On Rosenberg and Kaplan's "Physicalism and Antireductionism in Biology"

I've just finished reading Alex Rosenberg and D. M. Kaplan's "How to Reconcile Physicalism and Antireductionism about Biology" in the current issue of Philosophy of Science. The title describes its contents perfectly, and I can unequivocally recommend it to anyone interested in the topic (which is more than I can say for my foray into the subject). I'll quote the key part of the introduction:
In considering the relation between the PNS [Principle of Natural Selection] and physical science, three alternatives suggest themselves:
a. The PNS is an underived law about biological systems, and is emergent from purely physical processes. ...
b. The PNS is a derived law; it is derivable from some laws of physics and/or chemistry. ...
c. The PNS is an underived law about physical systems (including non-biological ones), and from it the evolution of biological systems can be derived... This is an alternative no one has canvassed, and one which we shall defend here.
Before continuing, let me give their version of the PNS:
For all x, y, and E: If x is fitter than y in environment E at generation n, then probably there is some future generation n', after which x has more descendants than y.
Note that the domain of quantification has no restrictions.

Their basic rationale for calling the PNS a law of physical science -- in particular, of chemistry (59, 62) -- is this: if there is a kind of chemical molecule that (in some sense) replicates itself and has higher rates of 'survival' than other molecules in a given reaction (say, as a reaction moves towards equilibrium, this kind of molecule is favored), that molecule will be subject to the PNS. (It's not a law of physics simply because (sub)atomic particles cannot be construed as replicating.) Calling the PNS a law of chemistry instead of biology "is just a picturesque way of drawing attention to the fact that selection for effects only begins to operate at the level of chemical interactions... Similarly, we call the second law of thermodynamics a law of physics, even though it obtains for all systems -- physical, chemical, and biological -- since it is at the level of the physical that it begins to operate" (61). As an example of such chemical natural selection, they point to current models of origins of life research.

So far, so good. But I'm less comfortable with the other half of their claim in c. above, viz., that from the physico-chemical PNS a fully biological PNS can be derived. They rephrase this point later in the article:
According to this view, at each level of the organization of matter there turns out to be a PNS, and each one should be in principle derivable from the PNS for the immediately lower level or some other lower level(s), all the way back down to the PNS for molecules. (61)
The problem is that they do not explain how this (in principle) derivation would proceed. They phrase the point slightly differently elsewhere (the PNS's "operation at higher levels of the aggregation of matter is a consequence of the operation of the underived PNS for molecules together with the rest of physical law" (62)), but they never actually spell out the derivation beyond this -- as far as I can tell. (In one place (top of p.62: "The rest is natural history"), they appear to hint that any higher-level PNS is a 'consequence' of chemical PNS in a historical sense: because the PNS acted on the primordial soup, today's organisms came into existence -- yet that is completely unlike any sort of reduction any philosopher of science that I know of has talked about. So I assume they can't mean that.)

So the question to Rosenberg and Kaplan is: does natural selection operate on the biological realm (whether it be genes, individuals, or even groups) because it operates on the chemical-molecular level? -- where that 'because' has the same force as the one in 'This mole of gas has a higher temperature than that one because this one's molecules have a higher mean kinetic energy.' Here's one way to press this worry. Look at their definition of the PNS quoted above, and convert it into 'the PNS for molecules':
For any molecule x and molecule y, if x is fitter than y, then...
Now they say that from this (and perhaps other PNSes), we should be able to derive higher level PNSes:
For any gene x and gene y, if x is fitter than y, then...
For any organism x and organism y, if... then...

But these higher-level PNSes don't appear to follow at all. Certainly, each follows from the general PNS quoted at the beginning; but from the fact that all chemical molecules behave a certain way, you cannot infer that organisms will behave a certain way -- unless organisms are chemical molecules. Rosenberg and Kaplan might say at this point: but all we are is an aggregate of chemical molecules: that is just the physicalist thesis which we profess in the title of our paper. But 'Aggregates of As are B' does not in general follow from 'As are Bs,' even for the most determined physicalist.

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