It is a combination. You are either born coordinated or you are not born coordinated. That is obvious. If you are coordinated, I believe, you can make yourself an athlete. An athlete CAN be born, however, they are not all that common. The athlete that works their tails off in the weight room, on the filed, and in the classroom is the athlete that gets noticed because he gets exposure. The athlete that relies solely on their talent and does not go to camps most likely will not be seen. These days, more and more coaches are looking at camps and combines more than they ever have. These camps and combines give the coaches an opportunity to see that "athlete" go up against other "athletes".
You can go D1 even if you don't have the size, IMHO. The problem that comes with that is that you have to leave no doubt that you are better than the player ahead of you that is taller. Coaches want tall OL, tall WR, tall DB, and tall QB. They want that and yes, that is in your genes, however; if you a 6'0 OL and you leave no doubt that you are better than the OL that is 6'5 ahead of you, the coach will play you BECAUSE THEY WANT TO WIN. Their job is to win football games and of the 6'0 OL is doing better than the 6'5 OL, the coach will play him. The coach will put the best 11 on the field that gives them the best chance of winning the game. Period. No "ifs", "ands", or "buts" about it.
Athletes can be made. They can be born. An athlete will not be athlete if he doesn't work. I dont care what you say to convince me otherwise. An "athlete" to me is some one who does whatever they can do to better. When you hear of an athlete, you don't hear of a lazy athlete. You hear of a working athlete-only because they put themselves in to position to be recognized.
You are born with size. No one can change that. But every thing else is justified by the athlete's desire to get better. Plain and simple. I am a firm believer that you can coach speed. It's very hard to get faster, but you can get faster. I've witnessed it myself along with doing it myself.