EXPERIMENTAL STUDIES OF THE DISTRIBUTION OF GENE FREQUENCIES IN VERY SMALL POPULATIONS OF DROSOPHILA MELANOGASTER. II. BAR

followed by fixation of type in 15 vials a.nd of Bar in one vial in F 2• In generation 10, type had become fixed in 95 lines and Bar in three, while only 10 lines remained unfixed. It is evident that selection strongly favors type but that Bar can occasionally drift into fixation in spite of this. A x2 test of the distributions of unfixed and newly fixed classes by generation from generation 4 to 10 (grouping generations 7 and 8, 9 and 10, classes 5 and 6, 7 to 12) yields a value of 22.2, 24 degrees of freedom and a probability between .50 and .70. It is evident that equilibrium of the form of the distribution was substantially reached by generation 4 and that an average of 22.0% of the unfixed classes became fixed per generation with respect to type and 0.7% with respect to Bar.