Increase in Number of Species 157 



T. leivisi group to the highly speciahzed venereal species T. equiper- 

 diim. Observations made to the present indicate that this entire 

 development is the result of strictly apomictic evolution because no 

 sexual stages have ever been observed in this branch of the Protozoa. 

 Quite possibly similar series occur in simple plants, especially the 

 Fungi Imperfecti, but little seems to be known about the relation- 

 ships of these forms. 



In the plants the greatest pyramiding of apomictic species ac- 

 companies hybridization combined v^ith polyploidy. A few examples 

 of this combination occur in the weevils. In the weevil subfamilies 

 Otiorhynchinae and Brachyderinae numerous apomictic polyploid 

 species have evolved, including triploids, tetraploids, and penta- 

 ploids. It is probable that in these weevils a diploid parthenogenetic 

 species evolved first and that polyploidy followed as the result of 

 occasional fertilizations of the parthenogenetic eggs by sperms from 

 a male of one of the related diploid bisexual species (White, 1954). 

 The earthworms of the family Lumbricidae contain several short 

 polyploid series of species (White, 1954). 



In the plants, however, large numbers of hybrid and polyploid 

 apomictic species have evolved which together form morphological 

 and ecological "bridges" between the parental diploid sexual parents 

 (Stebbins, 1950). Such a situation has been investigated in detail 

 for many genera including Crepis, Poa, and Taraxacum (Babcock 

 and Stebbins, 1938; Stebbins, 1950). 



IN BISEXUAL ORGANISMS 



Because bisexual organisms form interbreeding systems in which the 

 whole population is the local genetic unit, an increase in the number 

 of species results from circumstances which do not affect apomictic 

 organisms. These circumstances include geographic isolation, hy- 

 bridization, ecological isolation, and spontaneous genetic isolation. 



Geographic Isolation 



The two essential features of species fission by geographic isolation 

 are (1) a species comes to occupy a divided or disjunct range in 

 which practically no genetic interchange occurs between the isolated 

 portions, and (2) the population of each isolated portion gradually 

 changes genetically until the population of one segment is no longer 

 genetically compatible with that of another segment. A divided 

 range may be initiated either by critical range movements caused 



