CHROMOSOMAL MUTATIONS 



FiGLTRE 84. Pairing in Salivary Gland 

 Chromosomes Heterozygous for In- 

 versions. Top row, a single inversion; 

 second row, two independent inversions; 

 third row, two inversions of which the 

 shorter is included within the larger; bot- 

 tom row, two overlapping inversions. 

 (From Dobzhansky, "Genetics and the 

 Origin of Species," 3rd Ed., Columbia 

 University Press, 1951.) 



ABCDE F 



IEDCBF. . 



ABCDEFGHI 



AEDCBFGHI 



lEZ 



AEDCBFHGI 



ABCDEF 



AEDCBF 



'I* 



AECDBF 



ABCDEFGHI 

 AEDCBFGHl 



IT" 



aemgFbcdi 



ABCDEF 



ABCDEFGHI 

 AEOCBFHGl 



ABCDEF 

 AECDBF 



ABCDEFGHI 

 AEMGFBCDI 



Epling have described race B as a distinct species, D. persimilis, reserving 

 the original name for race A. 



Phylogeny of the various local populations of this group has been 

 studied by means of the analysis of inversions. When two inversions are 

 present in the same chromosome, it is possible that they may be independ- 

 ent (ABEDCFGJIHKL), or the second may be included within the first 



(ABC JIHEFGDK L), or they may be overlapping (AB GFIHCDEJ KL) 



( Figure 84 ) . The last type is of especial interest because here the order of 

 events can be determined. Thus, if there are three arrangements known 

 for a particular chromosome, (1) ABCDEFGHIJKL, (2) AB GFEDC HI- 



JKL, and (3) AB GFIHC DEJKL, it is obvious that either (1) or (3) could 



have been derived from the other only with (2) as an intermediate step. 

 Most of the chromosomal variability in the pseudoobscura group occurs 

 in chromosome III. Actually, there are thirteen different arrangements of 

 this chromosome which are known only in D. pseudoobscura, seven which 

 are known only in D. persimilis, and one, called Standard, which is found 

 in both species. By detailed analysis of overlapping inversions, starting 

 with the assumption that Standard was the primitive arrangement, it has 

 been possible to work out a nearly complete phylogeny for this group 

 (Figure 85). Only a single gap occurs, between Standard and Santa Cruz, 

 a variety of pseudoobscura. But even this gap is not entirely unfilled, for 

 the necessary intermediate chromosomal pattern is found in the closely 

 related species D. miranda. 



223 



