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 aeneocau da). Their ranges overlap for 3,000 km along the Andes with M. tyrianthina, which lives at lower elevations and is differentiated into several subspec ies that show clinal variation. The genus also includes M, phoebe in semiar id western Peru and M. iracunda in the Perija Mountains of the northern And es. The group could be a good model to study relative differences in divers ification between montane forest as such and the narrow transition zone tow ard the barren highlands. Analysis of nucleotide sequences from three diffe rent mitochondrial gene fragments (cytochrome b, ND2, and ND5) show that Me tallura forms a monophyletic group whose sister taxon is the genus Chalcost igma. The treeline forms of Metallura, including the morphologically diverg ent M. phoebe group in a clade sister to M. tyrianthina, confirming the ide a that montane forest and treeline forms are sister taxa in a strict sense. Neighboring treeline species show greater morphological and genetic differ entiation relative to neighboring montane forest forms. The split between m id-elevation and treeline forms is estimated to have occurred during the Pl iocene, suggesting that much of the Metallura radiation took place during t he Pleistocene.