Andean geese have developed a mutation in their hemoglobin that has led to a vast increase in hemoglobin-oxygen affinity. More specifically, Hiebl et al. found that the Andean goose has developed mutations that lead to five amino-acid substitutions in the alpha-chain and five substitutions in the beta-chain of their hemoglobin. A particular substitution in the Andean goose beta-chain has led to the elimination of a Van der Waals interaction between the alpha-chain and the beta-chain. This has destabilized the T-state (the deoxygenated state of hemoglobin), which has led to a higher affinity for being in the R-state (oxygenated state of hemoglobin). Overall, this mutation increases the hemoglobin-oxygen affinity of the Andean goose.
^Hiebl, I; Braunitzer, G & Schneeganss, D (1987). "The primary structures of the major and minor hemoglobin-components of adult Andean goose (Chloephaga melanoptera, Anatidae): the mutation Leu----Ser in position 55 of the beta-chains". Biological Chemistry Hoppe-Seyler. 368 (12): 1559-1569. doi:10.1515/bchm3.1987.368.2.1559. PMID3442599.