I believe that every character that doesn't yet have the optimates or populares trait, has 1/5 chance every turn to acquire the populares trait and 1/5 chance to acquire the optimates trait. Once one of them is acquired the other one can't be acquired anymore, they're antitraits. So that would mean that in normal circumstances, you would get a nice number of populares and optimates, and onely seldomly a character never gets one of those traits during his life.