I'm not convinced that ornamentation does have much to do with evolution of tunes, at not in European traditions.

I doubt if the influence of songs on tunes has been adequately investigated yet. I play a number of instrumental tunes that were used for songs by Hamish Henderson. I annoy the heck out of piping purists because the way I play those tunes mixes in the adaptations Henderson made to make them fit his words. This isn't because I belong to a purely oral culture - I know the originals, I've got them in books and could play them if I wanted to, but I don't. I am far from alone in doing that.