Macroevolution vs. microevolution is a false dichotomy dreamed up by the Creationism/ID crowd. It's the way they allow themselves to acknowledge incontrovertible evidence for evolution, like acquiring antibiotic resistance, while still denying evolution.
Unless you can give me a strict definition of the boundary between macro and micro, then they're all the same governed by the same laws.
And then there is natural selection, which decreases the amount of variation of a species.
It's very important to keep different things separate.