E. MAYR 379 



Different Levels of Speciation in Different Local Populations 

 of the Same Polytypic Species. The amount of genetic diverg- 

 ence may differ in various isolated populations of a polytypic 

 species. This inequality in the level of speciation takes different 

 forms. Particularly spectacular are the cases of circular overlap, 

 an increasing number of which are cited in the literature. Other 

 cases are sympatric species, which are completely distinct at cer- 

 tain localities but hybridize freely at others. Lorkovicz ( 1953) has 

 suggested broadening the term semispecies (Mayr, 1940) to 

 include all isolated populations which on the basis of some cri- 

 teria have reached species level, but not on the basis of others. 

 Grant (this symposium) has made a similar suggestion to 

 broaden the application of the term semispecies. All these diffi- 

 culties demonstrate the fact that the clear-cut alternatives of the 

 nondimensional situation are absent in a multidimensional sys- 

 tem. 



The species indicates a discontinuity above the level of the 

 individual, but a new difficulty is introduced by the fact that 

 there may be several such discontinuities, not all of them being 

 species. If we designate as isolate any more or less isolated popu- 

 lation or population segment, we can distinguish in sexually 

 reproducing organisms between geographical, ecological, and 

 reproductive isolates of which only the latter are species. Among 

 asexually reproducing organisms every clone and, in fact, every 

 individual is an isolate. Here, obviously, the species and the iso- 

 late can even less be synonymized with each other than in 

 sexually reproducing organisms. 



Asexuality and the Species Problem 



The essence of the biological species concept is discontinuity 

 due to reproductive isolation. Without sexuality this concept 

 cannot be applied. Asexuality then is the most formidable and 

 most fundamental obstacle of a biological species concept. In 

 truly asexual organisms there are no "populations" in the sense 

 in which this term exists in sexual species nor can "reproductive 

 isolation" be tested. Students of the species problem have neg- 

 lected asexual situations for a number of reasons. The geneticist 



