OK, if we have readers, we must have writers...
Sober's claim is that
1. Selection is an evolutionary force, and
2. Fitness is a supervenient property.
Recall that "supervenience" is a term of philosophy of mind that means that a property may be realised physically in many ways, but that if any two physical systems are identical [in all possible worlds, *shudder*] they must perforce have that property.
I think these two claims are in contradiction to each other. If fitness, the property of increasing relative to other alleles in a population, is a supervening property over the physical substrates of the actual organisms, then I fail to see how selection can be viewed as a physical
force.
But if selection is not a physical force, what kind of force can it be? It must be, like fitness which proceeds from its operations, a supervening
process. In short, selection is not something that occurs, so much as something that
sums up what occurs. And the summation is something that happens in the process of providing an explanation.
Sober's own view of selection, and of explanation, here, implies that selection is only a force in a metaphorical sense. I get that feeling from his original discussion, that "evolution is a theory of forces" means something more like, "in explaining an evolutionary event, we must separate and assess the actual kinds of evolutionary processes, as a prolegomenon to a full explanation". If this is how he meant it, then I think he escapes the attack of Walsh
et al.; if not, he should have drawn that conclusion. It is, in any event, how investigation into selective events proceeds in science.
Consider the famous cases of selection research, such as the Kettlewell moth study or Endler's study on Jamaican guppies. They did not posit some force like "selection"; they instead posited actual physical events - like predation by birds on physical trunks with physical properties (light reflection, and visual acuity).
That is the nature of selection - it is a summary of the kinematics of a population.
But, one might object, Fisherian and subsequent mathematics shows there is a proper theory, indeed a fundamental theorem, in play - surely this makes selection a force? No - a mathematical model describes the ways a system will behave assuming that it can vary only along certain defined parameters. An equation is not a theory; it is a model. The model develops into a theory only when it can provide an account of the underlying physical processes, which is why, despite its tentative nature, Newton's theory of universal gravitation was a theory and not just an equation - there was expected to be a physical underpinning, and there turned out to be one. But selection posits no such physical underpinning - in fact we have extremely good empirical reasons to think that nothing physical need be common to all selection events other than the ordinary physical properties of physics in general.
Selection, as I noted, provides a way to proceed. It organises what would otherwise be a blooming buzzing confusion of empirical data, and enables one to narrow down the sorts of variables one must look at. So, too, does a large slab of our background knowledge - that heredity is in play, and so on. But these abstractions are heuristic until the rubber meets the road in each particular case. I am saying that selection is a heuristic principle, and not a full explanation.
This may be controversial...
