FIGURE S7. Illustration of an example case of ancient population subdivision that could lead to an identical data signal as that created by genetic introgression between mosaic birds and Z. chrysops. Note that substantial levels of persistent ancestral subdivision (across speciation events) are needed to account for such patterns of asymmetry of tree signal (Slatkin and Pollard 2008). Given most Andean birds’ turbulent Pleistocene evolutionary history, this alternative scenario is probably unlikely.