This link leads to an interesting article in Science Daily about bees in New York that have developed a natural resistance to varroa mites via natural selection. As a side note, this phenomenon is similar to how the USDA Russian strain of bees developed their natural resistance to varroa in the Primorsky region of Russia.
Another important point of the article is the importance of maintaining genetic diversity in the bee gene pool. This helps the bees select genes via natural selection (ie. good ones survive, bad ones die) that help them combat varroa. Continuously procuring your bees from the same source probably is not the wisest choice. The industrial queen raisers who supply queens for packages are probably not motivated in this direction. So it will be up to you individual beekeepers to increase the genetic diversity in your apiary by varying your source and type of packages and queens.