The selection of invisible mutations

Abstract

An inbred line of Drosophila melanogaster was selected, in two separate experiments, for the manifestation of polygenic characters over periods of twenty-one and fifty-three generations respectively. In one case change with selection, when it occurred, was smooth, but in the second experiment it proceeded in the form of sudden steps separated by periods in which selection was ineffective. These jumps are due to the recombination of the mutations to which individual polygenes have given rise. Masked by the non-heritable fluctuations of the character, these mutations, having small individual effects, accumulate until recombinations give rise to more extreme variants, which fluctuation can no longer hide. Selection then becomes effective in producing a change in the character.