The best answers to questions like these are economic in nature, and not confined to any particular musical definitions, because "pop" is always evolving and changing.
At any given point in time, in any given location or culture, "pop" music is the newest emerging style of music that is being commercialized, marketed and sold on a large scale. What you define as "pop" in the USA in 2014 is going to be very different from what is defined as "pop" in the USA in ten or twenty years.
Another word for "pop music" is commercial music. It is entirely a creation of the 20th century, and it did not exist before the rise of radio.
"Pop" has meant very different things in every nation in the world where there was commercial broadcast radio. Pop music in Kenya in 1975 was very different than that in Mexico in 1935 or in Italy in 1995. But all forms of pop music have this in common: they are created to earn money in a larger music industry.
Pop music is a music industry phenomenon that began in the early 1900s when commercial radio stations began broadcasting pre-recorded music.
Then professional music publishing companies began commissioning the composing of specific kinds of music which would be produced and recorded by professional music labels in certain styles. These pieces of music would be specifically tailored to be the kinds of music that particular radio stations would be eager to broadcast to appeal to a certain audience of listeners. This was in turn driven by the kinds of advertisements (commercials) that various manufacturing companies would create and pay the radio stations to broadcast between the playing of pieces of pre-recorded music.
This happened in parallel in different nations and cultures all around the world. In every nation where there was broadcast radio and an economy that would support selling mass-produced recordings, completely different kinds of "pop" would arise and develop.
Originally, each radio station would play all kinds of music, but as time went on, certain radio stations would specialize in just one genre of music, in order to appeal to a certain narrowly-defined demographic of the listening public, again to attract paid advertisement from certain companies wishing to sell certain specific categories of goods to the public, in order to create a tightly-managed profitable business for the radio station.
In some nations, radio stations paid royalties to the publishing companies and the record labels whenever a particular piece of pre-recorded music was broadcast. At any rate, the record companies would mass-produce recorded discs (singles and albums) and later cassettes, CDs or other kinds of media, and make them available for sale in local music stores in the areas where a particular radio station broadcast to the public. This was carefully synchronized to marketing campaigns to get those certain pieces of music played on certain radio stations within a certain time frame, to enhance the potential for sales. It was all carefully organized with investments and business and marketing plans.
To the degree that radio stations would specialize in playing a certain style of music that appealed to a certain narrow demographic of listeners, the record labels and publishing companies would oblige by deliberately seeking out and financing musicians who could create music tailored to the desires of those specific radio stations. Thus the various sub-genres of pop music, everywhere in the world, were created in a kind of feedback loop between broadcast radio and the record labels. Styles and genres of pop music always changed and evolved, but always in response to the market pressures of selling recordings and making money.
This still goes on today, even though there are many more outlets for marketing and selling music than traditional broadcast radio stations and traditional music stores that sold discs and tapes.
There are only two other economic categories of music besides pop music: folk music, which is music that amateurs make for their own enjoyment and which they don't expect to make much money from, and classical music, which is intellectually sophisticated music that is expensive to create and perform but is paid for by patronage, which is to say the direct infusion of money from wealthy noblemen, governments, churches, corporations and the like. Classical music does not earn a profit overall.
So where did the styles of pop music originate? The earliest pop music in each culture usually was adapted from existing folk music and in some sense, classical music.
It has also always been the case that folk musicians and classical musicians have gotten themselves involved in the commercial music industry, record labels, and broadcast radio, television, and motion pictures. To the extent that they do so, they tend to transform their music from folk or classical into pop, according to the pressures put upon them by the publishing companies and record companies and the music industry, in the hope of profits. It's all about the money.
