Amazonia and the tropical Andes comprise the two richest regions of endemism in South America, each with highly distinctive faunas found nowhere else. Yet these faunas are not entirely independent, and many predominantly Amazonian lineages contain one or several Andean species. Analysis of DNA markers suggest that some Andean lineages originated in the Amazon, and that some Amazonian lineages were the result of colonization/diversification by Andean species. Fifth-year CEB student Nate Upham and his advisor, Field Museum Curator of Mammals Bruce Patterson, explored these biogeographic transitions in an article entitled “Transitions between Andean and Amazonian centers of endemism in the radiation of some arboreal rodents", published 9 Sept 2013 in BMC Evolutionary Biology. Using three major lineages of arboreal spiny rats (Echimyidae) that are well documented in the Field Museum’s mammal collections, the team documented phylogenetic patterns in rodents and generalized them with surveys of diversification in various other groups, including butterflies, frogs, birds, and other mammals (including the newly described olinguito). They show that biogeographic transitions between regions are infrequent but have been reciprocal, from the Andes to Amazonia and vice versa, and happening more or less continuously over the last 12 million years. This work comprises the second chapter of Nate’s dissertation, supervised by Bruce. Here is a link to the Open Access article.