EVOLUTION AS SEEN IN THE CLASSIFICATION OF ANIMALS 325 



substances in human blood cells (p. 121). As the profiles in Fig. 14.4 

 indicate, the three populations differed from one another in percentage of 

 individuals possessing each of the seven antigens. This is the same sort of 

 racial difference we noted in percentages of individuals belonging to the 

 different human blood groups. Whenever genetic tests are made a genetic 

 basis for cellular antigens is always disclosed. Hence we may feel confident 

 that Fig. 14.4 represents genetic difference between populations of wild 

 animals living only a few miles apart. 



Many other examples of microgeographic races might be cited (see 

 Dobzhansky 1950, p. 168 ft'.). Microgeographic races are of great interest 

 since they represent subpopulations of a type postulated as important in 

 the process of species formation (see speciation, pp. 484-491). 



Microgeographic races are not clearly distinguishable from geographic 

 races (subspecies). In fact the three populations of Columbia River valley 

 mice analyzed serologically may be considered to represent different sub- 

 species at least in part (Dice, 1949). Again we see in this difficulty in draw- 

 ing lines eloquent testimony to the fact of evolutionary change. 



Evolutionary Significance of Subspecies 



We have presented several contrasts between species and subspecies: 

 ( 1 ) Members of different species do not ordinarily interbreed when they 

 come into contact; members of different subspecies within one species ordi- 

 narily do so. (2) Different species frequently occupy separate territories, 

 but the latter may overlap, in which case intermediate forms are usually 

 not found; different subspecies occupy separate territories which do not 

 overlap, and if the territories come into contact intermediate or transitional 

 forms are frequently found. (3) Structural differences between species are 

 usually greater than those between subspecies. 



We should note, however, that exceptions are found to every one of the 

 above statements. In practice, then, it is frequently difficult to be sure 

 whether two groups of animals should be classed as belonging to two sub- 

 species in one species or whether they should be regarded as two distinct 

 species in one genus. Thus again the judgment of the individual biologist 

 comes into play, and with it opportunity for much disagreement in details 

 of classification. 



The principal interest in the subspecies or geographic race from the 

 standpoint of evolution lies in the fact that it seems to represent a small step 

 in the development of diversity. Most clusters of subspecies (Fig. 14.3) 

 probably arose when descendants from some parental stock migrated out 



