Races and the Origin of Species 



253 



same phenotypic result may be produced by 

 different genotypes because of gene inter- 

 action in dominance and epistasis (see 

 Chapter 7). 



It should be reemphasized that the num- 

 ber of races recognized is a matter of con- 

 venience. For some purposes it may be ade- 

 quate to separate mankind into only two 

 races, while for others, as many as two 

 hundred have been recognized. Most an- 

 thropologists usually recognize about half a 

 dozen basic races, or define about thirty, 

 when finer details of some populations are 

 to be considered. Regardless of the number 

 of races defined, however, each is best 

 characterized in terms of the genes it con- 

 tains. Since the people in a population are 

 either A, B, AB, or O in blood type, and there 

 are no intermediates, there is no average 

 genotype for ABO blood group. Nor is 

 there an average genotype for any other 

 polymorphic gene. Because there is no 

 average genotype for a population, a race 

 must be defined in terms of the relative fre- 

 quency of alleles contained in its gene pool, 

 and since there is no average genotype for a 

 race, there is also no average phenotype. 

 You see, therefore, the invalidity of trying 

 to picture a typical (average) member of any 

 race. 



A knowledge of the distribution of genes 

 for ABO blood types in diff'erent popula- 

 tions the world over provides important 

 information to geneticists, anthropologists, 

 and other scientists. To what can the dif- 

 ferent distributions be attributed? Since 

 people do not choose their marriage partners 

 on the basis of their ABO blood type, and 

 since there does not seem to be any pleio- 

 tropic eff"ect which makes the possessor of 

 one blood type sexually more attractive than 

 another, we may conclude that mating is at 

 random with respect to ABO genotype. 

 There is some evidence, however, that in 

 other respects not all ABO genotypes have 

 the same biological fitness. It is possible 



that diff'erential mutation can also explain 

 part of the diff"erences in distribution. But, 

 the greatest shift in ABO gene frequencies 

 in different populations in the past few thou- 

 sand years has probably been the result of 

 genetic drift and migration. In fact, the 

 paths of past migrations have been traced, 

 utilizing the gradual changes in the fre- 

 quencies of ABO and other blood group 

 genes in neighboring populations. 



It has already been mentioned (p. 235) that 

 different paracentric inversions are found in 

 natural populations of D. pseudoobscura. 

 Even so, all of these flies found in nature are 

 very similar phenotypically, even though they 

 differ with respect to their chromosomal 

 arrangements. This fly is common in the 

 southwestern part of the United States 

 (Figure 29-1), and different populations 

 located there have been sampled to detect 

 the relative frequency of these inversions.^ 

 It is found that California populations are 

 rich in the inversion types called Standard 

 and Arrowhead. Eastward, in nearby Ari- 

 zona and New Mexico, the populations con- 

 tain very few Standard chromosomes and 

 have the Arrowhead arrangement almost 

 entirely. Finally, in still more easterly 

 Texas, there are no Standard, some Arrow- 

 head; most chromosomes are of Pikes 

 Peak type. 



The shift in the frequency and type of in- 

 versions in different geographic regions can- 

 not be explained as the result of differential 

 mutation, since the spontaneous rate at 

 which inversions arise is extremely low. 

 Moreover, there is no reason to believe that 

 the gene flow among these populations has 

 changed appreciably in the recent past, so 

 that migration rates have probably had a 

 relatively small influence upon genotypic 

 frequencies; nor is there any reason to 

 attribute to genetic drift a major role in 

 causing the differences in inversion frequency 



* Based upon work of Th. Dobzhansky and col- 

 laborators. 



