Selection, Drift, and the “Forces” of Evolution

Abstract

Recently, several philosophers have challenged the view that evolutionary theory is usefully understood by way of an analogy with Newtonian mechanics. Instead, they argue that evolutionary theory is merely a statistical theory. According to this alternate approach, natural selection and random genetic drift are not even causes, much less forces. I argue that, properly understood, the Newtonian analogy is unproblematic and illuminating. I defend the view that selection and drift are causes in part by attending to a pair of important distinctions-that between process and product and that between natural selection and fitness. 1. Introduction. Philosophers and evolutionary biologists often appeal to an analogy with Newtonian mechanics to understand the interaction of natural selection, mutation, migration, and random genetic drift. 1 According to this traditional approach, factors such as selection and drift are viewed as "forces" that are the possible causes of evolutionary change. Perhaps the most developed use of the Newtonian analogy to discuss evolutionary theory occurs in Elliott Sober's book, The Nature of Selection (1984): All possible causes of evolution may be characterized in terms of *

Natural Selection as a Population-Level Causal Process

“…Even though we always expect a smaller population to exhibit greater fluctuations than a larger one from one generation to the next, the outcome that in fact occurs may be different (the small population may exhibit smaller fluctuations than the large from one generation to the next). (Stephens [2004] says that he is distinguishing between 'process' and 'product,' but it would probably be more accurate to say that he is distinguishing between 'expectation' and 'product').…”

“…Whereas the previously named PGI 1-1, PGI 1-4, and PGI 4-4 genotypes are real (and will be discussed in further detail below), PGI i-i, PGI i-j, and PGI j-j are imaginary. 8 For other arguments supporting the claim that natural selection is a causal process, see Sober ([1984]), Hodge ([1987]), Millstein ([2002]), Stephens ([2004]), and Shapiro and Sober ([forthcoming]). 9 Towards the end of Matthen and Ariew's essay, there seems to be a relaxation of their claim that there is a dichotomy between statistical trends and forces; Matthen and Ariew ([2002], p. 82) 'concede that stochastic causation occurs at the [population] level.'…”

“…However, their arguments commit two mistakes. First, as Stephens ([2004]) has argued, these authors conflate the concept of 'force' with the concept of 'cause.' At best, they have shown that natural selection is not a force, but this fails to demonstrate that it is not a causal process.…”

“…Others have developed critiques of WALM's position (see, e.g., Stephens [2004], Rosenberg and Bouchard [2005], Shapiro and Sober [forthcoming]). Thus, I will focus primarily on arguing for my alternative position.…”

How Do Natural Selection and Random Drift Interact?

“…Recently, some authors have attacked this realist interpretation of "force" talk (Walsh, 2000;Ariew, 2002, 2005b,a;Walsh et al, 2002). Aspects of Sober's conception have been defended (Stephens, 2005;Reisman and Forber, 2005;Shapiro and Sober, 2005). 1 However, advocates of a realist interpretation have not presented a clear, unified conception of what it is for forces of evolution to interact.…”