CHROMOSOMES AND EVOLUTION 99 



regions being present in the tetraploid condition. It 

 should eventually be possible to ascertain the diploid- 

 tetraploid ratio in different species of Drosophila. 

 The evolutionary significance of duplications is, as 

 Bridges points out, that they offer ' a method of 

 evolutionary increase in length of chromosomes with 

 identical genes which could subsequently mutate 

 separately and diversify their effects '. It has already 

 been pointed out that mutations which may be lethal 

 when homozygous in a diploid are often quite viable 

 when present twice in a tetraploid. Thus mutations 

 will be more likely to establish themselves in the 

 duplicated part of the chromosomes than in the 

 remainder. The diploid-tetraploid ratio may thus 

 be one of the factors determining the rate of evolution 

 in a species or group. Perhaps the shark Scaphano- 

 rhynchus owstoni which has existed unchanged from 

 the Upper Cretaceous to the present day has almost 

 no tetraploid regions in its chromosome set, and 

 perhaps duplications may be only very rarely pro- 

 duced in some groups (or be almost always lethal, 

 which comes to the same thing). 



Formation of Species in the Genus Drosophila 



The genus Drosophila has now been investigated 

 sufficiently for it to be possible to make some definite 

 statements as to how new species have arisen (the 

 problem of species-dichotomy as distinct from the 

 problem of species -differentiation). 



In the first place the chromosome number varies 

 considerably from species to species (Table X). Those 

 species with a high proportion of rod-shaped chromo- 

 somes have the highest numbers, those (like D. 

 ananassae) with all the chromosomes V-shaped have 

 the lowest numbers. But it is clear from the Table 

 that not all the changes in chromosome number can 

 be accounted for by fusion of rods to form V's and 

 vice versa ; even if we assume that all V's have two 

 spindle attachments there is still a variation in 



