Connectivity and evolution between forest macrorefugia: a niche and morphological study of South American birds

Macrorefugia are geographical regions where conditions (e.g. climate) remain favourable for persistence of organisms over time, enabling species to persist and diversify during adverse environmental periods. Our goal was to address whether isolated forest regions, such as the Andean and the Atlantic forests, act as a macrorefugia system promoting biological diversification (i.e. in complex morphological traits) and to evaluate biogeographical hypotheses of their connection. We studied 24 bird lineages with disjunct populations or sublineages in both the Andean and Atlantic forest realms, modelled their niches across current and past periods, and evaluated connectivity between regions. Subsequently, we tested whether bird morphological variation is predicted by geographical isolation between forest regions, climatic factors, and landscape resistance. Our findings support the hypothesis that the Andean and the Atlantic forests acted as a macrorefugia system, driving biological diversification. Refugia isolation alongside climate were significant drivers of morphological variation in the studied birds. The main biogeographical connections between these forests occurred during glacial periods, primarily through the southern Cerrado and its transition with the Chaco. Finally, we identified niche conservatism between disjunct populations and lineages, suggesting that it might be a lagging factor for phenotypic divergence between macrorefugia.