Still Dead. Feeling No Better.

Thomas Nagel’s recent attack on Darwinism raises important metaphysical questions about methodology, which Eric has begun to explore. Here, I want to muse on a no doubt unintended effect of Nagel’s argument—a rumoured small boost in the regard accorded to Fodor’s earlier attack on Darwinism (aided by Massimo Piattelli-Palmarini, whose complicity in this is a mystery to me). True, Fodor's little dagger looks philosophically cautious by comparison to Nagel's WMD. My purpose here is simply to remind you, dear reader, that like Generalissimo Francisco Franco, Fodor’s negative critique is Still Dead. And it's feeling No Better. 

 

Here's Fodor's critique. Suppose that, according to orthodox Darwinian theory, some species O is small because smallness is adaptive in its habitat. Suppose further that O’s coat is textured, but that this trait is adaptively neutral. We want to explain why O’s coat is textured. Well suppose that we know of a genetic linkage between size and coat-texture in O. We arrive, therefore, at a “spandrels” thesis: size was selected for; texture was a “free-rider”.

This leads us straight into Fodor’s Ambush.

(1) The small Os were fitter than the others, and in accordance with PNS were favoured.

(2) The textured Os were fitter (by virtue of being small) and were favoured.

Since PNS says only that fitter animals are favoured, and since both small Os and textured Os are fitter, both smallness and texture were favoured. PNS does not distinguish between the adaptive trait and the free-rider.

It doesn't, but is this a defect? There is an old distinction of Elliott Sober's Nature of Selection that is relevant here. Distinguish between a source law, which tells you about the causes of change, and a consequence law, which tells you what happens when certain causes are in place. Newton's Second Law is a consequence law: it tells you what happens when a (net) force works on a mass, but it does not tell you how the said force was generated. Source laws have to be invoked for the latter purpose: the Law of Gravitation, Coulomb's Law, and so on.

Similarly, PNS is a consequence law. It tells you what happens when fitness differences exist. It does not tell you how those fitness differences were generated. PNS does not distinguish between size and coat texture in the above example, because it doesn't tell you that (or why) coat texture is non-adaptive. That job is for source laws—or, since it is often misleading to talk about laws in evolutionary biology—source models.

Fodor is surprisingly obtuse on this point. In the Talking Heads conversation, he allows that a story like mine (or Sober's, in that dialogue) holds water. But he insists that somehow it is outside the ken of evolutionary biology. He seems to think that PNS is the whole of evolutionary biology. It isn't.

So far, I think, I have said nothing controversial. I am just summarizing Sober (and many others). But I would like to end on a slightly more partisan note. Fodor sometimes suggests that modern-day evolutionary biology makes natural selection into a cause. Just as the planets trace their paths in space under the causal influence of gravitation, so (according to Fodor's version of evolutionary biology) populations trace paths in phenotype quality space under the causal influence of natural selection. But if this were so, that is, if natural selection was the force acting on populations, causing them to evolve, then you would expect that the equivalence of (1) and (2) does indeed render evolutionary biology unable to distinguish between the traits responsible for evolution and those that are inert free-riders. And that would not be good.

Now, I have argued often that natural selection is not a cause. PNS sums up all of the the causes of evolution in the common currency of fitness. Given a fitness difference—i.e., given that one type of organism has a propensity to produce more offspring than another type—it is a necessary truth that the first type will have a higher probability to increase its proportion in the population relative to the second type. But (and this is just to repeat the point about source and consequence laws) this does not identify the cause of that increase in proportion. 

It is not uncommon among philosophers in biology to treat natural selection as if it were directly a cause of evolutionary change. This is unfortunate. It's Fodor's error.