Evolution is a process by which change occurs through replication. Variation can be introduced into a population during the replication process. Some of the resulting variants may be replicated more rapidly than others, and so the characteristics of the population and individuals within it change over time. These processes can be recognised most obviously in genetics and ecology; but they also arise in the context of cultural change. We discuss two key questions that are crucial to the development of evolutionary theory. First, we consider how different application domains may be usefully placed within a single framework; and second, we ask how one can distinguish directed, deterministic change from changes that occur purely because of the stochastic nature of the underlying replication process.