This is the Helgason 2009 paper on drift in Icelandic mtDNAm and how as a result, the mtDNA of the ancient samples was closer to that of the source populations than to that of modern Icelandic people. Unfortunately it's not the one to which I was referring, which I still can't find, but it's interesting none the less.
http://www.plosgenetics.org/article/info:doi/10.1371/journal.pgen.1000343
The study to which I had referred had to do with statistical patterns over a smaller period. (I still haven't found it, Epoch.)
Generally, I think that selection is extremely important, particularly as it relates to mtDNA. I think the huge expansion of the "H" clades since the Neolithic is a big factor in the changed mtDNA map of Europe. Clearly, that has something to do with the expanding populations made possible by the agricultural revolution, but I think it's also partly a reflection of certain selective advantages in terms perhaps of climate adaptability, but also as regards disease protection and fertility, both in women who are mtDNA "H", and men who carry it.
I think I remember a study that found that mtDNA "H" provides an advantage in terms of resistance to infection...perhaps someone has a citation handy. At any rate, with all of the benefits of farming in terms of expanding populations (I think hunter-gatherers have spent most of history on the verge of extinction) raising domestic animals for food is a filthy business even today with our increased knowledge of the importance of sanitary conditions. MtDNA "H" as well as other mtDNA clades involved with the early development of animal husbandry might have increased resistance to diseases carried or spread by the animals. (Hasn't even tuberculosis been linked to transmission from animals? ) I've also wondered what effect that might have had in times of plague, not only Justinian's, but also the Black Death.
This is John Hawks' take on all of this:
http://johnhawks.net/weblog/reviews/genetics/mtdna_migrations/selection-mtdna-iceland-2009.html
In terms of the "Y", the other issue is fertility rates. I know I read something about reduced sperm motility in men carrying certain mtDNA clades.