Written by By Selena Roberts, CNN

The Doobie Brothers have been together for over 45 years. But as the well-known classic rock band celebrates its 50th anniversary this year, it is clear that age is neither a problem nor a limiting factor. And, despite the ever-changing musical landscape, the Doobie Brothers’ first rule remains the same.

“The one thing that we always try to do is take requests,” says Tom Johnston, the band’s founding bassist and lead singer. “Maybe our fans still want to ask us some of the songs that we’ve never played before, some of the hits that we’ve had for 50 years.”

The Doobie Brothers look back at 50 years of music. Credit: Brian Finke

Johnston credits his bandmates — including founding vocalist Michael McDonald and lead guitarist Patrick Simmons — with maintaining a high standard.

“The thing that keeps us on our toes is the fact that we never consider a tour as a happy tour,” he says. “Every tour, we always push each other in a way that the fans should never feel like, ‘Well, that’s it, we’ll hit the road and go.’ We always have a war to fight to make sure that we can still do what we do after 50 years, and that’s write songs, play instruments, sing those songs in a way that they’ve always gotten a reaction from our fans.”

Talking with McDonald and Johnston about their careers as musicians for over five decades, it is obvious that their passion and talent never waned, no matter what the competition was, or the particular challenges of each era.

Johnston admits that his approach to maintaining the band’s musical legacy is simple: “The easiest way that you can lose any combination of musicians is if you burn out or break up, and it makes everyone unhappy.”

There’s no denying that the rise of bands like A Tribe Called Quest, Rage Against the Machine and Public Enemy later in the 1990s had a serious impact on the rock landscape. But the Doobie Brothers’ interest in experimental music remained strong, and they still have albums that seem ahead of their time: “Find Out Who Your Friends Are” (1995), “Soul Kitchen” (2005) and “Sounds of Blackness” (2011).

The band’s 50th anniversary tour also gives us a view of what’s on the horizon for the Doobie Brothers. Though it is always interested in collaborating with new talents, the founding members say that they are not looking to write new songs, but rather to make “old songs you don’t have to worry about putting CDs on your shelf.”

“One of the things that we’re always trying to do when we tour, or the time that we are in a studio, is get other musicians in and make it a little bit more interesting,” says Johnston. “So when you walk in, the sound is going to be a little different from what you’ve heard in the past because we got another sound system, maybe with a different member of the band. So we’re always trying to be a little bit more interesting, and that’s kind of the goal.”

So, what’s next for the Doobie Brothers?

Well, for now, that goal is a much more fun and varied one. This month, the band plays the Greek Theatre in Los Angeles as part of its 50th anniversary tour.

“It’s a huge opportunity for us to continue to do what we do for the fans and for music,” says Johnston. “I’m pretty excited for it.”