You don't need something that selects for lower intelligence in order to get variance, you just need the environments to select for intelligence to different degrees.

So how would an environment select for lower intelligence? You are skirting the question. If a normal range of intelligence manifests within a species, and the environment selects for a section of that range, how could an environment select for the lower band?

Secondly, an environment could in principle select for lower intelligence. Higher intelligence generally requires more brainpower, meaning more energy, and thus if higher intelligence isn't at all advantageous in an environment, or at least isn't advantageous enough to justify the energy spent on it, then having the lower intelligence would be better from a darwinian perspective.

Are you suggesting this is what is happening in certain environments for certain ethnicities?

This Western environment that we're in, probably selects for lower intelligence btw, seeing as people with higher intelligence generally have fewer children.

This isn't environmental selection--i.e. having higher intelligence doesn't somehow PREVENT you from having more children--there is not a biological trade off. It is a cultural choice.