Comments on Assessing the Dedifferentiating Effect of Gene Flow

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Abstract
In the neo-Darwinian view, gene flow between animal populations was considered a major conservative force retarding their differentiation. The influx of alien genes into a local population exerts a dedifferentiating effect by directly altering allelic frequencies and presumably by selecting for genes that coadapt easily. Ehrlich and Raven (1969) suggested the alternative that selection is both the primary cohesive and disruptive force in evolution and predicted that gene flow eventually might be discovered to play a rather insignificant role in evolution as a whole. Their suggestion has not stimulated empirical research. Instead, frequent citation of their opinions has contributed to the development of a neo-orthodoxy that attributes little importance to gene flow as a cohesive force. Another often-cited proponent of this view is Endler (1973, 1977). The conclusions of Ehrlich and Raven and of Endler apparently have been accepted uncritically, and objections to them are presented. The new orthodoxy is based on a recognition of the efficacy of selection, on simplistic models, and on opinion, not on studies in nature. Science should be done by experiment, and, for the question of the effect of gene flow, experiment means comparing differentiation with and without an intervening barrier.