p-ter has an excellent review of a new paper, Localizing Recent Adaptive Evolution in the Human Genome. Imagine if you will a flat pristine stretch of snow covered field which exhibits a perfect 2-dimensional symmetry. Now, note what happens when a few snow blowers criss-cross the field. That's the sort of thing I imagine when attempting to analogize selective sweeps going through human populations over the past few tens of thousands of years. Money shot:
...In general, we find that recent adaptation is strikingly pervasive in the human genome, with as much as 10% of the genome affected by linkage to a selective sweep.