THE ORIGIN OF VARIATION 



A A^ ^\ If ^ '^ ^ 



^ CC ^mi^ >w? - ., -^^ ^ ** ^ sophila pinicola, D. mel- 



^^^ jV ^^' 1 1 '^^ ^li^^ anogaster, D. texana, AND 



^\k^ U ^11'^ |l D. pfl/usim. (From White, 



Figure 83. Chromosome 

 Sets of Males of Dro- 



pinicola melanogaster texana palustris "Animal Cytology and 



Evolution, 2nd Ed., 

 Cambridge University 

 Press, 1954.) 



of species like palustris and melanogaster is incorporated into one of the 

 three large chromosomes of pinicola. 



In Drosophila, mutations iDased upon all four types of chromosomal re- 

 arrangements are known. Examples of mutant effects of duplications and 

 of translocations have been discussed above. Notch wing is a well known 

 sex-linked mutation based upon a deficiency near the left end of the X 

 chromosome. Curly and dichaete wings are based upon inversions of the 

 second and third chromosomes, respectively. Goldschmidt and others have 

 found that about half of the classical mutant genes of Drosophila are in 

 fact position effects. Of the other half, many have not been adequately 

 examined, and it is to be expected that a good portion of them will also 

 pro\'e to be position effects when they are properly studied. This leads to 

 the question: Are the remaining mutants really bona fide genes in the 

 sense discussed in Chapter 13, or are they also position effects of rear- 

 rangements which are too small for detection by present methods? Gold- 

 schmidt has taken the latter position, while the majority of geneticists 

 have tried to reconcile the facts of position effect with the classical theory 

 of the gene. 



That closely related species may differ by chromosomal rearrangements 

 of the types discussed above is well established, as in Makino's loaches, 

 which are distinguished by two translocations. Sciara ocellaris and S. reij- 

 nolchi (fungus-gnats) have been shown to differ by a number of small 

 deficiencies and duplications. White and others have shown that in many 

 insects for which salivary gland chromosomes are not available differences 

 between the chromosome complements of related species are nonetheless 

 most easily interpreted in terms of several types of rearrangements. This 

 cannot, of course, preclude the possibility that gene mutation might also 

 play a role, even a predominant role, in the differentiation of these species. 



Overlapping Inversions and Phylogeny. By far the most complete 

 study of chromosomal differences separating natural races and species is 

 that of Dobzhansky and his collaborators on Drosophila pseudoohscura 

 and its allies. As originally described, the pseudoohscura group comprised 

 D. pseudoohscura and D. miranda, two closely similar species. But D. 

 pseudoohscura was subdivided into races A and B on the basis of a series 

 of differences so refined that they can usually be distinguished only on 

 the basis of statistical analysis of populations. These differences, difficult 

 though they may be to diagnose, are nonetheless highly consistent, and 

 they include intersterility. After years of detailed study, Dobzhansky and 



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