Abstract

The humeral calli in D. tryoni range from completely yellow to almost entirely brown. This variation is largely genetic and the character responds readily to selection in the laboratory. In field populations, selection favours flies with little or no brown pigment on the calli (around 3% brown) and there is a strong natural selection against the darker forms. However, field populations of D. tryoni show only a limited response to this selection, since flies with dark calli are regularly present in field samples. One possible explanation is that the loss of genes through selection is balanced by recurrent gene exchange with D. neohumeralis, a species in which selection favours flies with dark calli. Gene exchange between the two species is restricted, because their periods of sexual activity do not coincide. This mating barrier is incomplete however, since limited gene exchange can be induced in the laboratory. D. tryoni is sympatric with D. neohumevalis for only part of its range in eastern Australia. It is shown that callus variability in D. tryoni is uniform over its entire range and is therefore independent of possible gene exchange with D. neohumeralis. An alternative mechanism is proposed to explain the maintenance of callus variability in D. tryoni, namely, a stabilizing selection that operates against flies at both extremes of its callus range.