Stabilizing and disruptive selection on a mutant character in Drosophila. I. The phenotypic variance and its components.

N an earlier paper (SCHARLOO 1964b) it was shown that artificial stabilizing I selection on the expression of a cubitus interruptus mutant causes a decrease in phenotypic variance and that artificial disruptive selection with random mating of the selected individuals causes an increase in phenotypic variance. Further analysis showed that under disruptive selection the increase in phenotypic variance was a consequence of an increase in additive genetic variance. The decrease of phenotypic variance in the stabilizing line, however, seemed to be caused by a decrease of both the genetic and the environmental components. The within-fly variance in both lines was unaffected. KAMSHILOW (1939), WADDINGTON (1960), WADDINGTON and ROBERTSON (1966), and RENDEL and SHELDON (1959) succeeded in diminishing the sensitivity of mutant expression to environmental factors but these authors did not apply simple stabilizing selection. KAMSHILOW and WADDINGTON practised selection directly against environmental effects, and RENDEL and SHELDON performed selection for low variability within families. PROUT (1962) emphasized that although it is assumed in theoretical discussions ( SCHMALHAUSEN 1949; WADDINGTON 1942, 195 7) that stabilizing selection would decrease the sensitivity of developmental pathways to both genetical and environmental differences, there was no experimental evidence to support this view. In his experiments in which stabilizing and disruptive selection was practised on the duration of larval development in Drosophila, he found no change of environmental variance under the influence of stabilizing selection and an increase in environmental variance in his disruptive line in which-instead of random mating-compulsory mating of opposite extremes occurred. He suggested that the latter kind of selection was particularly suited to change the environmental variance. In THODAY’S extensive experiments (see THODAY 1965) on the effect of disruptive and stabilizing selection on chaeta number in Drosophila, attention was almost exclusively focussed on the consequences for the genetical variance. In view of the considerable importance in the theory of evolution of the influence of stabilizing and disruptive selection on sensitivity to environmental and genetical differences, it was decided to compare the effect of stabilizing selection and both types of disruptive selection.