 What, then, are the statisticalist issues that random drift is entangled with? The concerns raised by Walsh, Lewens, and Ariew (2002) and Matthen and Ariew (2002) have their origins in claims made by Sober (1984) in his classic The Nature of Selection. Sober characterizes evolutionary theory as a theory of forces, with its zero-force state described by the Hardy-Weinberg equation of population genetics (see the  population genetics entry  for an explanation of the equation); in such a state, there is no selection, no mutation, no migration, no meiotic drive, random mating, and infinite population size. Thus, the Hardy-Weinberg equation is an idealized model that never obtains in the real world. It is a bit difficult to see where drift fits into the equation, which is no doubt the source of much of the confusion over how to define drift. In his 1984 book, Sober alternatively characterizes drift in terms of random sampling (the process, which occurs, Sober explains, during gamete formation and the founding of new populations) and sampling error (the outcome, i.e., the deviation from fitness expectations). From the point of view of process, at least some types of sampling could be understood to be part of the Mendelian process. The Mendelian process includes the “process wherein organism produce gametes and gametes produce organisms” (Sober 1984: 35), which Sober says is not treated as a force, but rather as the background against which evolutionary forces are described. But Sober does not take this route. Instead, Sober contrasts drift as sampling error (again, the outcome—presumably introduced when one relaxes Hardy-Weinberg assumptions to allow for finite populations) with selection, mutation, and migration; all are forces, Sober asserts, but drift is a different sort of force. It is not “deterministic” and it does not have a definite direction (although it does have a magnitude, determined by the population size). That is, given trait frequencies and fitness values, Sober suggests, selection predicts a specific outcome for the next generation in a specific direction (and is in this sense “deterministic”), whereas drift could yield an increase any of the types present in the population (and is in this sense directionless). Moreover, Sober states, you cannot say how much drift has contributed a change relative to the change introduced by selection; to do so would be as impossible as trying to say, when flipping a fair coin ten times and obtaining six heads, how much of the result was due to the fairness of the coin and how much was due to the fact that it was tossed ten times. He thus concludes that “if drift is an evolutionary force, it is a force of a different color” (1984: 117); he calls it a force, he says, mainly to indicate its causal role.