Environmental Heterogeneity: A Problem of Contradictory Selection Pressures, Gene Flow, and Local Polymorphism

Abstract

The interaction of gene flow with contradictory selective pressures caused by environmental heterogeneity was investigated by computer simulation. Generally there is a range of intermediate values of gene flow over which local polymorphism occurs. The length and location of the range depends, mostly upon the magnitude of the local selection pressures and upon the net effect of the overall average, or global, selection regime. For the cases studied, variables are introduced to characterize the magnitudes of the local and global selection pressures. The variable Z measures the net effect of the global regime near equilibrium relative to the square of the local selection pressures. In general, the range over which local polymorphism occurs will be long when the condition Z .mchlt. 1 is met. Within limits, asymmetry, in terms of the proportions of different local selection regimes within a mosaic environment, increases the range. Increased environmental complexity, in terms of the number of different kinds of local selection regimes, has a similar, but smaller, effect. Extension of these results to studies that used different analytic models of gene flow is discussed. Inferences are drawn concerning the evolutionary significance of gene flow.