Concepts of Drift and Selection in “The Great Snail Debate” of the 1950s and Early 1960s

Recently, much philosophical discussion has centered on the best way to characterize the concepts of random drift1 and natural selection, and, in particular, on the question of whether selection and drift can be conceptually distinguished (Beatty 1984; Brandon 2005; Hodge 1983, 1987; Millstein 2002, 2005; Pfeifer 2005; Shanahan 1992; Stephens 2004).2 These authors all contend, to a greater or lesser degree, that their concepts make sense of biological practice. So, it should be instructive to see how the concepts of drift and selection were distinguished by the disputants in a high-profile debate; debates such as these often force biologists to take a more philosophical turn, discussing the concepts at issue in greater detail than usual. Moreover, it is important to consider a debate where the disputants are actually trying to apply the models of population genetics to natural populations; only then can their proper interpretations become fully apparent. (Indeed, I contend that some of the philosophical confusion has arisen because authors have tried to interpret population genetics models through the mathematical equations alone, without any consideration of the phenomena that the models were intended to represent or the phenomena to which they were applied). A prime candidate for just such a case study is what William Provine (1986) has termed “The Great Snail Debate,” that is, the debate over the highly polymorphic land snails Cepaea nemoralis and Cepaea hortensis in the 1950s and early 1960s. The debate yielded one of the best, if not the best, of the early attempts to demonstrate selection and drift in natural populations.3 Moreover, this early debate over Cepaea is sometimes seen as one of the first skirmishes of the ongoing neutralist/selectionist debates; below, I will argue that the terms “neutralist” and “selectionist” are somewhat misleading, and that the debate over Cepaea concerned the relative contributions of drift and selection.