Research is an entirely different ball-game (or at least it is from where I'm standing!) to the kind of skills used as an undergrad. Therefore, I think as long as you're not a dimwit, the expectations that a supervisor might have are more to do with your maturity level, ability to work alone etc. I think it's perfectly normal to be doubting yourself at the beginning, but if you *were* unsuited to that particular PhD project, it's unlikely that you would have been chosen for it. Hope this is helpful!