When Caesar released the ALZ-113 gas at the primate shelter, he boosted the intelligence of all its inhabitants, and most of those apes have been the core members of his tribe in the years following.
You know, like how it is with humans.
So Caesar’s tribe is just the tip of the proverbial iceberg when it comes to ape society in this brave new world, but as Caesar learned when he battled Koba in Dawn of the Planet of the Apes, just because apes are all smart doesn’t mean they’ll all get along.
But as the ALZ-113 virus (contracted by Will Rodman’s neighbor, Douglas Hunker) spread across the world and killed off most of the human population, it would also seem that it functioned just like the concentrated gas and increased the intelligence of all apes elsewhere, whether they were wild or lived in captivity.