[L]ots of things, including random chance, can cause differential reproductive success.
Darwinian evolutionary theory does not specify particular causes for differential reproductive success. Anything will do, and quite a few very different things have done at one time or another - from slightly lighter colored bark to rocks falling out of the sky.
Right. All kinds of contingencies can be responsible for differential reproductive success. So, what are the necessary and sufficient conditions whereby we attribute the success to natural selection? Or to luck? Or to genetic drift?
My point is that the science gets sloppy when we say that we surmise various mechanisms but can't determine which ones contribute how much to any particular outcome. Shouldn't a science be picky about distinguishing one causal agent from another? For example, what are we to make of the notion of "fitness" -- or should we just drop it, since we have no criteria for distinguishing it from reproductive success? Is it just a synonym for reproductive success? Or what about "adaptation"? To say that phenotypes are adapted to their environments is just to say that there are such phenotypes (tip o' the hat to J. Fodor). At the very least, couldn't the theory of evolution clean up its terminology and get rid of extraneous jargon?