In the absence of natural selection, negative mutations accumulate more readily. (It turns out they accumulate anyway under natural selection, which is fighting a sort of rear guard action against their inevitable accumulation.) We see a similar phenomena in domestic cats versus wild felines in studies of neuron development.
No, it’s all selective breeding. And even then, certain characteristics do not go away.
You’re referencing human specie, right?
Well, not just that, but the domestication process itself would weed out the very smartest of animals, because you don’t keep breeding the ones that are constantly trying to escape, disobey you, assert dominance over humans, etc. You want to keep breeding the more “beta” animals instead, because they will naturally look to the humans for leadership.