Pop music is generally associated with a 'populist movement' and these are not new. In fact populist movements can easily be traced back to ancient Rome and probably way further back than that. One of the more recent populist movements occurred in the sixteen hundreds or there about. There was also a 'Free Love' movement in Germany just prior to the Nazi take over.
So this sets the stage for pop music in this country. Pop music as we know it is a mixture of the Blues and Folk Music, plain and simple. One would have to give a lot of credit to the invention of the electric guitar as well, which was used as early as nineteen thirty-one, with early proponents such as Les Paul. The electric guitar, however, was first used in the Big Band, which was jazz. So jazz or Blues mixing with Folk Songs, however you want to look at it.
The basic musical practice of pop music is the same as Blues or Jazz. In jazz, musicians use a chord chart and improvise or make-up their parts. In Blues, the musical practice is much the same except the chords involved are much simpler. These are three chords, known as the Blues Chords. These are also the most used chords in pop music. Jazz could have considered a bit of a more specialized application of the Blues performance practice and this is generally attributed to Black music.
I think the most interesting thing is that there was this same populist / snob division going on way back, that people who do not look very far back in history only attribute to today's pop music. Way back, jazz was derided as a low, debased musical form that would degenerate society. There are also songs that go way, way back such as "Momma Do not 'Low," which have lyrics that say' momma do not allow no guitar playing 'round here.'
Further back then that, we have Plato saying that new and novel fashions in music should be avoided because they will unsettle a societies most important conventions. But, as I often point out, what society today does not need to have its most important conventions unsettled? The thing to consider, though, is that pop music today is the same as it was back in the nineteen fifties. That's because it is manipulated through the conventional media channels who, like Plato, do not want anything of new and novel fashion because that would unsettle their most important convention which is their profits.