Increase in Number of Species 199 



zero if it relies on several polyploid gametes being produced as 

 spontaneous mutations by different parents of opposite sexes of 

 the same species at the same time and place. 



In contrast, a somatic polyploid mutation could produce a whole 

 polyploid organism or fruiting structure. If this were hermaph- 

 roditic, it would produce polyploid gametes of both sexes in close 

 proximity, and if this individual were also capable of self-fertiliza- 

 tion, the stage would be set for the production of a potentially 

 viable and fertile polyploid population. Such a population would 

 immediately comprise a new species because it would form an 

 independent phylogenetic line, genetically compatible within itself 

 but rigidly isolated genetically from its original parent. 



The occurrence of polyploidy in species reproducing bisexually 

 bears out the role of chance as has just been outlined. The infre- 

 quent cases of polyploid species in bisexually reproducing metazoan 

 animals are confined to a few groups which are hermaphroditic, 

 chiefly the turbellarian flatworms and the leech and earthworm 

 groups of the Annelida. Even in some of these examples (White, 

 1954) there is considerable doubt as to the polyploid nature of the 

 species. In many of the turbellarians, for example, the gametic 

 chromosome numbers for related species are two, four, five, and 

 eight, or two, three, and six, or three and six. This suggests strongly 

 that the species with four and eight chromosomes are tetraploid and 

 octoploid, respectively, and those with six are tetraploid, but it does 

 not prove it. The added chromosomes could be the result of other 

 processes. There seems to be no doubt concerning the tetraploid or 

 at least polyploid nature of some earthworms belonging to the 

 family Lumbricidae. White (1954) suggested that they may have 

 arisen originally as parthenogenetic polyploids which later became 

 bisexual, but this again is only a surmise. Gates (1952) suggested 

 that several mammals are polyploid, but White attacked the 

 reliability of his circumstantial reasoning. 



In the plants, examples of polyploidy are legion and emphasize 

 the importance of this type of evolution in this branch of the biotic 

 world. Two factors appear to be the basis of this diff^erence between 

 plants and animals. In the plants, hermaphroditic structures 

 (monoecious organisms) are the rule rather than the exception, and 

 in many groups infertile hybrid individuals may reproduce vegeta- 

 tively for many years. The combination of both conditions occurring 

 together should give the greatest proportion of polyploid types. 

 Stebbins (1950) pointed out that such is partially the case because 

 the highest incidence occurs in the perennial monoecious herbs 



