Evolutionary dynamics of the elevational diversity gradient in passerine birds

Low-elevation regions harbour the majority of the world’s species diversity compared to high-elevation areas. This global gradient
suggests that lowland species have had more time to diversify, or that net diversification rates have been higher in the
lowlands. However, highlands seem to be cradles of diversity as they contain many young endemics, suggesting that their rates
of speciation are exceptionally fast. Here we use a phylogenetic diversification model that accounts for the dispersal of species
between different elevations to examine the evolutionary dynamics of the elevational diversity gradient in passerine birds, a
group that has radiated globally to occupy almost all elevations and latitudes. We find strong support for a model in which passerines
diversify at the same rate in the highlands and the lowlands but in which the per-capita rate of dispersal from high to
low elevations is more than twice as fast as that in the reverse direction. This suggests that while there is no consistent trend
in diversification across elevations, part of the diversity generated by highland regions migrates into the lowlands, thus setting
up the observed gradient in passerine diversity. We find that this process drives tropical regions but for temperate areas, the
analysis could be hampered by their lower richness. Despite their lower diversity, highland regions are disproportionally important
for maintaining diversity in the adjacent lowlands.