With elevated aerobic metabolism, hummingbirds have exceedingly high oxygen demands. Yet 63 species thrive at high-altitude in the Andes, where oxygen is scarce.
These different hummingbirds independently evolved hemoglobins with enhanced oxygen-binding. They repeatedly did so via the same molecular changes.
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While hummingbirds have convergently adapted to high altitudes similarly, other birds have not. Avian adaptation to alpine air has transpired innumerable times, but the molecular means to do so has differed considerably. These discrepancies reflect differences in evolutionary history.
Even in cases where adaptive phenotypic change is predictable, the molecular pathways to these changes may not be. ~ Indian biologist Chandrasekhar Natarajan et al