As fun.’s “We Are Young” featuring Janelle Monae ascends to No. 1 on the Billboard Hot 100 this week, we also got news that Toby Keith’s former country smash, novelty song “Red Solo Cup,” is going for ads at Top 40 radio as well.

Those are just two examples of the changing landscape at Top 40 radio, which is, slowly but surely, expanding its parameters once again.

For the last several years, Top 40 has been dominated almost exclusively with hip-hop and rhythmic leaning tunes.  To be sure there have been a few exceptions, such as Train’s “Hey Soul Sister,” Taylor Swift’s “You Belong With Me” and, of course, Adele’s three chart toppers, but by and large, even a Katy Perry or Britney Spears track would get bolstered by a remix with a rapper or with some heavy rhythmic component. For awhile there, it seems like Lil Wayne was on every song because his presence definitely gave the song a better shot at rising up the chart.

Looking at the songs that reached No. 1 on the Billboard 100 last year, only Adele’s “Rolling In the Deep” and “Someone Like You,” as well as Maroon 5’s “Moves Like Jagger” featuring Christina Aguilera and Katy Perry’s “Firework”  weren’t built around a thundering bass or hip hop. In 2010, pretty much every song to hit No. 1, other than Bruno Mars’ “Just The Way You Are,”  was uptempo and bass heavy. In 2009, Owl City’s “Fireflies” was the only non-rap/rhythmic song to top the charts. The last rock group to hit No. 1 before fun. was Coldplay in 2008 with “Viva La Vida.”

For a very long time, most acts that were pure rock or adult contemporary have had little chance of getting very far up the Hot 100, but it seems as if the door is cracking open (which could be good news for the likes of someone like John Mayer, who releases his first album in a few years later this year, as well as for Jason Mraz.)  Adele has not gotten enough credit for creating music that Top 40 simply couldn’t deny despite her not going to Kanye West or Lil Wayne or Nicki Minaj as a featured guest. When was the last time before “Someone Like You” reach No 1 that you heard a pure torch ballad on Top 40?

Now, with Adele,Katy Perry and Rihanna's help, there’s room for purer pop on radio (Rihanna effortlessly shape shifts between pop and a rhythmic hybrid of pop and hip-hop). For example, British boy band, The Wanted, is exploding at radio with “Glad You Came” (which got a boost from “Glee”). Although after opening like a Coldplay tune, the song, No. 4 this week, very quickly segues into electropop so that gives it a very big advantage at Top 40. “Glad” is The Wanted’s first top 10. Same for Gotye’s “Somebody That I Used To Know” featuring Kimbra, which soars to No. 9 this week. If the song already sounds familiar, it should: “Somebody” has more than 100 million views on YouTube and has been around for more than nine months, and spawned some incredible cover versions by Walk Off The Earth and Ingrid Michaelson. Sometimes Top 40 radio leads, but often, as in this case, it follows.

So what does this mean going forward: It’s exciting to see new acts coming onto the chart but it’s not  a new development.  Last year, 20 acts scored their first top 10 hit either as the main artist or as a featured artist. The number was 19 in 2010. This year looks like it will easily reach that stat. Many of them we’ll never hear from again simply because that is the nature of Top 40: to capture the moment and part of that means creating One Hit Wonders. With few exceptions, Top 40 has always been about the song, not the artist. It's way too soon to predict where we're headed other than a lessening of hip hop's grip on Top 40.

Despite Billboard’s declaration that fun. is the first rock band since Nickelback to see its debut single reach No. 1, we’ve got some ways to go before we see more rock at the top of the Hot 100. There’s a very wide chasm between “We Are Young” with its operatic swirls and pop flourishes, and what most folks call rock, such as the Foo Fighters. For example, The Foos’ 2011 smash “Rope,” which is one of its most successful songs ever, reached No. 1 on Billboard’s Alternative Songs, Mainstream Rock Songs and US Rock Songs charts. It peaked at No. 68 on the Billboard Hot 100. As of now, the only way they are getting into the Top 10 on Top 40 radio is with a duet with Ke$ha.

What do you think it means? Is it all cyclical or are we seeing a major change? 

Follow Melinda Newman on Twitter @HitfixMelinda