Abstract
The discovery of homosequential species of Hawaiian Drosophila which share identical polytene chromosomes and metaphase karyotypes (Carson et al., 1967), established that overt chromosomal change need not accompany speciation. These homosequential species are morphologically distinct and are, therefore, not sibling species. The genetic divergence that has obviously occurred must therefore have involved only gene mutations. Within the genus Drosophila not all homosequential species, as defined by polytene chromosome banding patterns, share identity of metaphase karyotypes. Thus in the D. pachea group homosequential species show striking heterochromatin differences in their metaphase karyotypes (Ward and Heed, 1970). Since heterochromatic sequences are generally underreplicated relative to euchromatin in polytene chromosomes of Drosophila (Cordeiro et al., 1975; Gall et al., 1974; Rudkin, 1972), no differences in polytene chromosome structure are evident despite the considerable divergence revealed by mitotic karyotypes. Thus where possible, analysis of both diploid and polytene chromosomes should be carried out. In the Simuliidae mitotic karyotypes are remarkably conservative being identical in species of several different genera (Rothfels, 1956). On the other hand, the analysis of polytene chromosomes has shown the existence of sibling species
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