Building on George Price's formal account of selection, we present an abstract theoretical account of behavioral selection that integrates the domains of individual learning and evolution. From the perspective of the multilevel model of behavioral selection (MLBS), we argue that the covariance based law of effect (CLOE) qualifies as a fundamental principle of behavior in that it provides a general formal framework for selectionist thinking and model building. We demonstrate the feasibility of our approach by means of a covariance based model of choice behavior that explains the effects of changeover delays on operant matching.