 Contemporary population genetics takes place in a very different scientific landscape to the one inhabited by Fisher, Haldane and Wright. They were working in the pre-molecular biology era, when the “gene” was a purely theoretical entity, posited to explain observed patterns of inheritance, but whose structure and molecular composition were unknown. Genetic variation could thus only be observed indirectly, though the phenotypic variation to which it (sometimes) gave rise. This meant that there was very little empirical data against which population genetic models could be tested; so the enterprise remained an essentially theoretical one. In the intervening century, the gene has gone from being a theoretical posit to being an entity whose molecular structure and functioning is understood in great detail. Since the 1980s, the technology for gene sequencing, i.e., determining the sequence of nucleotide bases in a length of DNA, has become increasingly fast and cheap. This has allowed population geneticists to directly study the genetic variation found in natural populations, by sampling a number of individuals and sequencing a gene of interest (or in some cases the whole genome). As a result, population genetics is now a “data rich” science, the polar opposite of the situation when the field was founded. Despite this, many of the models, techniques and conclusions provided by the earlier theoretical work remain directly relevant today (Charlesworth and Charlesworth 2017).