Can polymorphism be maintained by selection favouring an intermediate optimum phenotype?

Populations of Drosophila melanogaster containing the special chromosome, M-5, were subjected to artificial selection to canalise scutellar bristle phenotype at four bristles. At the end of the experiment allele frequencies at three enzyme loci were measured. In the selected population, these frequencies were found to have diverged from their starting values less than would have been predicted on the basis of random genetic drift. This was not found in control populations or in populations selected for low bristle number.