 The respective roles of drift and natural selection in molecular evolution was the subject of the selectionist versus neutralist controversy in the 1960s and 1970s, as noted in the Introduction (cf. Dietrich 1994). The neutralist camp, headed by Kimura, argued that most molecular variants had no effect on phenotype, so were not subject to natural selection; random drift was instead the main determinant of their fate. Kimura argued that the apparently constant rate at which the amino acid sequences of proteins evolved, and the extent of genetic polymorphism observed in natural populations, could best be explained by the neutralist hypothesis (Kimura 1977, 1994). Selectionists countered that natural selection was also capable of explaining the observed polymorphism. The controversy ended without a clear victory for either side, in part due to paucity of data. However the opposition between selection and drift remains a central topic in molecular population genetics today, where there is an abundance of data on DNA sequence variation in natural populations. Sophisticated methods have been developed to allow researchers to hunt for signatures of past selection in the genomes of modern organisms. It has become clear that there is indeed much neutral molecular variation in DNA sequence (in part due to “synonymous” mutations that leave unchanged the amino acid sequence of the protein that the gene codes for). However, there is also much evidence showing that the genomes of contemporary species have been substantially influenced by natural selection (Casillas and Barbadilla 2017, Kern and Hahn 2018). Moreover, the idea that drift is the sole determinant of a neutral variant’s frequency, as the original neutralists held, is not necessarily true. Another possibility, championed by J. Gillespie (2004), is that a neutral variant’s spread in a population may be heavily influenced by selection at linked loci, a process known as “hitchhiking” or “genetic draft”; see Skipper (2004) for discussion. A recent evaluation of the selection versus drift issue concludes that “the extent to which DNA sequence evolution is caused by selection versus drift remains an important unanswered general question” (Charlesworth and Charlesworth 2017, p.6).