I always assumed the reference in the chanty was to women of mixed black and white race, though I don't really know. The song has a reference to an "immigrant Irish girl", "two Irish sailor boys" as well as "Irish immigrants" bound around the Horn, none of which (on the surface, at least) would indicate anything to do with "yellow gals" as we've been discussing. Still, I don't know any other way to interpret the name of the song.Paddymac, do you know any sources for your "developing evidence" of gradual gene drift from Europe to North America? It's certainly possible (and you're dead right about the variance of color among "Native Americans"), but I've not seen any discussion of this -- scholarly or unscholarly -- in recent years.
Or is it that the myth of Prince Madoc and the "Welsh Indians" may still be alive and well! It was, among other things, the fact that Indians were occasionally found who had nearly white skin, blue eyes and blonde or even red hair, that gave rise to that fanciful tale that has had at least nine lives. Too bad the lunatic fringe of "white supremacists" can't realize that there is no such thing as "racial purity". Archaeologists are now indicating that we all may have Neanderthal blood flowing in our veins!