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CHAPTER 29 



These two races may continue to become 

 more and more different in their gene pools 

 because of mutation, natural selection of 

 genotypes which further increase adaptive- 

 ness, and genetic drift. As this differentia- 

 tion process continues, the genes which 

 make the two races adaptive in their own 

 territories may, via their pleiotropic effects, 

 make matings between the two races still 

 less likely to occur, or may cause the hybrids 

 of such matings to be less adaptive than the 

 members of either parent race. The greater 

 the degree of reproductive isolation, which is 

 an accidental or incidental by-product of 

 the different genotypes which are adaptive 

 in the two territories of the two races, the 

 greater would be the selective advantage of 

 other mutants which further increase the 

 reproductive isolation between the two races. 

 If the two races continued to diverge geneti- 

 cally in this way, they would eventually form 

 separate and different gene pools, at which 

 time they would have changed from two races 

 of the same species to two different species. 

 Note that speciation is an irreversible process, 

 for once a gene pool has reached the species 

 level, it can never lose its identity via cross- 

 breeding with another species. 



In this generalized account of how specia- 

 tion usually occurs, races have acted as 

 incipient species. But, you must also recall 

 that, under other circumstances, what are 

 two races may also often become a single 

 race. Thus, for example, while several thou- 

 sand years ago different allopatric popula- 

 tions of human beings were definitely dif- 

 ferent races which might have formed dif- 

 ferent species had the same conditions of life 

 continued, some of these races have subse- 

 quently merged into one race because of 

 civilization and migration. 



What are the principal means by which 

 races are known to become reproductively 

 isolated from each other? The barriers lead- 

 ing to complete reproductive isolation in- 

 clude the following: 



1. Geographical. Races may become sepa- 

 rated by water, ice, mountains, wind, 

 earthquakes, and volcanic activity. 



2. Ecological. The habitat of the two races 

 may be different or may become more 

 different than it was originally, because of 

 changes in temperature, humidity, sun- 

 light, food, predators, and parasites. 



3. Seasonal. In adapting to seasonal 

 changes, one race may become fertile at 

 a different time than the other, even if 

 their territories overlap, or if the races are 

 sympatric. 



4. Sexual or ethological. The races may 

 show preferences for intrarace mating. 

 (In domesticated forms this preference 

 may be decided upon by man.) 



5. Morphological. The sex organs of the 

 two races may be incompatible to various 

 degrees. 



6. Physiological. The sex cells of one race 

 may fail to fertilize those of the other, so 

 that the hybrid zygote is not formed at all, 

 or is formed infrequently. 



7. Hybrid inviability. Even when hybrid 

 zygotes are formed, their development 

 may be abnormal, so that they fail to 

 complete development. 



8. Hybrid sterility. Finally, even if hybrids 

 complete development and even if they 

 are hardy, they may be sterile. 



Environmental differences in geography, 

 ecology, or season which act to separate 

 races do not automatically produce geno- 

 typic differences among them. They do 

 furnish, however, the environmental varia- 

 tions which provide a means of selecting 

 genotypes which are adaptive, i.e., which 

 have the greatest reproductive potential 

 under the different conditions. Of course, 

 mutation must occur to provide the raw ma- 

 terials for this selection of more adapted 

 genotypes, and since no single genotype is 

 equally well adapted to all conditions, the 

 races will come to contain different geno- 



