The analogy here with biology would be the influence of man upon biological evolution, unnatural selection if you like.

Isn't that a different analogy? If an analogy with evolution through natural selection is being made for a human process then I think the human aspect has to count as 'natural' so far as the anology is concerned.

However, if not doing that then Pamela's mention of an issue within biology of "How does "unnatural" selection (criteria imposed by human intervention) relate to, and interact with, natural selection?" is interesting. I wonder if some of the more overtly commercial aspects of change in songs are similar to, say, the creation and propogation of garden palnts or fancy poultry. One aspect of that is creating a fashion for people to follow. If that is a better comparison it could be that social psychologists (or ethnomusicologists) are the ones with better conceptual tools.

Going back to Sharp, rather than modal tunes being polished by selection amongst 'the folk' doesn't what we know about them suggest that they are a more like an early-evolved form that has survived with little obvious change because there has always, somewhere, been niche for them. Jack Campin's penatonic slime is doing very well all over the world.