So, the answer to your question is going to vary somewhat depending on precise time period and geographic location. That's true of any broad historical question. The answer to yours will also vary depending on the precise definition of "pop culture" one decides to use. Unfortunately, I can't offer you an AH-worthy answer on the fifteenth century. What I can offer you, however, is an answer that is based in the nineteenth and early twentieth century United States (and also mostly applicable in Europe), which is the time period in which we see the development of what I would call "popular culture."

So first, some terms. One might generally consider popular culture to be the culture of the masses, the ordinary people. This is opposed to "high" or "elite" culture. To be simplistic, we might posit that in the late twentieth century United States, television was a ubiquitous form of popular culture, while theater was "high culture" (one might quibble and ask about musical theater - as we shall see, in practice the boundaries between the two are porous, and many people have devoted large amounts of their time to defining and otherwise considering those boundaries). Popular culture should not only be contrasted with high culture. It also, in a classic definition, should be contrasted with folk culture. What is the difference between popular and folk culture, you might ask? Well, the difference is industrial capitalism. Before capitalism, in this formulation, there were indeed bifurcations in culture. The elites had their culture, which in Europe was the culture of the Church, and the universities, and generally included literature, painting, sculpture, and others of the high arts as we still understand them today. And then you had folk culture, which was autochthonous (meaning originating from below, from the earth), and was indigenous to the largely illiterate common people. This might have included popular songs, tales, and other aspects of the oral tradition, as well as the sort of vernacular architecture of everyday life, popular religious cults, etc. And really, there was not too much that these spheres of culture had to do with one another. The elite, content with economic domination, left the ordinary people alone in their culture. And without literacy, the common people had no access to their culture. However, with the advent of industrial capitalism, greater and greater numbers of ordinary people began to achieve literacy, as well as other means of accessing elite culture. This caused elites, disgusted by the lack of exclusivity attendant to their culture, to consciously attempt to push it in new and innovative directions. This is what is known as "avant-gardism," the conscious attempt to push culture in a new, forward-thinking direction. However, what quickly develops is an attendant process, which we might call "kitschification," whereby the new advances in high culture are claimed by popular culture, which takes the forms of high culture and removes its political and social meaning, remaking it as a pure commodity for consumption. The driver of this process is capitalism, which seeks to quiet the masses through the ever-more-insistent creation of novelty. Thus there is a kind of constant race between avant-garde and kitsch, with every advance of the former being swallowed and evacuated of meaning by the latter. One example is the avant-garde painter Piet Mondrian, whose radical experiments in painting were taken up by fashion designer Yves St. Laurent, which inspired a host of imitations, at ever-decreasing price points and further and further evacuated of Mondrian's original political and social purpose.

So, before I move on, I should mention that the previous paragraph was based entirely on an essay by the influential critic Clement Greenberg called "Avant-garde and kitsch." I had not really intended to spend that much time on him, but I found myself writing quite a bit. I'm teaching 20th century art this semester, so I suppose I have the issues surrounding avant-gardism close in my mind. Anyway, if you think Greenberg's ideas sound both kind of dreary and also super elitist, well, you're right. Greenberg was a Jewish Marxist who got started in the 1930s; it was a gloomy time. And for Greenberg, as for many avant-garde artists and their fellow travelers, avant-gardism was at its core a political movement, one that married formal changes in art to radical politics. So the appropriation of high art imagery by capitalism was for him at its core a tragedy, as it made that art no longer able to serve its political function. Greenberg did not despise folk culture, the old culture of the pre-industrial common people. He despised industrial pop culture for serving as a distraction to the masses, for making them dependent on its commodities for entertainment, rather than allowing them to rely on their inherent interests. Thus, while he was undoubtedly an elitist, he was an elitist for reasons that we might ultimately diagnose as hopeful or forward-thinking

I can also provide you with an example of how this played out in practice, which is the nineteenth century U.S. At the beginning of the nineteenth century in the US, there was little (or at least much less) distinction between high and low culture than there was at the end of the twentieth century. To take one example: the theater. While in recent times the theater has mostly been the province of elites (especially here, where the government largely does not subsidize the theater), in the early nineteenth century it was the most popular art form among people of all stripes. In fact, the most popular theatrical productions were undoubtedly Shakespeare (remember, even Huck Finn is familiar with Shakespeare), with huge crowds gathering to see famous stars like Edwin Forrest and Junius Brutus Booth (father of John Wilkes Booth, incidentally) perform their version of famous plays, in particular Hamlet and MacBeth. In fact, one of the largest riots before the Civil War Draft Riots in New York was as a result of an early attempt by New York's moneyed classes to keep the rabble out of the theater. The working class of the city were generally fans of Forrest, who was a forceful and boisterous American type, while the elites generally preferred the more genteel English actor William Charles Macready, the two of whom were engaged in a fierce rivalry. Forrest had taken to following Macready around and appearing in the same plays as him, in order to challenge him. Eventually, tensions around this broke out, with members of New York's elite building their own theater (with a strict dress code that included kid gloves) specifically to keep Forrest's largely Irish, immigrant fans out. These fans, egged on by their superiors in various Bowery gangs, rioted, breaking windows, throwing garbage, and generally disrupting the performance. This went on for several days, until the city called out the militia, who fired into the crowd, killing and wounding dozens. The elites praised the city for taking proper measures, and moved their new opera house even further uptown, to 15th street. Increasingly, even if they enjoyed the same plays, New York's elites and workingmen did so in separate spaces. Theater remained broadly popular for some time longer, but its bifurcation ensured that the two spheres would continue to grow apart, with the elites enjoying their Shakespeare and other European imports, while the masses tended to enjoy melodrama, vaudeville, and other forms we now recognize as "lower."

Another way in which this played out was with visual art. In the 1840s and early 1850s there existed an organization in New York called the American Art-Union that sought to encourage American artists, basically by buying their works and then distributing them via lottery. Until the lottery, however, it displayed its works in its free gallery, which was located downtown and featured at least one night a week of evening open hours, so that the city's working men might also take advantage of it. The Art-Union was closed as an illegal private lottery, however, and by 1870 when a group of wealthy New Yorkers decided to open a permanent art museum in the city, they did so on the Upper East Side, far from the Five Points and other working-class neighborhoods. The intervening years had seen the Draft Riots and a great deal of other civil unrest, and the elite were increasingly walling themselves off into the upper parts of the city, protected by an increasingly professionalized police and national guard. This physical walling was accompanied by further and further splits in culture, until the cultures of the two groups were largely mutually unrecognizable. Thus enter Greenberg and kitsch, and the increasing domination of capitalism over the culture of the lower classes.

Sources:

Greenberg, "Avant-garde and kitsch."

Lawrence Levine, Highbrow/Lowbrow

Sven Beckert, The Moneyed Metropolis

David Grimsted, Melodrama Unveiled and American Mobbing