Experimental studies of the distribution of gene frequencies in very small populations of Drosophila melanogaster: I. Forked. 1954.

For quantitative study of the sort of random drift due to inbreeding it is de sirable to experiment with segregating genotypes that can be classified without risk of error and with populations that are all of a very small definitely known size. With respect to the first condition, it is much the most satisfactory if all segregat ing genotypes can be distinguished ac curately. This, however, limits severely the number of loci that can be studied. Something can be done where only domi nants and recessives can be accurately classified. Most pairs of alleles with such con spicuous difference in effect as to satisfy the first condition, turn out to be subject to such enormous differences in selection that the accumulation of random devia tions, implied by the term random drift, is largely prevented. Selection pressure and the effects of random processes are roughly comparable in magnitude at a given gene frequency if the change in fre quency in a generation which the former tends to bring about (Δq) and the variance increment due in a generation to the latter (σδq) are of the same order (Wright, 1931, 1948). Where the ratio (Δq/σδq) is as high as 10 in absolute value, there is not much accumulation of random devia tions and hence little random drift. The size of experimental populations in studies of the latter, should be small enough to meet this condition. They should be con stant for convenient comparison with theory. The present experiments (all with Drosophila melanogaster) were designed to meet these conditions. Three series were performed. About 120 lines were started in each. In the first series, the sex-linked mutation forked (f) competed with its type allele. Four females (1 f/f, 2 f/+, 1 +/+) and 4 males (2 f/0, 2 +/0) were put in each vial. The second series in volved the sex-linked semidominant mu tation Bar (B) and its type allele. Each initial vial contained 4 B/+ females and 4 males (2 B/0,