Yannis Pitsiladis is a biologist at the University of Glasgow who collects the DNA of champions, both sprint and endurance athletes, in an attempt to determine if there are genes which are necessary for success in certain sports. David Epstein quotes him in this great article on SI.com which attempts to shed some light on the age-old "nature vs nurture" debate.
All in all the article is very thorough and well worth the read. Epstein presents both sides of the argument, showing that indeed both nature AND nurture are critical for success. In the end, though, he postulates that perhaps the most important gene is the one that makes an individual push herself to work hard. Either way, there is plenty of evidence to back both sides; either you have the ability to improve, or you are destined to a predetermined fate. I guess that leaves it up to you, then, to determine which you would like want to believe!