The effects of disruptive and stabilizing selection on body size in Drosophila melanogaster. II. Analysis of responses in the thorax selection lines.

An analysis was made of changes in mean and variance in some thorax selection lines. The decrease of mean thorax length in the stabilizing selection lines (S) was a consequence of a directional selection component, caused by the skewness of the frequency distributions. The slight or temporary increase of the phenotypic variance and the large increase of the mean value in the disruptive selection lines with random mating (D(R)) could be attributed to differences in reproduction between small and large flies (egg production and mating success). Phenotypic variability was high in two disruptive selection lines with compulsory mating of opposite extremes (D(-)). The mechanism of the change in variability was different in these replicate lines. In D(-)-1 the change was obtained by an increase of the environmental and the nonadditive genetic components of the variance. In D(-)-2 almost exclusively an increase of additive genetic variance occurred.