[maro]

If the set A of which we are trying to determine P(A) were so clearcut as the set of all five playing cards constituting a poker hand, we would not be having this discussion. I submit that we now know far too little to know what A is, even in sketch form. So the only thing we can "compute" (at least in concept) is the probability of one river crossing, to refer to my metaphor. Having some idea what that probability is (on average, under stipulated assumptions), we can reason backwards and determine what the size of A must be (or be like in orders of magnitude) for the evolutionary hypothesis to be true. I would adjust your poker analogy by saying that the poker hand the probability of which we are trying to get a fix on is also an unknown. In those circumstances, the best we could do is compute the probability of an unordered set of five playing cards, drawn without substitution, call it F, and say that P(A) equals #A * F.

[edsheppa]

I submit that we now know far too little to know what A is

I agree.

the best we could do is compute the probability of an unordered set of five playing cards, drawn without substitution, call it F, and say that P(A) equals #A * F.

You will get the wrong answer which makes the effort rather pointless. OTOH a probabilistic argument can effectively eliminate some models; asking for more is asking for trouble.