Selection for Canalization of the Scute Phenotype. II.

Abstract

Whether selection for progeny groups with low phenotypic variance could breed a population in which a variable character was converted into one with a very constant phenotype was studied. The original population chosen was a homozygous scute stock of prosophila melanogaster. Before selection the mean scutellar bristle number was approximately 2 in females and 1 in males with a standard deviation of 1 bristle. Fifty generations of selection for progeny groups with low variance of scutellar bristle number and a mean of 2 bristles resulted in a population approximately 95% of which had 2 bristles. Selection for progeny groups of low variance and selection for a mean of 2 bristles both contributed. Both forms of selection pressure reduced sensitivity of scutellar bristle number to temperature change. The distinction between the 2 selection pressures is discussed. If the phenotype, x, is related to the sum, m, of all influences working on a developmental process so that x = f (m), then selection for progeny groups with a low variance operates mostly through f and selection for a mean of 2 bristles mostly through the standard deviation of m. f and m are under multigenic control which responds to selection.