Is Wright's shifting balance process important in evolution?

In 1997, we published a Perspective (Coyne et al. 1997) that questioned the validity and importance of Sewall Wright's shifting balance theory of evolution (SBT). Our review of both theory and data led us to reject the idea that Wright's shifting balance process has played a major role in adaptive evolution. We supported instead the view of Darwin (is quantified by Fisher) that the main engine of adaptation is natural selection acting on differences among individuals-without genetic drift, population subdivision, and differential migration playing the vital roles hypothesized by the SBT. Peck et al. (1998) and Wade and Goodnight (1998) each claim that our dismissal of the SBT is premature. Peck et al. (1998) offer a theoretical defense of Wright, claiming that phase III (the movement of populations to higher adaptive peaks) may act more frequently than we proposed. Wade and Goodnight, on the ether hand, defend the SBT by discussing experimental studies of group selection and aspects of population subdivision and epistasis. Here we respond to both papers, and conclude that neither offers substantial support for the SBT.