This paper proposes an evolutionary model of science policy. The paper draws upon evolutionary economics and associated applications to theory of the firm on routines and the role of knowledge in decision-making. This strand of literature is called the growth of knowledge perspective, because routines are assumed to embody useful knowledge about problems to be solved and potential solutions to them. This paper develops a conceptual model, based upon this literature and Campbell’s evolutionary epistemology. The paper proposes that the equivalent of firm routines is, in the science policy context, public policy alternatives such as policy instruments. Moreover, the paper develops notions of science policy fields, and puts them in an evolutionary model in order to understand the creation of variety, retention and selection amongst policy alternatives. © 2017 Inderscience Enterprises Ltd.