Races and the Origin of Species 



255 



vation races. By means of such experiments 

 it was shown that each race is adapted to 

 the conditions of its habitat. 



In different parts of the territory occupied 

 by a species, there are different inorganic 

 and organic (including organisms) environ- 

 ments. It is clear, from what we have just 

 described, that no single genotype would be 

 equally well adapted to all the different en- 

 vironments encountered within this territory. 

 One way in which a cross-fertilizing species 

 may, as a whole, attain maximal biological 

 fitness is for it to remain genetically poly- 

 morphic and to become differentiated into 

 geographical populations or races which differ 

 from each other genetically. 



In all of the examples discussed so far, the 

 different races of cross-fertilizing species 

 have occupied geographically separate terri- 

 tories, and are said to be allopatric. Some- 

 times, however, different races may be 

 sympatric, that is, they may be found in the 

 same territory. In the absence of geographi- 

 cal separation, what factors operate to keep 

 sympatric races from hybridizing to become 

 one race? We can look for the answer to 

 this from studies of the fate of races, origi- 

 nally allopatric, which have become sym- 

 patric. Man offers one example of this kind 

 of change. Several thousand years ago, 

 mankind was differentiated into a number of 

 allopatric races. Since then, the develop- 

 ment of civilization, and improved methods 

 of travel, have made such races sympatric in 

 part. But gene exchange in the now-sym- 

 patric races may be prevented by social and 

 economic forces, so that some of these races 

 may retain their identity. Domesticated 

 plants and animals offer another example of 

 what may happen when allopatric races be- 

 come sympatric. Consider the case in dogs. 

 Many different breeds, or races, which 

 were originally allopatric, may now all be 

 found living in the same city. Yet these 

 now-sympatric races do not exchange genes 

 with sufficient frequency to form the single 



breed, or race, called mongrel, because their 

 reproduction is controlled by man. It should 

 be realized that, under other circumstances, 

 allopatric races, which become sympatric, 

 may form a single polymorphic race via cross- 

 breeding. 



A species of cross-fertilizing organisms 

 usually consists of a number of races adapted 

 to the different environments in which they 

 are found. All these races are kept in genetic 

 continuity by interracial breeding and hybrid 

 race types, so that the species, as a whole, has 

 a single gene pool within which no portion 

 is completely isolated from any other. How 

 does one species differ from another? Let 

 us restrict our attention to cross-fertilizing 

 species. We shall require that two groups of 

 organisms be genetically discontinuous from 

 each other in order to be considered different 

 species. (This means that each species has, 

 in effect, a gene pool which is so isolated from 

 the gene pool of another species, that neither 

 the one nor the other can lose its identity via 

 cross-breeding, or backcrossing subsequent 

 to cross-breeding.) In addition, the gene 

 pools of different species must be isolated 

 from each other for genetic, and not merely 

 environmental, reasons. 



New species have arisen during the course 

 of past evolution, and since evolution con- 

 tinues today, new species are forming at pres- 

 ent also. By what mechanisms do new 

 species of cross-fertilizing individuals arise? 

 The mechanism considered most common is 

 the production of two or more species from 

 a single species. How could this come about? 



Suppose different portions of the same 

 species become somewhat independent of 

 each other reproductively, that is, they form 

 different populations, and, while there is 

 some interbreeding, most of the breeding is 

 intrapopulation. In the course of time, the 

 two populations may diverge genetically so 

 that each is adapted to its own territory. We 

 might now want to call these two popula- 

 tions different races of the same species. 



