Darwin’s theory of evolution by natural selection explains the appearance of design in the natural world. Darwin reasoned that because natural selection favours the accumulation of traits that improve survival and reproduction in biological populations, individuals will consequently evolve to appear as if designed to maximise their reproductive success. In contemporary evolutionary theory, this logic is captured in abstract mathematical terms by the Price equation – a mathematical statement of the change in phenotypes in a population brought about by the action of natural selection. This equation derives from an abstract description of selection first given by George R. Price in 1970. It forms the foundations of many theoretical frameworks in evolutionary biology, including quantitative genetics, kin selection theory, and multi-level selection theory. In recent years, however, the explanatory power of the Price equation has been called into question, and a disconnect between major theoretical frameworks in evolutionary theory has emerged. In this talk, I will revisit the foundations of Price’s work and propose that an expanded version of his framework leads to a unification of different branches evolutionary theory.