, and differences in stature and 

 head shape, in delimiting races. Our knowl- 

 edge of genetics should caution us, however, 

 that the use of phenotypic criteria only may 

 be quite unsatisfactory for classifying races. 

 For the environment itself can cause pheno- 

 typic differences (see Chapter 1), and the 



