[maro]

Limited by time and probability. I would think that the "range" of the river crosser making one river crossing within unit time is a probabilistic cloud centered about the original position. The cloud is "denser" (higher probabilities) in the center and falls off thereafter. With unlimited time, the cloud grows bigger and bigger, but we are not dealing with unlimited time.

[Nebullis]

This has all been modeled before. Here is an example using GAs. In vitro models of actual molecules has been done with RNA (structure is simpler than proteins), and in vivo with the examples I mentioned above.