Genetic Drift in Irradiated Experimental Populations of Drosophila Melanogaster.

HE investigation reported in the present paper is of the effects of genetic T drift in three experimental populations of Drosophila melanogastar, two of which received treatments with gamma rays. Although there has been considerable theoretical discussion of genetic drift, few experimental studies of this phenomenon have been carried out. This paucity of experimental verification exists in spite of the fact that WRIGHT (1931 and other papers) has described several concrete situations where the effects of genetic drift could be studied and measured experimentally. One of these situations may be found in a single population in which there are a number of loci with a pair of alleles at each locus. If all these alleles are subject to the same evolutionary forces (mutation, selection, migration, etc.) then the variance of allele frequencies may be taken as a measure of genetic drift. In other words, genetic drift will tend to cause the alleles at some loci to be represented more times than the average and some less than the average. It is this model which will be used in the present investigation of the effects of genetic drift on the behavior of recessive lethal alleles at various loci on the second chromosome of Drosophila melanogaster. This approach is essentially the same as that used by DOBZHANSKY and WRIGHT (1941), WRIGHT, DOBZHANSKY and HOVANITZ ( 1942) and IVES (1945). In their studies the assumption was made that a Drosophila chromosome carries a certain number of loci which mutate to recessive lethal alleles at finite rates. Each locus may then be represented by either a lethal or a nonlethal allele. Samples of chromosomes were taken from the natural populations studied, and analyzed to determine which of them carried recessive lethal alleles. Strains carrying the lethal bearing chromosomes were then crossed among themselves to determine which chromosomes carried allelic lethals. This investigation of allelism showed the number of times different loci were represented in the sample by lethal alleles. In turn this information allowed evaluation of the amount of genetic drift which occurred in the populations studied. The authors cited above carried out their studies on the third chromosome in certain natural populations of D. pseiidoobscura and on the second chromosome in populations of D. melanogaster. The investigation reported in the present article is concerned with studies