On genetic drift as an evolutionary force

ROFFÉ, ARIEL

San Pablo

Congreso; ISHPSSB 2017 Meeting; 2017

Universidad de San Pablo

In 1984, Elliott Sober posited an analogy between classical mechanics and evolutionary theory, according to which both theories have similar explanatory structures. That is, they would both explain their respective phenomena via a zero-force law (the principle of inertia and the Hardy-Weinberg law, respectively) and a set of forces that can alter the zero-force state (selection, mutation, migration and drift, in the evolutionary case), which can also add up to produce complex effects (Sober, 1984). More recently, McShea and Brandon have argued against this view, claiming that there is an evolutionary factor (genetic drift), which Sober had identified with a force, that has no structural analogue in Newton's theory (Brandon, 2006; McShea & Brandon, 2010). They present two arguments in favor of this thesis. Firstly, they claim that drift does not have a direction, and since newtonian forces are vectorial quantities that have both a magnitude and a direction, drift should not be considered a force. Secondly, they argue that drift as a factor is "constitutive" of evolutionary processes, and not "imposed" on them, as newtonian forces are on the processes on which they operate. Hence, again, drift should not be equated with a force. In this presentation I evaluate these two arguments. I will reject the first, by showing that there is a significant sense, not considered before, in which drift can be said to be a directional factor (even an a priori specifiable one), as it can bias populations to go towards some specific region of the evolutionary space. Contrarily, I will show why we must accept a version of the second argument. This is accomplished via an explication of the concept of "constitutivity" at stake, by means of a formal reconstruction of population genetic theory. The reconstruction will show that drift must be incorporated into the formal apparatus in a different manner than the rest of the evolutionary factors, and that it plays a structurally different role in the evolutionary explanations in which it figures. Lastly, some general consequences are drawn from this discussion, that go beyond the question of the adequacy of the force analogy.