Races and the Origin of Species 



243 



figure 18—1. Distribution of inversion types 

 in D. pseudoobscura collected in the South- 

 western United States. (After Th. Dohzhan- 

 sky and C. Epling.) 



these inversion types show different adaptive 

 values in the laboratory, it is reasonably cer- 

 tain that they do so in nature too. Accord- 

 ingly, natural selection is primarily responsi- 

 ble for the inversion differences among the 

 three geographic populations, which can be 

 defined as three different races. 



Similar results have been obtained with 

 three California races of the cinquefoil plant 

 species, Potentilla glandulosa, which live at 

 sea level, mid-elevation, and the alpine zone. 

 The sea level race is killed when grown in 

 the alpine environment, whereas the alpine 

 race grown at lower elevations proves less 

 resistant to rust fungi than the lower-eleva- 

 tion races. Such experiments show that dif- 

 ferent races are adapted to their own habitats 

 but not to others. The inorganic and organic 

 environment — including its organisms — is 



different in different parts of the territory 

 occupied by a species. Clearly, then, no 

 single genotype will be equally well adapted 

 to all the different environments encountered 

 within a particular territory. One way in 

 which a cross-fertilizing species can attain 

 maximal biological fitness as a whole is to 

 remain genetically polymorphic and sepa- 

 rate into geographical populations or races 

 which differ genetically. 



Whenever, as in all of the examples dis- 

 cussed so far, different races of a cross- 

 fertilizing species occupy geographically sep- 

 arate territories, they are said to be allo- 

 patric; different races occupying the same 

 territory are said to be sympatric. In the 

 absence of geographical separation, what 

 factors operate to keep sympatric races from 

 hybridizing to become one race? One may 

 find the answer by considering the fate of 

 races — originally allcpatric — which have be- 

 come sympatric, a kind of change which has 

 occurred in man. Several thousand years 

 ago, mankind was differentiated into a num- 

 ber of allopatric races. With the develop- 

 ment of civilization and improved methods 

 of travel, many of these races have become 

 sympatric. Gene exchange in the now- 

 sympatric races, however, is sometimes in- 

 hibited by social and economic forces, so 

 that some of these races continue to main- 

 tain their identity. Domesticated plants and 

 animals provide another example of what 

 can happen when allopatric races become 

 sympatric. Many different breeds, or races, 

 of dogs originally allopatric are now found 

 living in the same locality. Yet these now- 

 sympatric races do not exchange genes with 

 sufficient frequency to form a single mongrel 

 breed, or race, because their reproduction 

 is controlled by man. It should be realized 

 that, under other circumstances, such allo- 

 patric races which become sympatric can 

 form a single polymorphic race via cross- 

 breeding. 



