Increase in Number of Species 155 



( 1 ) In some cases, an asexual mutant may arise from a normally 

 sexual parent. Such action probably explains the origin of the cu- 

 rious oligochaete species Enchytraeits fragmentosiis which repro- 

 duces only by fragmentation (Bell, 1959). In organisms having an 

 alternation of sexual and asexual generations, a strain might lose the 

 sexual generation and so become an apomictic species. The try- 

 panosomes in the Protozoa probably arose in this fashion. Examples 

 are much commoner in the plants, in which many species scattered 

 through many families persist and reproduce only vegetatively by 

 runners, underground stolons, bulb division, or other means. In a 

 large proportion of these plants, the apomictic species originated 

 as an intersprecific hybrid individual which retained its vegetative 

 viability but which was infertile. Manton (1950) has found that the 

 European Eqtiisetiim litorale, E. trachyodon, and E. Moorei belong 

 to this category. The most conspicuous cases reported in nature are 

 polyploids. The lily Fritillaria camscliatcensis is a tetraploid which 

 reproduces only by offsets from its bulbs. Many sterile grass poly- 

 ploids reproduce only by underground stolons or runners. 



(2) A truly remarkable array of mechanisms have evolved which 

 produce apomictic species by the complete suppression of normal 

 fertilization. In some cases meiosis never occurs in gametogenesis; 

 in other cases the products of meiosis may re-unite before the re- 

 duced cells divide, thus restoring the 2n chromosome number; in 

 others the early cleavage nuclei unite in pairs to bring about the 

 same result; in still others the egg unites with a polar body to restore 

 the chromosomal complement. These and other ways of circumvent- 

 ing cross-fertilization are described by Stebbins ( 1950 ) and White 

 (1954). To this category belong the greatest number of existing 

 apomictic species. 



Splitting of Apomictic Species 



In apomicts having only mitosis in their life histories, theoretically 

 the occurrence of a single, distinctive, viable mutation would result 

 in a new homozygous type. In apomicts having meiosis plus chromo- 

 some number restitution, there might be a possibility for gene re- 

 combinations; hence the ehmination of heterozygotes by natural 

 selection might be necessary for the formation of a distinctive spe- 

 cies. Although the splitting of apomictic species seems so simple, 

 in general only a few cases occur here and there among the various 

 taxonomic groups. In many instances either only one species in the 

 genus is obligatorily apomictic, as is the sawfly Diprion polytomum. 



