The Lost World of South America is a unique landscape of flattop mountains that are home to hundreds of endemic species. These flattop mountains, or tepuis, were formed after millions of years of erosion of the high-altitude Guiana Shield plateau. The tepui summits, isolated by their surrounding cliffs that can be up to 1000 m tall, are thought of as “islands in the sky,” harboring relict flora and fauna that underwent vicariant speciation due to plateau fragmentation. High endemicity atop tepui summits supports the idea of an ancient “Lost World” biota. However, recent work suggests dispersal between lowlands and summits occurred long after tepui formation, but neither view (i.e., ancient vicariance vs. recent dispersal) has strong empirical support owing to a lack of studies. I tested diversification hypotheses of the Guiana Shield highlands by estimating divergence times of Tepuihyla, a Guiana Shield endemic genus. Diversification among the different species did not support the Lost World Hypothesis of summit diversification, but rather recent dispersal approximately 50 million years after tepuis formed. This study was the first to explicitly test these hypotheses with a tepui endemic vertebrate, and as such a significant contribution to our understanding of the evolutionary history of this region. After increasing sampling, I focused on three of the most recently diverged lineages of Tepuihyla, in order to examine population genetics, phylogeography, and species delimitation atop these summits. I found high levels of lineage sorting in spite of low divergences in both nuclear and mitochondrial genes. I also found an unexpected pattern of nuclear versus mitochondrial diversity, suggesting the possibility of a recent mitochondrial selective sweep. Species delimitation analyses support the existence of a cryptic, undescribed summit species. Finally, I obtained a Single Nucleotide Polymorphism matrix with next-generation sequencing in order to observe more fine-scale population structure atop the Chimantá massif, a formation composed of ten tepui summits and intermediate altitudes separating them. I found high levels of population structure and assignment atop different tepui summits on the massif, indicating that even at extremely low levels of divergence the landscape complexity may be fomenting population isolation even at the smaller scale of within-massif divergences.