Biogeographic Boundaries and Angiosperm Phylogenetic Endemism: Highlighting the Evolutionary Complexity of the South American Transition Zone

Abstract

Aim: Biogeographic transition zones represent areas of overlap, replacement or partial segregation between distinct biotas, yet their evolutionary role in biotic assembly remains poorly understood. We analysed spatial patterns of phylogenetic endemism in the South American Transition Zone and their associations with geomorphology, geodiversity and climate to test whether it reflects the overlap of temporally disjunct lineages, a cradle of diversification, or a museum of persistence. Location: South American Transition Zone (STZ). Taxon: Angiosperms. Methods: We compiled 1,450,545 georeferenced records for 4995 native angiosperm species intersecting the STZ, mapped onto an equal-area grid of 2137 hexagonal cells (100km width). Using a dated phylogeny, we calculated species richness, phylogenetic diversity (PD), relative PD, phylogenetic endemism (PE) and relative PE. Categorical Analysis of Neo- and Paleo-endemism (CANAPE) identified cells harbouring neo-, paleo-, mixed, or super-endemism. Environmental associations were assessed by positioning CANAPE categories in climate space with kernel density centroids and tested with PERMANOVA across present and paleoclimatic intervals (Late Holocene, Last Glacial Maximum, Last Interglacial, Pliocene). Pairwise contrasts of lineage ages and climate variables among CANAPE categories were quantified with Cliff's Delta. Results: Significant PE concentrated along the Andes. Neo-endemism occurred in cooler, drier, and highly seasonal climates within geologically active areas of high topographic diversity, mainly in the northern and central Andes. Paleo-endemism was linked to wetter, climatically stable, and geologically stable regions of lower geodiversity, primarily beyond the transition in the southern Andes. Mixed endemism predominated across most of the transition zone, while super-endemism occurred in cool, dry environments with very high seasonality. Main Conclusions: The STZ constitutes a zone of overlap where ancient and recent lineages converge and also functions as a cradle of diversification. Consistent links between long-term climate, Andean geomorphology and endemism types indicate a complex evolutionary interface rather than a transient biogeographic feature.