Why do you not want black people in North America but are OK with them being in South America?

There are multiple issues with this question. First off, while some of us hold the opinion that blacks have no place in North America whatsoever and should all be deported back to Africa, a lot of us (myself included) think that it wouldn't be reasonable to kick them out (because they've been living in North America for centuries, and because their ancestors were brought over against their will rather than being invasive immigrants) and instead propose a black ethnostate (if it was up to me in the already black-dominated parts of the Deep South), with returning to Africa remaining voluntary.

Regarding blacks in South America, I only care about the white-dominated parts of South America (Southern Brazil, Uruguay, Argentina and to some degree Chile), from which non-white immigration should be prevented as much as possible, just like with any majority white country. For the non-white parts of Latin America, I personally don't give a damn if blacks want to live there, but I'd totally understand if those countries don't want them either; it's up to the peoples and governments of those countries to decide whether or not to allow in blacks (or any other group of people).

Racial differences within humans are tiny, whilst differences between species are greater.

The differences between humans, chimpanzees and gorillas are also tiny compared to the differences between apes in general and non-apes, so by that logic, why shouldn't we integrate chimpanzees and gorillas into our societies and interbreed with them?

Even if a change in the racial mix of the world happens, so what? Could it just be evolution?

Evolution would imply that the non-whites moving into white nations are conquering us by themselves and/or outsmarting us. Moving into another country because its treasonous government lets you in, just to go on welfare and then pump out 10 babies, most of whom will in turn grow up just to do the same, isn't conquering nor does it require any useful skill (let alone intelligence), in fact it's pure parasitism.