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Human Subspecies and Clash of Evolutionary Levels
The Morlocks would also apply in your specific example (the "pretty" Eloi counterparts are optional)
Worth noting that if they've actually read the Trope Maker (The Time Machine) and paid attention, there will be nods to the real commentary there: The Eloi are just pampered, mindless cattle living in a peaceful arcadia built and maintained by the intelligent morlocks.
Edited by Scorpion451
I can't find that trope where humans evolve into two completely different species: One that is advanced, clean, and utopian, and another that is savage, dirty, and Axe-Crazy.