Finally, original population substructure and subsequent processes of genetic drift and founder effects can introduce errors into the estimation of coalescence dates of mitochondrial and Y chromosome haplogroups [18]. (journals.plos.org)
Apparently the variant, known as haplogroup D, was favored by natural selection, although no one is sure of its function (Science, 9 September 2005, p. 1662). (sciencemag.org)