A logical reason for humans to turn into dwarves/gnomes?

I am creating a fantasy world where on one continent there are humans, dwarves and gnomes. All 'species' can interbreed (much like Neanderthals and Homo Sapiens), thus they need to have close similar ancestors for it to make sense. The planet is like earth in terms of atmosphere, gravity etc.

Is there any logical reason as to why humans would (d)evolve to become physically smaller to represent a dwarf and or gnome without needing to involve magical influence?

I have already considered:

1. Magical influence. (Perhaps a magical crystal that enhances gravity?)
2. Living in a cramped environment (low ceiling, thin hallways) for a longer period of time.
3. Diet?
4. A birth defect becomes common?

I'd love to get some input on this.

posted about 5 years ago

Finn‭

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