Jim Bender links to my post where I express some frustration with tree based thinking in terms of human ancestry. He says, “I take that to mean that there is a great deal of mixing between the so-called races over time” [my emphasis]. Terms like “great deal” are insufficiently precise. Fst 0.2 is often used as a boundary number between low and moderate levels of population substructure, but it is somewhat arbitrary (1 migrant per generations yields 0.2). My overall point was that beyond a few generations in the past our intuitionally grounded concepts start to deviate from how gene genealogies work. 50 generations into the past I have an x number of distinct ancestors, but a y number of these ancestors show up many times at the tips of branches. In talking about populations the easiest citations are mtDNA and NRY, the male and female lineages…which causes problems in adding empirical support to overall assertions derived from an understanding of the details of how the tree plays out as you progress back in time, because mtDNA and NRY isn’t always a good match (concordance) with the rest of the genome.