Mutations, in the broad sense, are the source of genetic variation for evolutionary change, whether through the agency of natural or artificial selection. The contributions of new mutations to genetic variation in artificial selection lines are thought to be small, as estimates of the rate of generation of new genetic variation by mutation have been only of the order of 10-3 times the environmental variance for the character per generation. However, several occurrences in selection lines have led the authors to question this view. They set out to measure the contribution of 'mutations' (from whatever source) to selection response for a quantitative character by selecting in lines from a largely homozygous base stock of Drosophila melanogaster. In 2 of the low lines response due to bobbed alleles was detected. This has allowed the authors to implicate unequal crossing over as a source of quantitative genetic variation and to question current views concerning the rate at which new genetic variation arises.