Cladistic biogeography of the Mexican transition zone

Biogeographic relationships among nine montane areas of endemism across the transition zone between North and South America are analysed cladistically based on phylogenetic hypotheses of thirty-three resident monophyletic tax a of insects, fish, reptiles, and plants. Areas of endemism include the Arizona mountains (AZ), Sonoran Desert (SD), Sierra Madre Occidental (OCC), southern Sierra Madre Occidental (SOC), Sier ra Madre Oriental (ORI), Sierra Transvolcanica (TRAN), Sierra Madre del Sur (SUR), Chiapan-Guatemalan Highlands (CGH), and Talamancan Cordillera (TC). Area relationships are summarized using Brooks Parsimony Analysis and Assum ption 0, with the former resulting in more defensible biogeographic hypothe ses. Areas of endemism are dividable into two monophyletic groups; a northern gr oup including AZ, SD, OCC, and ORI, and a southern group consisting of TC, CGH, TRAN, SUR, and the isolated southern regions of the Sierra Madre Occid ental (SOC). The northern set of areas are characterized by recent, probably Pleistocene , isolation and prevalent widespread species, whereas the southerly areas p robably diverged after Pliocene closure of the Panamanian isthmus. The southern areas are redundantly represented on many of the taxon-area cl adograms by endemic species, indicative of much higher levels of endemism i n the Sierra Transvolcanica and further south. Use of a general area cladogram in such a transition zone permits explicit exploration of biogeographic patterns and establishes a predictive framewor k for taxonomy and conservation prioritization.