FLOATING AND FIXED DISCONTINUITY 



the sequences themselves. This is shown in Fig. 78, from which it 

 will be further seen that not merely are the inversions of this species 

 relatable to one another, but that they must be descended from an 

 ancestral type, now lost from pseudoohscura so far as is known, 



D.MIRANDA 



D. PSEUDOOBSCURA 



Olympic Oaxaca 



Estes Park 



Chiricahua IE 



-I 



Texas 

 T Arrowhead 

 Cuernavaca Pikes Peak 



D. PERSIMILIS 



Cowichan 



Standard 



Hypothetica 



Fig. 78. — The phylogenetic relationships of the third chromosome sequences of the 

 Drosophila species pseudoohscura, pcrsimilis and miranda. The hypothetical sequence is 

 closely resembled by that found in miranda. Standard is the only sequence which 

 occurs in two species: with its derivatives it floats in pseudoohscura but is fixed in 

 persimilis. 



The regional names associated with the sequences relate chiefly to the places 

 along the Pacific coast of North America where they were first found (after 

 Dobzhansky and Sturtevant, 1938). 



which was common also to the related species persimilis (once 

 called D. pseudoohscura, race B) and miranda. 



Each of these sequences in pseudoohscura is itself heterogenic, 

 that is to say it exists in genically diverse forms in nature, and 

 the different combinations within the sequence display relational 

 balance with respect to each other. Thus any one chromosome may 

 give rise to a poor, even a lethal, individual when homozygous; 

 but when heterozygous with another chromosome of the same 

 structural type, the resulting individual will be perfectly viable 



313 



