Results from Structure runs on 13 nuclear microsatellite loci for successively smaller genetic clusters ( K = 2 for each run). Numbers correspond to sites listed in Table 2. (A) The first run split the individuals broadly into coastal and interior groups, with mixed assignment for individuals in the contact zone, which were removed from further runs. (B) The coastal group showed little geographic structure. (C) The interior group split into a US Interior cluster and a Mexico cluster. (D) An Edwards Plateau group split from the rest of US Interior; (E) A northern Mexico group split from a southern Mexico group; (F) The southern Mexico group showed some evidence for differential assignment between Oaxaca and Guerrero populations.