BREEDING SYSTEMS, REPRODUCTIVE 

 METHODS, AND SPECIES 

 PROBLEMS IN PROTOZOA* 



T. M. SONNEBORN:f Indiana university, 



BLOOMINGTON, INDIANA 



The so-called modern biological species concept defines a species 

 as a group of actually or potentially interbreeding populations. 

 As the other papers in this symposium attest, this concept is 

 widely accepted as the only valid species concept. Yet it is of 

 admittedly limited applicability. In obligatorily self-fertilizing 

 organisms, this concept of the species contracts to include but a 

 single individual. In exclusively asexual organisms, the concept 

 admits of no species at all. On this concept, perhaps half or more 

 of the species recognized by students of the Protozoa are invalid 

 and, in principle, cannot be modified so as to become valid. In 

 like manner, practically every phylum of Invertebrates and all 

 major plant groups contain numerous organisms in which species 

 do not exist at all or in which each individual is a separate spe- 

 cies. This greatly limits the applicability of the modern biological 



Note: Superior numbers in text refer to Addenda, page 314. 



* Contribution #618 from the Department of Zoology, Indiana Uni- 

 versity. 



f The work of the author and his collaborators, much of which has not 

 previously been published, was supported by grants from Indiana Uni- 

 versity, the Rockefeller Foundation, the American Cancer Society, and the 

 Atomic Energy Commission (Contract No. AT ( 11-1) -235 ) . The following 

 people have critically read part or all of this manuscript: Mary L. Austin, 

 G. H. Beale, T. T. Chen, Ruth V. Dippell, A. C. Giese, L. Gilman, E. Han- 

 son, Mrs. Margaret Johnson, R. F. Kimball, E. Mayr, D. L. Nanney, Ursula 

 Philip, J. R. Preer, and R. Siegel. I am immensely indebted to these experts 

 for checking the accuracy of my statements, for generously providing me 

 with unpublished information and permission to cite it, and for numerous 

 criticisms and suggestions. 



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