Enzyme variability in theDrosophila willistoniGroup. V. Genic variation in natural populations ofDrosophila equinoxialis

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Abstract
SUMMARY: We have studied genetic variation at 27 loci in 42 samples from natural populations of a neotropical species,Drosophila equinoxialis, using standard techniques of starch-gel electrophoresis to detect allelic variation in genes coding for enzymes. There is considerarle genetic variability inD. equinoxialis. We have found allelic variation in each of the 27 loci, although not in every population. On the average, 71% of the loci are polymorphic – that is, the most common allele has a frequency no greater than 0·95 – in a given population. An individual is heterozygous on the average at 21·8% of its loci.The amount of genetic variation fluctuates widely from locus to locus. At theMdh-2locus arout 1% of the individuals are heterozygotes; at the other extreme more than 56% of the individuals are heterozygous at theEst-3. At any given locus the configuration of allelic frequencies is strikingly similar from locality to locality. At each and every locus the same allele is generally the most common throughout the distribution of the species. Yet differences in gene frequencies occur between localities. The pattern of genetic variation is incompatible with the hypothesis that the variation is adaptively neutral. Genetic variation inD. equinoxialisis maintained by balancing natural selection.The amount and pattern of genetic variation is similar inD. equinoxialisand its sibling species,D. willistoni. Yet the two species are genetically very different. Different sets of alleles occur at nearly 40% of the loci.