What is Great Replacement Theory?

 [Copied over from a tweet stream.]

So now I'm trying to figure out what Great Replacement Theory is. It seems to be one part true and one part sort-of true.

The true part is that we expect a low-melanin blond-red hair (LMB) phenotype to become less common barring genome hacking. That's been well expected since at least the 1970s (perhaps 1870s?). These are recessive genes and migration and differential birthrates mean they will become uncommon.

The sort-of true part is that libs like me don't care about this. It's not just that my white skin is a PITA (thin, burnt, premalignant), it's also that we have a lot more to worry about. Like civilization for example.

The mostly untrue part is that libs/dems are conspiring to accelerate the decline of the LMB phenotype.

It's true we hope the GOP's white nationalism will discourage "non-white" (whatever whiteness is) voters, it's true we encourage immigration as a generally good thing for a low birth rate America that has benefitted from attracting worldwide talent, it's true that we enjoy and appreciate novelty and diversity, and it's true that we think it would quite good if all this led to the GOP to morph into a non-racist opposition party.

But it's not true that this is an explicit conspiracy.  It is, perhaps, an emergent result of our not caring that much about preserving a particular phenotype, our interest in preserving human and American civilization and our affection for novelty and diversity. I can, however, see why people who are passionately attached to LMB phenotypes would confuse this emergent result with a diabolical conspiracy.