[Nebullis]

Let's try yet another metaphor.

Okay. (Why all the metaphors?)

Just for fun let's call the river an entropy barrier, the crossing a fitness barrier and the banks phenotypic spaces. Just as there is a considerable degeneracy between genotype and phenotype, there is degeneracy between phenotype and fitness. As you move up or down the river you encounter widenings and narrowings at the surface as the depth of the river changes. At a few very rare locations, the banks of the river are close enough that the river can be easily crossed.

Neutral mutations allow the organisms to explore the banks (phenotypic space) free of fitness constraints (necessity of crossing the fitness barrier) until a natural river-crossing is found.

[Southack]

Your thought experiment in that last post was one of the better evolutionary metaphors that I've seen. Nice job.

[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.