Races and the Origin of Species 



257 



types. Completion of reproductive isolation 

 may be accomplished by the remaining bar- 

 riers listed. 



Reproductive isolation may be due genet- 

 ically to either genie action or chromosomal 

 behavior, or both. Thus, the many genes, by 

 which the two incipient species differ, may 

 produce seasonal, sexual, morphological, and 

 physiological barriers, as well as hybrid in- 

 viability. Although hybrid inviability is due 

 to developmental disharmony consequent 

 upon the presence of two genetically differ- 

 ent genomes in each cell, hybrid sterility may 

 be due, also, to an additional factor. The 

 two races may have become quite different 

 with respect to the arrangement of their 

 genes, by means of structural changes within 

 and between chromosomes, so that during 

 meiosis, synapsis between the two different 

 genomes is irregular. Failure of proper 

 pairing will mean that the segregation proc- 

 ess will be abnormal and the products of 

 meiosis aneuploid. You should recall that 

 aneuploidy in pollen is lethal, while aneu- 

 ploid gametes in animals usually produce 

 dominant lethality of the zygotes they form 

 at fertilization. 



Are morphological differences a good indi- 

 cation of species differences? It would seem 

 likely that the more divergent two forms are 

 morphologically, the more likely they are 

 to differ physiologically, and also the more 

 likely that these differences derive from very 

 different gene pools which are isolated from 

 each other. We would certainly expect, 

 simply from a comparison of horse and 

 mouse morphology, that these are different 

 species. However, when the two groups 

 being compared are more closely related in 

 descent, it is found that morphology is not 

 well correlated with reproductive isolation. 

 Thus, for example, European cattle and the 

 Tibetan yak are quite different in appearance 

 and are usually placed in different genera. 

 Yet these two can be crossed, and in 

 Tibet, many cattle have yaklike traits. On 



the other hand, consider D. persimilis and 

 D. pseudoobscura. These two species were 

 once considered races of the same species, 

 and are so similar morphologically that one 

 can differentiate between 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, and have originated from different 

 races of a single species in relatively recent 

 times. Sibling species have been found in 

 other Drosop/iila, mosquitoes, and other in- 

 sects, and have been found in plants, among 

 the tarweeds of the aster family, and in the 

 blue wild rye. 



D. pseudoobscura and D. persimilis illus- 

 trate two other principles relative to species 

 formation. Their study demonstrates that 

 any particular reproductive barrier usually 

 has a multigenic and/or a multichromosomal 

 basis, and that any two species are separated 

 not by one, but by a number of reproductive 

 barriers. Each of the barriers involved is in- 

 complete, but all together they result in 

 complete reproductive isolation, so that there 

 is no stream of genes between the two gene 

 pools in nature. In the particular case of 

 these two sibling species, the known barriers 

 include the fact that pseudoobscura lives in 

 drier and warmer habitats, and that females 

 accept the mating advances of males of their 

 own species more often than they do of males 

 of the other. Pseudoobscura usually mates 

 in the evening, persimilis in the morning; 

 when interspecific hybrids are formed, they 

 are relatively inviable, or, when viable, mostly 

 sterile. 



In the formation of new species from races, 

 the nature and origin of the reproductively 

 isolating mechanisms involved shows that 

 valid species do not originate by a single or 

 simple mutation, but arise as the result of 

 many different, independently occurring 

 genetic changes. Moreover, we have seen 



