Using data from three years (1994–1996), I tested whether differential migration occurs from demes of high mean fitness in the shining fungus beetle, Phalacrus substriatus. The results show evidence for differential migration, thus providing evidence from a natural population for a critical demographic assumption of many interdemic selection models. To predict the evolutionary response to interdemic selection through differential migration, the genetic basis of the variation among demes in mean fitness must be known because the observed patterns could also be explained by some demes having an intrinsically favorable habitat. Thus, the importance of differential migration through interdemic selection in natural populations cannot be unequivocally answered without experiments specifically addressing the question of what causes differences in mean fitness among demes.

Corresponding Editor: C. Boggs