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Molecular basis of hemoglobin adaptation in the high-flying bar-headed goose

During adaptive phenotypic evolution, some selectively fixed mutations may be directly causative and others may be purely compensatory. The relative contribution of these two classes of mutation depends on the form and prevalence of mutational pleiotropy. To investigate the nature of adaptive substitutions and their pleiotropic effects, we used a protein engineering approach to characterize the molecular basis of hemoglobin (Hb) adaptation in the bar-headed goose (Anser indicus), a hypoxia-tolerant species renowned for its trans-Himalayan migratory flights. We synthesized and tested all possible mutational intermediates in the line of descent connecting the wildtype bar-headed goose genotype with the most recent common ancestor of bar-headed goose and its lowland relatives. Site-directed mutagenesis experiments revealed effect-size distributions of causative mutations and biophysical mechanisms underlying changes in function. Trade-offs between alternative functional properties revealed the importance of compensating deleterious pleiotropic effects in the adaptive evolution of protein function.