Modeling human evolution--to tree or not to tree?

Underhill et al. (this issue) report 19 new Y chromosome markers from a survey of 718 human genomes and use these data to construct a gene genealogy. According to the data, most of these biallelic markers were restricted to a few populations in specific regions of the world, but a few occurred at varying frequencies in all populations sampled, implying that they are older mutations. Underhill and colleagues constructed a genealogy of these polymorphisms using a novel combination of parsimony methodology and marker frequencies in 10 extant regional samples. In the discussion that follows, we distinguish the separate histories of chromosomal regions, individuals, and populations, as the interpretation of the branching process implied by a tree-like relationship