 Whatever one’s view of this debate, it is striking that virtually all of the central concepts of population genetics were devised in the pre-molecular era, when so little was known about what genes were; the basic structure of population-genetic theory has changed little since the days of Fisher, Haldane and Wright (Charlesworth and Charlesworth 2017). This reflects the fact that the empirical presuppositions of population-genetic models are really quite slim; the basic presupposition is simply the existence of hereditary particles that obey the Mendelian rules of transmission, and that somehow affect the phenotype. Therefore, even without knowing what these hereditary particles are made of, or how they exert their phenotypic effects, the early population geneticists were able to devise an impressive body of theory. That the theory continues to be useful today illustrates the power of abstract models in science.