Are Random Drift and Natural Selection Conceptually Distinct?

Abstract

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.

Thinking about evolutionary mechanisms: natural selection

“…7 And by this we mean, roughly, that the various selective effects identified in Figures 1 and 2 are made more likely by their causes, but that they Skipper/Millstein,"Evolutionary Mechanisms",p. 25 of 38 are not guaranteed by their causes (Millstein 2002). It is not clear from Glennan's (2002aGlennan's ( , 2002b) discussions whether he intends to capture this kind of stochastic causal relationship.…”

(Mis)interpreting Mathematical Models: Drift as a Physical Process

“…On this view, drift is a term designating a set of physical processes, arguably, indiscriminate sampling processes (Beatty 1984;Hodge 1987;Millstein 2002Millstein , 2005Skipper 2007, Dietrich andMillstein 2008). Drift is not a purely mathematical outcome.…”

“…Although it is not our target here, it can be argued that problems arise for accounts of drift which incorporate both process and outcome, as these biologists' definitions do; to mention one problem, authors usually end up vacillating between drift-as-process and drift-as-outcome, leading to inconsistencies (as Millstein 2002Millstein , 2005. Even if it were the common definition among biologists, such definitions should not be accepted uncritically.…”

“…Ultimately, correctly interpreting mathematical models of drift by relating the models to the physical systems being modeled results in a conception of drift as an indiscriminate sampling process (Beatty 1984), i.e., a physical process where heritable physical differences between entities are causally irrelevant to differences in reproductive success (Millstein 2002(Millstein , 2005; see also Hodge 1987). Entities that can be indiscriminately sampled include, but are not limited to, "parents" (i.e., organisms) and gametes; Beatty (1984) dubs the former "indiscriminate parent sampling" and the latter "indiscriminate gamete sampling."…”

“…When one does take those factors into account, starting with the motivation for Sewall Wright's and R.A. Fisher's early drift models and ending with contemporary applications, a very different picture of the concept of drift emerges. On this view, drift is a term for a set of physical processes, namely, indiscriminate sampling processes (Beatty 1984;Hodge 1987;Millstein 2002Millstein , 2005. …”