VIP restriction site mapped on Zonotrichia tree. Phylogenetic tree for Zonotrichia from  showing branch lengths (% nucleotide differentiation). Mapped on the tree are presence (+), absence (-), or both presence and absence (+/-) of the Dra I restriction site in the VIP fragment. Data for the restriction site were derived from Table 1. Only the four North American Zonotrichia species show polymorphism for the VIP fragment, suggesting that the polymorphism arose in the common ancestor of those four species.