T. M. SONNEBORN 291 



a breeding test. In sexual organisms such divergence is associated 

 with complex and discontinuous genetic differences. In asexual 

 organisms also, such differences may be used as the sign of ir- 

 reversible divergence. 



To discover whether such complex discontinuities exist within 

 a taxonomic species of asexual organisms, one would have to 

 make detailed comparative studies of many strains of the species 

 obtained from throughout its range. The comparisons would have 

 to include studies of the life cycle, cytology, morphology, physiol- 

 ogy, and ecology. Many features would have to be examined 

 experimentally under the same conditions to distinguish pheno- 

 typic from genotypic variation. As with "sibling species," syn- 

 gens discovered within an asexual species would not differ in 

 readily identifiable traits. If there were closely related sexual 

 species, the genetics of traits that differ in the asexual strains 

 should be studied in the sexual relatives. In short, every possible 

 means of arriving at a sound judgment about the complexity of 

 the genetic differences and their discontinuities would have to 

 be utilized. This would be a great labor; but great labor is also 

 involved in the analysis of a sexual species into its syngens. The 

 results of such studies on the species of Paramecium, summarized 

 in this paper, represent the work of 20 years by an increasing 

 group of investigators and the work is far from complete. 



The concept of an asexual syngen as having passed the thresh- 

 old of irreversible evolutionary divergence, but not having 

 reached readily recognized differentiation, is thus the exact 

 equivalent of the sexual syngen and refers to the same level of 

 biological organization. In both cases, this level is the one which 

 embraces all the individuals that can potentially contribute to the 

 further evolution of the group. Thoday (1953) has presented a 

 comparable point of view. The evolutionary unit is as distinct and 

 isolated in asexual organisms as it is in sexual organisms, no more 

 and no less so. The syngen is sharp and definite in many out- 

 breeders; it becomes fuzzier and fuzzier the more inbreeding is 

 adopted. Similar differences in its distinctness may be anticipated 

 among asexual organisms. The difference between syngens in the 

 two cases is not in the concept or in its correspondence with 



