Genetic and Chromosomal Variability in Drosophila

The importance of genetic and chromosomal vari-ability in evolution may be demonstrated by com-paring the variability present in natural populations with the differences between species. This compares the potential variability with that useful in evolution of those forms where such systems can easily be studied. Many of the species of the Genus Droso-phila possess several characteristics which make them most uitable for such analyses. Many strains from different localities may be bred in the labora-tory to determine the genetic differences. The ordi-nary metaphase chromosomes so frequently used to determine the chromosome composition of a species can be examined. Differences in gene order and as-sociation can be made using the analyses of these differences in the Drosophila salivary gland chromo-somes, as demonstrated by Painter (1933). A com-parison of the metaphase and salivary gland con-figurations hows how many of the chromosome arms are euchromatin and how many are hetero-chromatin, as was done by Wharton (1943). When the number of heterochromatic arms present in metaphase and the number and size of the euchro-matic arms is determined in the salivary gland chromosomes, the changes from the primitive con-figuration of five pairs of rods and a pair of dots (five long strands and the dots in the salivary gland nucleus) may be analyzed. Figure 1 shows some of the types of chromosome r arrangements that may occur. These include exchange of equal chromo-some segments or symmetrical translocation, un-equal exchange or asymmetrical translocation, and the limiting case of this type, called a centric fusion. Both paracentric and pericentric inversions are il-lustrated as well as addition or subtraction of het-erochromatin. These types of heterochromatin mod-ification cause changes in the chromosome configur-ation as well as ize. Pericentric inversions in which