Abstract
Heritability estimates, based on 19 generations of selection for fast and slow mating speed, were not significantly different from zero at the 0.05 level in any replicate of selected lines in a population of flies descended from the Mather population in California. Only the combined heritability estimate of approximately 2% was significant. This indicated that very little additive genetic variance was present in the base population and that strong directional selection for rapid mating may have occurred in the previous history of the local population at Mather and/or during its many generations of laboratory propagation. Frequencies of third chromosome gene arrangements were monitored during the course of selection. Balancing selection, unrelated to that imposed for mating speed, and genetic drift appeared to be the major factors causing changes in chromosome frequencies. Present differences in adaptive value of third chromosome variants in nature may be associated with nonadditive effects on mating speed, as well as effects on other components of fitness.