Distinguishing Drift and Selection Empirically: “The Great Snail Debate” of the 1950s ROBERTA L. MILLSTEIN Department of Philosophy University of California, Davis One Shields Avenue Davis, CA 95616 USA E-mail:
[email protected] Forthcoming in the Journal of the History of Biology -- no doubt, there will be some (hopefully small) changes in the proofing process Distinguishing Drift and Selection Empirically p. 1 Abstract: Biologists and philosophers have been extremely pessimistic about the possibility of demonstrating random drift in nature, particularly when it comes to distinguishing random drift from natural selection. However, examination of a historical case - Maxime Lamotte's study of natural populations of the land snail, Cepaea nemoralis in the 1950s - shows that while some pessimism is warranted, it has been overstated. Indeed, by describing a unique signature for drift and showing that this signature obtained in the populations under study, Lamotte was able to make a good case for a significant role for drift. It may be difficult to disentangle the causes of drift and selection acting in a population, but it is not (always) impossible. Keywords: adaptationism, Arthur J. Cain, conspicuous polymorphism, Cepaea nemoralis, random genetic drift, ecological genetics, evolution, Philip M. Sheppard, Maxime Lamotte, natural selection, selectionist Pessimistic Introduction The process known as “random drift”1 is often considered to be one of the most important chance elements in evolution. Yet, over the years, biologists and philosophers have expressed pessimism about the possibility of demonstrating random drift in nature. The following is just a sampling. In 1951, Arthur Cain argued: 1 Authors refer to this phenomenon variously as “random drift,” “genetic drift,” “random genetic drift,” or simply “drift,” without any apparent shift in meaning.