Strong Diversification at the Treeline among Metallura Hummingbirds

The hummingbird genus Metallura comprises nine species. Six of them live at the treeline and replace each other sharply along the eastern slope of the tropical Andes (williami, baroni, odomae, theresiae, eupogon, and aeneocauda). Their ranges overlap for 3,000 km along the Andes with M. tyrianthina, which lives at lower elevations and is differentiated into several subspecies that show clinal variation. The genus also includes M. phoebe in semi-arid western Peru and M. iracunda in the Perija Mountains of the northern Andes. The group could be a good model to study relative differences in diversification between montane forest as such and the narrow transition zone toward the barren highlands. Analysis of nucleotide sequences from three different mitochondrial gene fragments (cytochrome b, ND2, and NDS) show that Metallura forms a monophyletic group whose sister taxon is the genus Chalcostigma. The treeline forms of Metallura, including the morphologically divergent M. phoebe, group in a clade sister to M. tyrianthina, confirming the idea that montane forest and treeline forms are sister taxa in a strict sense. Neighboring treeline species show greater morphological and genetic differentiation relative to neighboring montane forest forms. The split between mid-elevation and treeline forms is estimated to have occurred during the Pliocene, suggesting that much of the Metallura radiation took place during the Pleistocene.