Levels of Divergence in Drosophila Speciation

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Abstract
Several investigators have found marked variations in the genie structure of different wild populations of D. melanogaster. The work of Dobzhansky and Sturtevant on the evolution of chromosome inversions in the 3d chromosome of D. pseudoobscura is reviewed. A discussion of interfertile races and subspp. includes in the order of increasing divergence D. affinis affinis and D. a. iroquois, D. athabasca athabasca and D. a. mahican, D. hydei hydei and D. h. yucatanensis, and D. macrospina macrospina and D. m. ohioensis (new subsp.). At the level of partial inter-subsp. and hybrid sterility 2 cases are presented. D. virilis virilis and D. v. americana differ in salivary and gonial chromosome structure, and several morphological and physiological characters. In crosses they show marked incompatibility. The hybrids of both sexes are partially fertile. D. lattivittata and a new and undescr. form living in the same Ohio swamp habitat differ by numerous morph. characters and show a high degree of incompatibility. However, the hybrids of both sexes are partially fertile. At the level of complete hybrid sterility in one sex, the [male], are the 2 races of D. pseudoobscura and D. miranda. Crosses of D. melanogaster and D. simulans, D. azteca and D. athabasca, and D. algonquin and D. athabasca give a few hybrids, sterile in both sexes. Thus Drosophila contains numerous examples of various levels of evolutionary divergence and offers unusual opportunities for the study of speciation.