Isozyme Variation in Natural Populations Ofdrosophila Buzzatii

Since the first demonstration that natural populations of Drosophila are polymorphic for a high proportion of protein and enzyme loci (Hubby and Lewontin, 1966; Lewontin and Hubby, 1966; Johnson et al., 1966), similar high levels of electrophoretically detectable genetic variation have been found for many species (e.g., Johnson, 1974; Lewontin, 1974; Selander and Johnson, 1973). The mechanisms by which this variation is maintained within populations and transformed into variation between populations remain uncertain. Recent experimental studies of natural populations have concentrated on the question as to whether the variation is maintained through some form of selection or whether it is selectively neutral. Some form of selection would be indicated, although not necessarily for the marker isozyme locus, if the frequency of a particular allele were found to vary consistently with change in some environmental component. Thus a number of attempts have been made to detect such associations (e.g., Johnson and Schaffer, 1973; Kojima et al., 1972; Rockwood-Sluss et al., 1973; Tomaszewski et al., 1973) or associations between heterozygosity and environmental variability (Bryant, 1974a,b). However, most Drosophila species show a relatively uniform spatial pattern of gene frequencies, even across wide climatic distributions. This uniformity could be due to the unifying force of migration opposing differential selection in different populations, or to similar selective forces throughout the distribution, i.e. a function of the physiological properties of the molecules,