CBS is splitting up the tribes for the 13th installment of its veteran reality series “Survivor” along ethnic lines, with blacks, Asian Americans, Hispanics and Anglos initially competing against each other in ethnicity-specific teams.

The castaways for “Survivor: Cook Islands” will initially be organized into four tribes divided along ethnic lines before merging in a later episode.

Host Jeff Probst defended the ethnic breakdown on CBS’s “The Early Show” Wednesday. Mr. Probst told “Early Show” co-host Harry Smith the idea “actually came from the criticism that ‘Survivor’ was not ethnically diverse enough.”

When Mr. Smith said “there were groans” from his colleagues about the “Survivor” news, Mr. Probst admitted that “at first glance when you hear the idea, it could sound like a stunt.”

But the show wanted to “turn this criticism into the creative,” he said. “It is a social experiment.”

The “Survivor” move has raised eyebrows even among seen-it-all reality executives.

“That’s a bold thing to do in this country,” said David Lyle, COO and general manager of Fox Reality Channel, upon hearing the news. “It seems somewhat inflammatory. But I daresay that’s what they want. It’s a a pretty confronting way to act in a world where ethnicity and tribalism and religious differences are seen as flashpoints.”