The effects of stabilizing selection on the time of development in Drosophila melanogaster

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Abstract
The time of development from egg to emergence of Drosophila melanogaster was subjected to stabilizing selection in one line and to disruptive selection with disassortative mating in another line. Forty generations of selection produced a loss of phenotypic variance in the stabilized line and an increase in variance in the disruptive line. Toward the end of the experiment the lines were tested for response to directional selection. These tests indicated that the loss of variance in the stabilized line was due entirely to a reduction of the additive genetic component while the increase of variance in the disruptive line was due entirely to an increase in the non-additive genetic and/or environmental component(s) of the variance. Thus the stabilized line is a case of normalizing selection with no canalization while the changes in the disruptive line were probably the result of a loss of canalization. The latter notion is supported by the fact that the disruptive line showed a loss of fitness.