Sorry the article didn't open. The summary doesn't explain much. A major example given of 'positive selection' is that Tibetans adapted to high altitudes in just 4,000 years. The gene is identified. This is a much faster rate than the known rates of random mutation and natural selection can provide. Therefore, it is concluded there are subtle mechanisms that can push adaptive evolution.- http://www.sciencemag.org/cgi/content/summary/329/5993/740