Fertile periods in pop music—the early '80s, for example—are usually marked by collaboration and cross-pollination: Chic's Nile Rodgers produces David Bowie. Hip-hop producer Arthur Baker remixes Bruce Springsteen. Michael Jackson drafts Eddie Van Halen for a solo, while brother Jermaine brings in the members of Devo. Prince plays keyboards on a Stevie Nicks song, but it's a minor tremor compared with the genre-busting in his own hits.

In 2013, a lot of people consider the concept of genre to be permanently busted. But if you were still looking for new examples, they abound on mainstream top 40 at the moment. It's hard to top a folk/EDM hybrid, sung and co-written by a retro R&B artist. Avicii's "Wake Me Up!" is No. 1 at triple A and No. 1 at mainstream top 40 this week. It's also a hit of some magnitude at adult top 40, alternative, dance, Americana and, momentarily, mainstream AC. And it's not the only song that fits everywhere by fitting nowhere.

The song that was No. 1 last week, Lorde's "Royals," started at alternative before conquering mainstream top 40, adult top 40, triple A and Americana. It's a lyric that could be on country radio, but it's over hip-hop beats. So even though it's gently chiding the materialism of hip-hop lyrics (and others), it's top three at rhythmic top 40 and scaling the R&B/hip-hop chart.

And if you keep working your way down the mainstream top 40 chart, you'll find:

The rapper who decides to sing from time to time (Drake).

The rapper who collaborated with another genre-defying artist and samples "Smells Like Teen Spirit" (Jay Z). He shares the charts with the rapper sampling a Billy Squier track (Eminem), although to be fair, the rap/rock hybrid goes back as far as the '80s songs mentioned above.

The chillout artist who finally found a foothold at top 40 through a dance artist/producer (Lana Del Rey).

The band with this week's only guitar-rock hit that began its career as an alternative act before being exiled to top 40, although Paramore hasn't really changed its sound that much since the mid-'00s.

Taylor Swift dueting with Ed Sheeran—not even his most provocative collaboration, given his One Direction co-write.

Miley Cyrus, Selena Gomez and every other teen idol (or defrocked teen idol) sampling dubstep. That's a 15-month-old trick wrested from alternative and active rock acts, although, as it happens, it took that long for Awolnation to scale the top 40 charts with a song that was pioneering at the time.

It’s undoubtedly the case that some of the recent genre-busting reflects an understanding by artists and the industry that listeners are sincere (in some manner) when they declare their favorite genre to be "a little bit of everything." But it's also a matter of necessity.

For starters, there aren't that many routes to top 40 for the pure version of any sound. At this moment, top 40 sets the agenda for other formats, not vice versa. Country is rich in its own sonic experimentation, but it rarely sends songs over. Alternative is rich in songs that sound like pop hits, but is still allowed to fill only a few slots. Hip-hop is already represented by the one artist still welcome at top 40 on a regular basis—and Jay-Z still needed to bring Justin Timberlake and Kurt Cobain.

As a result, a lot of artists from other genres are making pop singles because they can no longer cross to top 40 any other way. In a parallel universe, Taylor Swift's "Red" could as easily be the pop single, and "Everything Has Changed" would barely sound out of place on country. Usually, the dual singles track is a good stretch for the artist. But it means less sonic diversity for top 40. Six weeks ago, I was kind of hoping that "Wrecking Ball" would be the last brutal dubstep chorus. Now we're committed to another 18 months of them.

EDM as the new pulse of modern AC will continue for a while as well, from OneRepublic's "Counting Stars" to One Direction's just released "Story Of My Life." And if other acts weren't allowed to join the party, we would have missed the best single from the Fray in years. But they won't all be that good. Now, it's genre-busting. Soon, like the dubstep chorus, it will just be a regular part of the pop toolbox.