Races and the Origin of Species 



245 



3. Seasonal. Seasonal changes may cause 

 different races to become fertile at differ- 

 ent times even if their territories overlap, 

 or if they are sympatric. 



4. Sexual or ethological. Intrarace mating, 

 due to preference or domestication ef- 

 fected by man. 



5. Morphological. Incompatibility of the 

 sex organs between some races. 



6. Physiological. Failure of a race's sex 

 cells to fertilize those of another, so that 

 the hybrid zygote is formed infrequently, 

 or not at all. 



7. Hybrid inviability. Even when formed, 

 the development of hybrid zygotes may 

 be so abnormal that it cannot be com- 

 pleted. 



8. Hybrid sterility. A possibility even if 

 hybrids complete development and are 

 hardy. 



Although geographical, ecological, and sea- 

 sonal differences do not automatically initi- 

 ate genotypic differences, they furnish the 

 environmental variations which select from 

 the available genotypes those which are 

 adaptive; that is, those with the greatest re- 

 productive potential under the given condi- 

 tions. Of course, mutation must provide 

 the raw materials for natural selection; since 

 no single genotype is equally well adapted 

 to all conditions, different races come to 

 contain different genotypes. The remaining 

 barriers listed can complete reproductive 

 isolation. 



The many genes by which two incipient 

 species differ may produce seasonal, sexual, 

 morphological, and physiological barriers. 

 Hybrid inviability may result from develop- 

 mental disharmony caused by the presence 

 of two genetically different genomes in each 

 cell. Although hybrid sterility can be caused 

 by such genetic action, it also results when 

 two races become quite different with respect 

 to gene arrangement — because of structural 



changes within and between chromosomes — 

 so that during meiosis, synapsis between the 

 two different genomes in the hybrid is irreg- 

 ular. Improper pairing causes abnormal 

 segregation, which results in aneuploid meio- 

 tic products. Recall that aneuploidy in pol- 

 len is lethal, and that aneuploid gametes in 

 animals usually result in dominant lethality 

 of the zygotes they form. Consequently, 

 reproductive isolation can be based upon 

 either genetic activity or chromosomal be- 

 havior, or both. 



It seems reasonable that the more morpho- 

 logically divergent two forms are, the more 

 likely it is that they will differ physiologically 

 and that these differences will have orig- 

 inated in very different and isolated gene 

 pools. Simply by comparing horse and 

 mouse morphologies, one certainly expects 

 them to be different species; thus the occur- 

 rence of morphological differences is some- 

 times a good index of a species difference. 

 However, when the groups being compared 

 are closely related in descent, one finds that 

 morphology is not well correlated with re- 

 productive isolation. For example, Euro- 

 pean cattle and the Tibetan yak are quite 

 different in appearance and usually are 

 placed in different genera, but these two 

 species can be crossed. Moreover, in Tibet, 

 many cattle have yak-like traits, so that 

 widely different phenotypes do not neces- 

 sarily result in complete reproductive isola- 

 tion between closely related species. On the 

 other hand, consider D. persimilis and D. 

 pseudoobscura. These two species — for- 

 merly considered races of the same species 

 — are so similar morphologically that they 

 can be differentiated by their genitalia only 

 if very careful measurements are made. 

 Nevertheless, these two species have com- 

 pletely isolated gene pools in nature, even 

 where their territories overlap. Such mor- 

 phologically similar species are called sibling 

 species. They originated from different 



