Directional Selection

Directional selection is a mode of natural selection that shifts a population's phenotypic mean toward one extreme, driving evolutionary change in a consistent direction. Unlike stabilizing selection (which preserves the mean) and disruptive selection (which splits the population), directional selection is the engine of adaptation to changing environments. Classic examples include industrial melanism in peppered moths and antibiotic resistance in bacteria. In adaptive dynamics, directional selection corresponds to the gradual approach toward a singular strategy, where the fitness gradient is non-zero and points toward a convergence-stable point. The apparent simplicity of directional selection is misleading: it is the default mode only when the environment is stable and the fitness landscape is smooth. When either condition fails, directional selection gives way to more complex dynamics.