ISOLATING MECHANISMS AND SPECIES FORMATION 



Droso phila mplano fl aster 



Droso phila artificialis 



FictTRE 106. Translo- 

 cations OF Drosophila 

 artificialis with Re- 

 spect TO D. melano- 

 gaster. (After Kozhev- 

 nikov, from Dobzhan- 

 sky, "Genetics and the 

 Origin of Species," 1st 

 Ed., Columbia Univer- 

 sity Press, 1937. ) 



gametes 



gametes 



do serve an important function in isolating related populations one from 

 another. Kozhevnikov performed a particularly interesting series of exper- 

 iments on this problem. By combining in a single stock of Drosophila 

 melanogaster two translocations between the second and third chromo- 

 some, he has obtained a strain which he regards as a synthetic species, 

 Drosophila ariificialis (Figure 106), D. artificialis forms four types of 

 gametes, which combine to form sixteen types of zygotes. However, only 

 four of these survive, as the other twelve contain large deficiencies and 

 duplications. It is completely isolated from the parent species, D. melano- 

 gaster, for all zygotes formed by the "interspecific" cross are inviable, 

 because of the large deficiencies and duplications. But a "species" with 

 only 25 per cent viability would not be likely to fare well in nature. If, 

 however, an additional rearrangement should stabilize this genotype, so 

 that it would become viable in the homozygous condition, a good species 

 would for all practical purposes have been synthesized in the laboratory, 

 by means of a fortunate combination of a few chromosomal rearrange- 

 ments. 



While much progress has been made in identifying and classifying 

 isolating mechanisms, and while they have been successfully fitted into 

 the neo-Darwinian theory as a fundamental part of the mechanism of 

 evolution, yet very little progress has been made in analyzing the genetic 

 basis of isolating mechanisms. The chromosomal rearrangements are an 

 outstanding exception to this statement even though their exact role in 

 evolution is not clear. Yet everyone can agree with Dobzhansky that 

 ". . . there can be little doubt that chromosomal changes are one of the 

 mainsprings of evolution." * There is, however, a very general feeling 

 among geneticists that isolating mechanisms and phenotypic differentia- 



* Op. cit. 



289 



