198 Increase in Number of Species 



factors could have become established. These factors might also 

 have been associated with physiologically beneficial characters on 

 which natural selection would operate positively, increasing the 

 rate at which the intersterility factors would have become intensi- 

 fied and established throughout the entire isolated population. If 

 after this evolution the segregates of the old range rejoined, the 

 result would logically be exactly what Laven described. If this 

 sequence is correct, then this mosquito complex is another example 

 of species fission by typical geographical isolation. 



Polyploidy 



Polyploidy is the one known mutational method by which a new 

 species can evolve in one step. Certain peculiar conditions must 

 occur, however, before a polyploid mutation can become a poly- 

 ploid population. Polyploidy results when whole new sets of 

 chromosomes become incorporated into the genome. This happens 

 when the chromosomes divide but the cell itself does not. Thus, in 

 an organism having a haploid or gametic number of five chromo- 

 somes, mutation during gametogenesis might give rise to gametes 

 having 10 chromosomes. When one of these gametes united with 

 a normal gamete, it would result in a 3n or triploid instead of a 

 normal 2n or diploid individual. These triploids might be viable 

 but, with only one or two known exceptions, are always highly 

 sterile. As a result most gametic polyploid mutations lead to no 

 permanent lineage. 



Although few in number, the triploid species of animals are of 

 great interest in that all of them are parthenogenetic. The list in- 

 cludes a dozen or more species of weevils, four species of black 

 flies, a few species of earthworms, and possibly a few others (White, 

 1954; Basrur and Rothfels, 1959). How the combination of par- 

 thenogenetic condition and triploidy may have arisen is a complex 

 and unsolved problem. It has been discussed by White, Basrur 

 and Rothfels, and by Stalker (1956). Triploids and other odd n 

 {5n, In) plants occur as isolated perennial indix iduals in many 

 groups, as in the ferns (Manton, 1950) but, as with the animals, 

 never form bisexual populations. 



If it should happen that the polyploid gamete united with a 

 sister polyploid gamete, a viable and fertile individual frequently 

 would result which would have 20 somatic chromosomes (4/j, or 

 tetraploid) instead of the parental 10 somatic chromosomes (2n, or 

 diploid). The chance of this type of union happening is almost 



