It might work if intelligence wasn't important for members of lower castes but was for higher castes, perhaps due to the nature of their culture or work. Then low castes wouldn't select for intelligence but high castes would. Not saying that's what actually happened but it shows that selective pressure could still exist without inter-caste mobility.
Stop this nonsense about topics being inappropriate to discuss unless you're a nun. Nobody has a cultural taboo about discussing dog breeding or evolution of other traits. Just race and intelligence is taboo because people have been indoctrinated to believe that low intelligence marks you as an inescapably undeserving or a "bad" person. If you broaden your mind and stop treating intelligence as such an important aspect of life, then it won't be so taboo.
Stop this nonsense about topics being inappropriate to discuss unless you're a nun. Nobody has a cultural taboo about discussing dog breeding or evolution of other traits. Just race and intelligence is taboo because people have been indoctrinated to believe that low intelligence marks you as an inescapably undeserving or a "bad" person. If you broaden your mind and stop treating intelligence as such an important aspect of life, then it won't be so taboo.