Does natural selection happen to individuals or populations?

Answer: To populations!

Explanation: Evolution by natural selection is a process that takes several generations. We never say that individuals evolve. It's always populations that evolve.

Answer: populations

Explanation: Natural selection is a mechanism by which populations adapt and evolve. In its essence, it is a simple statement about rates of reproduction and mortality: Those individual organisms who happen to be best suited to an environment survive and reproduce most successfully, producing many similarly well-adapted descendants.