THE ORIGIN OF SPECIES 



validity of the concept. First, it is difficult to distinguish between cases 

 in which organisms cannot interbreed, and those in which they can but 

 do not for other reasons. Thus, the closely similar species Drosophila 

 pseudoobscura and D. persimilis do not interbreed in nature because of 

 a genuine sterility barrier. But the pariah and the Hindu do not inter- 

 breed, although there is undoubtedly no sterility barrier to prevent it. 

 Sterility barriers may exist within an undoubtedly single species, as in 

 the case of Lijmantria dispar in which racial crosses may result in hybrids 

 which are sterile because of intersexuality. Finally, there are many cases, 

 especially among plants, in which crosses between well recognized species 

 can be made easily. Frequently, the fertility of such crosses, or of the Fi 

 from them, is impaired partially, even greatly. But such cases make it 

 difficult to maintain that sterility is an absolute criterion of species dif- 

 ferentiation. 



Nonetheless, interspecific sterility remains the common element of most 

 definitions of species. Dobzhansky has defined species as the stage in the 

 evolutionary process "at which the once actually or potentially interbreed- 

 ing array of forms becomes segregated in two or more separate arrays 

 which are physiologically incapable of interbreeding." Goldschmidt stated 

 that groups which can be successfully crossed should be treated as a single 

 species for evolutionary studies, while acknowledging that taxonomists 

 may be justified on other grounds in separating them into distinct species 

 for purposes of formal classification. Mayr* has recently defined species 

 "as a group of actually or potentially interbreeding populations that is 

 reproductively isolated from other such groups," and again, "in the strict 

 sense of the word, speciation means the origin of reproductive isolating 

 mechanisms." This concept of species based upon gene flow between re- 

 lated populations is often called the biological species concept. It has 

 strong logical appeal, especially now that the successes of genetics in 

 analyzing the problems of evolution are so striking. Sonneborn, however, 

 has recently protested against it on two grounds. First, it is difficult to 

 apply it to more than a few of the more thoroughly studied organisms 

 because of the sheer volume of experimental study required for its ade- 

 quate application. And second, it is not applicable to those many species 

 of Protozoa, lower Metazoa, and plants which reproduce asexually or par- 

 thenogenetically. While some biologists have held that species in these 

 organisms are not comparable to those of sexual organisms, Sonneborn 

 makes a good case for the proposition that species in these groups are also 

 based upon accumulation of genetic differences under the control of nat- 

 ural selection, and hence that a species concept which excludes them is 

 unsound. 



Because of the above described situation, including the difficulties of 

 delimitation of species among asexually reproducing groups, many biolo- 

 gists have come to the conclusion that the species is an arbitrary unit in 

 the same way that the higher categories are. Yet many biologists feel that 



** Mayr, E., "Taxonomic Categories in Fossil Hominids," Cold Spring Harbor Sym- 

 posia on {hiant. Biol., 15, 112 and 11.5 ( 1950). 



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