The old nature vs nurture debate. No one knows the correct answer, and there is extreme cases on both sides of the argument.
Personally, I say that it's kinda 50/50. You can naturally be really good at a game, and become the best player with little practise. But thats not to say you can't be really bad and become the best player with a load of practise. I think the true great players have to have some degree of natural tallent backing up the skills they learnt.
There are extreme cases on both sides though, people who naturally are amazing at something they have never done, and people who sucked, but became amazing with practise. There is this young Asian child who plays the piano like no other, and I think he is like 13. He was amazing at it with next to no practise. He is one of few people who has pitch perfect ears aswell. He can say any note he hears perfectly, which is part of his natural talent. There is also this Russian woman who is the first, perhaps only female to become a Chess Grand Champion, she had no talant at chess, but spent years and years and years practising. Her father was out to prove that nurture plays a massive role in talent, and I guess he did.
Arbiter is 1337!