T. M. SONNEBORN 313 



well shown by the rapid selection of very diverse types within 

 a clone in Rhizopods and the selection of similar types from 

 initially different clones. Thus genetically simple morphological 

 differences as well as genetically simple physiological differences 

 are equally inadmissible as species differentials if species are to 

 represent equivalent evolutionary divergences in sexual and 

 asexual organisms. Many criteria now used in asexual organisms 

 to distinguish species are comparable to individual differences in 

 sexual organisms. 



Two major mechanisms of variation are recognizable in asexual 

 organisms. One is based upon mutation alone. It operates success- 

 fully in haploids that rapidly produce large populations. The 

 other works equally well in diploids. This involves having in the 

 genotype multiple loci for alternative, mutually exclusive, and 

 rapidly transformable traits. It is illustrated by the serotype sys- 

 tem in Paramecium and Trypanosomes. This mechanism operates 

 in purely asexual organisms, in sexual organisms with a well- 

 developed asexual phase, and in the asexual phase, that is, the 

 phase of development, in higher organisms. 



Speciation appears to be fundamentally the same in sexual 

 and asexual organisms. In both it is based upon genetic adapta- 

 tion to changing conditions. It may proceed more slowly and 

 less abundantly in some asexual organisms because of their 

 choice of relatively constant conditions of life, such as in a host 

 organism or in a considerable body of water. The asexual method 

 of reproduction has proved its adequacy if not its advantage 

 under certain conditions, by its widespread occurrence in nature. 

 It has means of rapidly spreading and multiplying advantageous 

 variants, and it avoids the wastage of recombination and the 

 swamping of variations which results from crosses. However, 

 short-term advantage is no guarantee of long-term persistence. 

 While some asexual organisms may be wiped out in times of 

 stress, others have succeeded on the evolutionary time scale by 

 adapting themselves to a constant or very slowly changing en- 

 vironment, such as life in a host. 



It is possible to adopt principles of classification into species 

 and syngens which are equally applicable to sexual and asexual 



