Why Jamaica Have The Quickest Sprinters

After I watched the 2012 London Olympics I couldn't stop thinking about how and why the Jamaicans are so fast. Many different people believe in many different things. The most believable story i heard a few weeks ago really had me thinking.
The reason why Jamaicans are so fast is because, after the slave trade had ended and the British had left the Americas the Jamaican people and people in some of the other islands starting breeding the physically fittest men and women. By doing this they hoped they could make a superior race which would be men of the physically fittest men and women. If this is true this may be the reason why the Jamaicans looked untouchable at the Olympics this year and the last 3 Olympics.

So can the Jamaicans be beaten by hard work and training or are other athletes simply not as physically advanced?