The Effect of Artificial Selection on Bristle Number in Drosophila ampelophila and its Interpretation

Abstract

About two years ago I began an experiment to test the effect of artificial selection in a bisexual form. Drosophila ampelophila was chosen after a careful search through the list of material which could be bred in the laboratory. There were two principal reasons for using this material. First, it is easily bred, and second, because of the four sets of linked genes described by Morgan and others, it is possible to better interpret how the results of selection have been accomplished. I wish to emphasize that the interpretation of the results is the important part of the problem. Practically every one admits that selection may be, in certain cases, effective. The present work is not a repetition of McDowell's, although the character, bristle number, has been used. He used the bristles on the thorax, while I have used the bristles on the scutellum. Some of our final conclusions may agree, yet, the work is very different and I believe my own conclusions have been carried to a more definite termination. McDowell did not attempt to link up the factors which he believed the cause of extra bristle number with any other genes. The experiment was started by mating a female with one extra bristle on the scutellum to a normal male (four bristles is the normal number). Both flies were taken from a mass culture which had been kept in the laboratory for about three months. Counts in this mass culture before the experiment began gave 612 normal flies and the one female with one extra bristle. The result of this cross (female five by normal male) was 226 normal flies and two females with one extra bristle. These two F1 females with one extra bristle were mated to F1 normal brothers. These two pairs gave F2 offspring as follows: 935 normal, 39 with one extra bristle, and four with two extra bristles, a ratio of extras to normals of 1: 21.7. From these F2 offspring, the flies with 55