Abstract
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.
Highlights
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 fitted a dynamic model that estimates the diversification rate associated with each elevational state as well as the transition rates between these states
In spite of clear differences in passerine diversity at different elevations, we find no difference in diversification rates across elevational zones
Summary
There are various reasons why rates of speciation may be faster at high elevations, including the fragmentation of habitats on different mountain summits[13]; ecological opportunity as new habitats were made available during recent mountain uplift[14]; exposure to higher levels of ultraviolet radiation, which boosts rates of mutation[15]; and the susceptibility of species inhabiting narrow thermal bands to becoming isolated by the expansion of glaciers during recurring ice ages[12,16] All of these factors are thought to be important in the tropics because limited seasonal variation enables the greater thermal stratification of species across mountain slopes, enhancing opportunities for geographic isolation and divergent ecological selection[17]. While some studies have supported the idea that diversification is faster in the highlands[18], other studies have found no evidence for differences in diversification rates across elevations[19], and whether rates of diversification vary consistently across elevations remains debated
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