Cytology and Phylogeny of Drosophila

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Abstract
The rediscovery of the polytene chromosomes in the salivary glands of the Diptera has allowed for a high order of accuracy in the identification of chromosomal rearrangements. Of these, paracentric inversions have been the most common and the most important during the evolution of the genus Drosophila. These inversions have been used in the construction of phylogenetic trees. Possible errors in the phylogenies result from observational mistakes, the segregation of independent inversions, cytological parallelism and cytological convergence. Observational errors are present in all scientific work. Experience has shown that the segregation of independent inversions, rather than detracting from the accuracy, has allowed for a better understanding of the evolutionary process. Convergence and parallelism presupposes indistinguishable inversions occurring in two different clades. Experimental work indicates that the adaptive value of an inversion is determined by the genetic material tied together by the action of the inversion as a suppressor of recombination. The genetic content varies from locality to locality: each inversion, per se, therefore, has no definable adaptive value. Survival of a newly arisen inversion is dependent upon its fortuitous occurrence in a chromosomal region which is preadapted for a recombinant suppressor. Moreover, given preadaptation, any one of a large number of distinguishable inversions could equally serve the purpose. It is therefore argued that the occurrence and survival of indistinguishable inversions in two phyletic lines is unlikely, and that phylogenies based upon these rearrangements have an exceedingly high degree of accuracy. Phylogenetic relationships among 46 species in the repleta group are shown and their evolution is briefly discussed.