You need to get onto the PhD programme in the first place - meaning you'd probably need a good degree and some sort of funding - which does select reasonably we

You need to get onto the PhD programme in the first place - meaning you'd probably need a good degree and some sort of funding - which does select reasonably well for ability. There's gonna be exceptions however (people who are otherwise capable but lack either of those, or people who are not the smartest but have managed to secure both)