The Phylogenetic and Biogeographic History of High Altitude Adaptation in Hummingbirds: Selection on Hemoglobin Proteins as a Function of Oxygen Supply and Demand

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Hummingbirds are exceptional for their small body size, high metabolic rate, and energetically costly mode of flight. These characteristics would seem to make hummingbirds ill-suited for harsh, oxygen-depleted, high-altitude environments. Paradoxically, hummingbirds reach their peak diversity in the equatorial Andes, and over 80 species reach elevations of 3000m or higher. Hummingbirds evolved about 35 million years ago, well before the Andes began ascending. Following Andean uplift, numerous independent hummingbird lineages colonized high-altitude habitats, and these invasions were accompanied by changes in hemoglobin, the respiratory protein responsible for uptake and delivery of oxygen. This research will examine hemoglobin genes for 275 hummingbird species in order to identify evidence of past selection that may have accompanied evolutionary shifts in elevation.Independent high-altitude human populations have evolved unique adaptive respiratory strategies, but it is unknown whether alternative strategies reflect differences in timing of colonization or distinct evolutionary trajectories. Hummingbirds represent an ideal system in which to evaluate these alternatives because independent lineages have undergone high-altitude colonization at staggered times over the last 15-20 million years. We will characterize hummingbird hemoglobin and myoglobin genotypes, and measure several hematological parameters, hovering metabolic rates, and hypoxia tolerance for a suite of species that span a 5000m elevational gradient. The results will refine our understanding of high-altitude respiration, hemoglobin evolution and pathology, and the relative roles of determinism and contingency in evolution. Partial funding for this study was provided by the Office of International Science and Engineering at NSF.