How Do Natural Selection and Random Drift Interact?

One controversy about the existence of so called evolutionary forces such as natural selection and random genetic drift concerns the sense in which such “forces ” can be said to interact. In this paper I ex-plain how natural selection and random drift can interact. In partic-ular, I show how population-level probabilities can be derived from individual-level probabilities, and explain the sense in which natural selection and drift are embodied in these population-level probabili-ties. I argue that whatever causal character the individual-level prob-abilities have is then shared by the population-level probabilities, and that natural selection and random drift then have that same causal character. Moreover, natural selection and drift can then be viewed as two aspects of probability distributions over frequencies in popu-lations of organisms. My characterization of population-level proba-bilities is largely neutral about what interpretation of probability is required, allowing my approach to support various positions on bio-logical probabilities, including those which give biological probabilities one or another sort of causal character.