> Isn't it possible that those without the beneficial cholesterol are the result of a mutation?
Possibly. More detailed examination of the DNA would show one way or the other. However, the fact that it's rare and can be traced argues to this being a new mutation.
My memory on this could be faulty, but I seem to remember that a study of the population of this town in Italy was able to trace the gene in question to a single common ancestor not that many generations ago. It would appear that, in this case, the continued diversification of our species is going to point the way.
For those wanting to turn this into another flame war on the evolution topic, be aware that this is not the rise of a new species. This is only an adaptation in an existing species and neither proves nor disproves evolution or creation.