“Big Brother” (7 p.m., CBS) returns for a fourth installment, this time with a surprising — and sadistic — twist. Instead of spending three months with perfect strangers, some of the houseguests will be shocked to discover that they are going to be incarcerated with ex-lovers. Will the former couples play nicely together, or drive each other crazy?

When “Big Brother” debuted a few summers back, much was made of the title’s reference to the all-seeing dictator in George Orwell’s novel “1984.” But after watching way too many reality shows, I’ve got a hunch that “Brother” may trace its literary roots back to Jean Paul Sartre. He’s the French existentialist philosopher who wrote the 1944 play “No Exit,” which depicts four strangers in a drawing room that they slowly discover is their own very private hell. One character, Garcin, sums up the situation (and the key to “Big Brother”) with the quotable phrase “hell is other people.” Another character, Ines, cuts right to the heart of the “reality TV” audience when she admits, “I need the suffering of others to exist.” Of course, Ines never had to wrestle with the notion of a $500,000 prize for being the last contestant to leave the padded cell.

• What if the earth suffered a cataclysmic climate change that caused mass extinction? “The Future is Wild” (7 p.m., Animal Planet) ponders what type of creatures might evolve to repopulate the planet five million years in the future. Using computer graphics, “Wild” offers a speculative vision of something called a saber-toothed snowstalker and a big, nasty rodent called the shagrat (paging Austin Powers!) that’s larger than a sheep. Next Wednesday, “Wild” looks at Earth 100 years in the future, and the July 25 installment ponders life 200 million years from now, when all of the continents have drifted together to form one huge land mass and a single enormous ocean covers three quarters of the weird planet’s surface.

Tonight’s other highlights

• A visit from Great-Grandma (Marion Ross) on “Gilmore Girls” (7 p.m., WB).
• Rita Moreno guest stars as Lulu’s mother on “The Guardian” (8 p.m., CBS).
• Eddie enlists a former child prodigy on “Keen Eddie” (8 p.m., Fox).
• The World Series of Poker (8 p.m., ESPN) unfolds in seven one-hour segments, airing every Tuesday beginning tonight.

Late night

Juliette Binoche, Joe Queenan and Blur appear on “Late Show with David Letterman” (10:35 p.m., CBS) … Jay Leno hosts James Woods, Kristanna Loken and Evanescence on “The Tonight Show” (10:35 p.m., NBC).