Recent discussions in the philosophy of biology have brought into question some fundamental assumptions regarding evolutionary processes, natural selection in particular. Some authors argue that natural selection is nothing but a population-level, statistical consequence of lowerlevel events (Matthen and Ariew [2002]; Walsh, Lewens, and Ariew [2002]). On this view, natural selection itself does not involve forces. Other authors reject this purely statistical, population-level account for an individual-level, causal account of natural selection (Bouchard and Rosenberg [2004]). I argue that each of these positions is right in one way, but wrong in another; natural selection indeed takes place at the level of populations, but it is a causal process nonetheless. argue for a third logical possibility: natural selection is indeed a causal process, but it operates at the population level.The issues at stake in this debate will be fleshed out in more detail in the discussion below. However, at the outset it is important to distinguish this debate from the more well known debates over the units (or levels) of selection; unfortunately, similar terminology makes this distinction difficult. One obvious difference between the debates is that the units of selection debates assume a causal basis for natural selection, whereas the existence of selection's causal basis is one of the points of contention in the debate at hand. With regard to the issue of levels, however, the difference between the two debates can best be seen by the following example.Suppose that a person believes, in terms of the levels of selection debate, that selection (either sometimes or always) acts on organisms. Such a person still might ask whether that selection Millstein, p. of 3 of 41process was acting on individual organisms or populations of organisms. 1 It is the latter question that concerns us here, and indeed, throughout this paper I have, for the sake of simplicity, assumed organismic selection, although I believe that similar arguments can be made for other units of selection. Now, it may turn out that the present debate has interesting consequences for the debates over the units of selection, but I will not explore such consequences here.Further terminological confusion arises because of the distinction being made between an individual and a population. Indeed, one might wish to speak of an 'individual' population rather than a class or type of population. However, the reader should understand that, in what follows, 'individual' refers to an individual organism, whereas a population refers to a particular spatiotemporal collection of organisms.Some philosophers might object to the very idea of population-level causality. In response to this concern, I will argue in section 2.1 that anyone who accepts the reality of frequency-dependent selection is already committed to population-level causality. Moreover, in section 2.2 I will argue that population-level causality is consistent with three commonly accepted accounts of causality; thus,...

The latter half of the twentieth century has been marked by debates in evolutionary biology over the relative significance of natural selection and random drift: the so-called "neutralist/selectionist" debates. Yet John Beatty has argued that it is difficult, if not impossible, to distinguish the concept of random drift from the concept of natural selection, a claim that has been accepted by many philosophers of biology. If this claim is correct, then the neutralist/selectionist debates seem at best futile, and at worst, meaningless. I reexamine the issues that Beatty raises, and argue that random drift and natural selection, conceived as processes, can be distinguished from one another.

This paper explores whether natural selection, a putative evolutionary mechanism, and a main one at that, can be characterized on either of the two dominant conceptions of mechanism, due to Glennan and the team of Machamer, Darden, and Craver, that constitute the "new mechanistic philosophy." The results of the analysis are that neither of the dominant conceptions of mechanism adequately captures natural selection. Nevertheless, the new mechanistic philosophy possesses the resources for an understanding of natural selection under the rubric.

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