Hits that changed what the hits were.

In the early-to-mid '90s, when Garth Brooks was the biggest superstar in any format and leading the charge for a younger-leaning, more rockin' country format, radio still resisted two of his singles. One was "We Shall Be Free" with its reference to being "free to love anyone we choose." The other was a slightly rewritten Aerosmith song, "The Fever."  

Today, the younger-leaning, way more rockin' country format accommodates a lot. The No. 1 song of the year is Darius Rucker's Americana remake, "Wagon Wheel." No. 2 and No. 3 are Lady Antebellum's "Downtown" and Miranda Lambert's "Mama's Broken Heart," and neither of them are formulaic. Earlier this year, the latter song's co-writer, Kacey Musgraves, got away with suggesting on "Merry Go Round" that small-town life might not be so great.

Then Musgraves released "Follow Your Arrow," with its suggestion that a female friend was free to "kiss lots of girls," if so inclined. Eric Church gave "The Outsiders" a very typical country lyric, but finished it off with an active rock rave-up. Musgraves doesn't yet have enough of a career to force country to follow her lead, but Church seemed to. Yet both songs are still struggling the way those Brooks songs did in the '90s.

Mainstream top 40, by comparison, has a slot for anything. If you're not rhythmic pop,you'll wait nine months for it, but there's still room for EDM and Americana and, in the case of Avicii's "Wake Me Up!" (or Pitbull's "Timber"), even on the same song. It was the first time in years that PDs had to code songs both country and dance in their music schedulers. As we look at "songs that made a difference in 2013"—songs with larger implications for what top 40 played—a lot of the heavy lifting was done in 2012. But certain evolutions continued this year, and these are the songs that drove them.

Swedish House Mafia, "Don't You Worry Child": As 2013 began, this song and its Mumford-but-on-keyboards break was unavoidable. Beyond that, "Don't You Worry Child" was the transformation from turbo-pop hits headlined by pop and R&B chart fixtures (Usher, Rihanna, P!nk) to DJ/producers as the star attractions. Typical of its peers, Swedish House Mafia could sell out arenas, and most people didn't even hear of the act until it was breaking up.

"Don't You Worry Child" was also the keystone in rhythmic pop's transformation from party rock anthems to something decidedly more sober. Rihanna and Calvin Harris found love in a hopeless place. Harris, Ellie Goulding and Florence Welch managed to find anxiety, even at 115 bpm. It made 2013 the year of the passive/aggressive dance record—lyrically the former, musically the latter. It was still "throw your hands in the air," but this time it was in dispair. And while it was a subtle transition at top 40, you couldn't help wondering if the new darker sound was really as mass appeal as its predecessor.

Robin Thicke, "Blurred Lines": Its research hook was the most provocative line in the song, meaning that female listeners had plenty of chances to be offended, but mostly weren't. And while Thicke murmured suggestively to the ladies, he kept their dates distracted with an R-rated video. In recent years, programmers have watched the miracle growth of top 40 and worried that adults would rebel against the edginess of the lyrics. This year, you might legitimately have wondered if a certain amount of low-grade titillation was part of the adult appeal.

Miley Cyrus, "We Can't Stop"; Anna Kendrick, "Cups (Pitch Perfect's 'When I'm Gone)"; Sage the Gemini, "Red Nose": Cyrus was synonymous with low-grade titillation this year, but these songs are all here as examples of programmers post-"Gangnam Style" trying to triage all the songs-with-streaming stories. "Cups," like many songs in this and other recent years, took a ludicrously time long to reach top 40. Sage the Gemini bubbled up with an organic 1992-style success story, then switched up and gave rhythmic top 40 a single that was easier to play. And before that TV appearance, Cyrus' streaming success story sped up acceptance of an artist who top 40 didn't always accommodate. But it's still hard for radio to distinguish the phenomena that should be acknowledged from the "Harlem Shake"-type memes that don't translate.

Lorde, "Royals," "Team": "Royals" fit everywhere, including the R&B/hip-hop format it seemed to scold, but went to alternative first, because labels still do that when they can. "Team" is the best case of a sound-alike follow-up equaling its model since Michael Jackson's "Don't Stop 'Til You Get Enough" rewrote and even improved "Shake Your Body (Down To The Ground)." 

Jay-Z, "Holy Grail" (featuring Justin Timberlake), "Tom Ford": There's nothing especially mass-appeal about these songs. They're not as radio-friendly as, say, J. Cole's "Crooked Smile." But while Cole languishes in the lower reaches of the top 40, "Holy Grail" has gone top five, and "Tom Ford" is grabbing some (once unimaginable) mainstream spins. Along with "Thrift Shop" and "The Monster," those songs are proof that top 40 audiences still want to like hip-hop. But with R&B radio so off top 40's radar, it's usually hard for anybody other than an established name to provide it. Then there was . . . 

Macklemore & Ryan Lewis, "Same Love," "Can't Hold Us": In Australia, "Same Love" was the follow-up to "Thrift Shop." So I initially considered it an act of capitulation when American radio was given "Can't Hold Us" as the safer choice. Eventually, however, that strategy gave us a hip-hop record as mainstream top 40's most-played song of the year, then another hit ripped from today's headlines in a way that no summer song had been for decades. By comparison, MCA Nashville tried a similar strategy with Musgraves, but couldn't find enough takers for the "safe" second single, "Blowin' Smoke," to give "Follow Your Arrow" the momentum it would have needed. So be grateful that "Same Love" became a song that made a difference in 2013.

What are the songs that had the greatest impact on top 40 in 2013 for you? Please leave a comment.