[maro]

Yes, that is a nice metaphor, and perhaps better than mine. In your metaphor, the question I would ask is whether the wanderings up and down the bank "looking" for a natural crossing are not inherently limited by the probability of the neutral mutations. I hope you would agree that there is some such limit. From there, it is a question of fact (or modelling) to determine what that limit is, and whether that limit is big enough/not big enough for natural river crossings to found as the evolutionary hypothesis predicts.

[Nebullis]

In your metaphor, the question I would ask is whether the wanderings up and down the bank "looking" for a natural crossing are not inherently limited by the probability of the neutral mutations. I hope you would agree that there is some such limit.

I'm not sure what limit you mean. An iso-fitness region is, of course, limited. But the limit for possible mutations (and fitness)is potentially so large it's pointless to talk about that. It keeps getting larger (and drives evolution, does it not?).

Perhaps you are unaware of the great potential for functional mutations generated by random change. Think of somatic mutations of the genes coding for immune recognition proteins. Think of the rapid evolution of genes coding for protein coats of viruses.