A new era in American talk show comedy begins Tuesday when Stephen Colbert officially steps into the shoes of television legend David Letterman at the helm of "The Late Show."

The man who shot to prominence on Comedy Central with an ultra-conservative alter-ego on "The Colbert Report" moves to CBS and a world of late night vastly different from the golden years of Letterman.

US networks face increasing competition, audience figures are down, young people watch less and less television and the success of Colbert's rival hosts at NBC and ABC has been fueled by clever video clips that go viral on the Internet.

The genre is "not as central as it used to be," explains Deborah Jaramillo, assistant professor of film and television at Boston University.

"The audience is much more fragmented" and young viewers watch shows on actual television sets less and less, she said.

When Letterman's rival Jay Leno stepped down at "The Tonight Show" on NBC last year, the channel -- an engine of late-night talk show evolution -- chose "Saturday Night Live" alum Jimmy Fallon to succeed him.

Young, debonair and with social media savvy, he has excelled at bringing along young viewers with him.

Before that, Jimmy Kimmel was shaking up things up at ABC since 2003 as host of "Jimmy Kimmel Live!"

Jeb and George

"Kimmel became successful in part because what he did was easily translatable to YouTube," said Dominic Caristi, professor in the telecommunications department at Ball State University in Indiana.

Colbert comes to CBS from Comedy Central, a cable network which was popularized by Jon Stewart's "The Daily Show" -- also good at nurturing an online profile -- and adult animated comedy "South Park."

By adapting to an Internet audience, the networks have repositioned themselves in a shifting media landscape.

In 2014, advertising revenue on late-night talk show slots grew 14 percent to $597 million, according to Kantar Media, a media analysis group.

At 51 years old, Colbert is older than the two Jimmys and the last to arrive. Some would have preferred to see the job go to someone like Stewart, who left "The Daily Show" last month.

Stewart told The New York Times he was certain CBS had made the right choice. "I'm not suited, and he is," he said.

Colbert will be expected to hold his own against the Jimmys, who excel in putting together skits with Hollywood stars or musical sequences with singers. Over the summer, he dripped out a series of short comic "Late Show" videos online prefacing his takeover.

One way in which he may carve out a niche would be to dabble in more social commentary than Fallon and Kimmel, Caristi suggests.

One early sign of that could be the inclusion of Jeb Bush, the Republican candidate for the White House whose poll numbers have tanked under the juggernaut that is Donald Trump, alongside Hollywood A-lister George Clooney as guests on the first show.

Can fill a gap

Vice President Joe Biden is booked for Thursday, and in the second week Colbert will host UN Secretary-General Ban Ki-Moon and Supreme Court Justice Stephen Breyer among his guests.

Colbert has already shown on Comedy Central that underneath his Catholic family persona, he is one of the most biting comics on American television.

"Colbert can maybe fill a gap," says Jaramillo, believing that he can attract young and older audiences alike.

The high financial stakes and conservatism of America's big networks can often restrict innovation, specialists say.

The set and set-up are sacrosanct: the guest's chair before the audience and the traditional starting monologue from the host.

"If you just look at the TV with the sound off, you probably won't notice any difference," quips Caristi.

It's an approach that irritates agents of change.

"We were hoping that CBS would go in a different direction and chose a woman or a person of color," said Jaramillo.

Apart from the exception that was Arsenio Hall, a black talk show host between 1989 and 1994, the big evening hosts have all been white men.

Trevor Noah, a South African comic who has a black mother and a white father, will replace Stewart as host of "The Daily Show" later this month.