 Fisher and Haldane were both strong Darwinians—they believed that natural selection was the most important factor affecting a population’s genetic composition. Wright did not downplay the role of natural selection, but he believed that chance also played a crucial role, as did migration between the constituent sub-populations of a species (See sections 4, ‘Random Drift’, and 3.3, ‘Migration’.) The respective roles of natural selection and chance (or random drift) in shaping genetic variation, both within and between species, became a major theme in population genetics, and is still a live issue today. The issue lay at the heart of the “neutralist versus selectionist” controversy of the 1960s and 1970. The neutralists, led by Motoo Kimura, argued that much of the molecular genetic variation found in natural populations was likely to be neutral, i.e., the different genetic variants at a given locus were mostly selectively equivalent (Kimura 1977, 1994). If true, this suggests a significant role for random drift. Though initially controversial, the idea of abundant neutral variation in DNA sequence is today quite standard (Jensen et al. 2019); though this is fully compatible with natural selection playing the major role in adaptive evolution (Kern and Hahn 2018).