Canalization of globins in the adaptive evolution of birds

The globin superfamily, central to oxygen (O₂) cascade dynamics, exemplifies how canalization—evolutionary stabilization of phenotypic traits—enables vertebrates to thrive in extreme environments. In birds, hemoglobins (Hbs) serve as a paradigm of this process, with structural and functional canalization underpinning their exceptional aerobic capacity and elevational diversification. Despite significant advances of globins in our understanding of avian aerobic adaptation, a comprehensive synthesis of functional diversity, molecular evolution, and structural innovation is essential to fully elucidate their canalized roles in O₂ homeostasis. Integrating perspectives on globin functional diversity and structural evolution, this review examines how chance (mutation/fixation biases) and contingency (historical genetic/epistatic constraints) shape Hb divergence and parallelism, thereby bridging molecular mechanisms with physiological adaptation in birds. We highlight how avian Hbs, canalized through compensatory substitutions and allosteric regulation, achieves a balance between evolutionary robustness and adaptive plasticity. However, critical gaps remain persist: the roles of understudied globins (e.g., neuroglobin, globin E) and the mechanisms of genetic assimilation in migratory taxa. We propose an integrative framework that incorporates ecological divergence (elevation, flight endurance), phylogenetic timescales, and systems biology to unravel how canalization directs adaptive compromise. By focusing on birds within the amniotes, this synthesis advances a cohesive model for vertebrate evolution, wherein canalized globins reconcile metabolic precision with ecological innovation. Ultimately, this review refines hypotheses of O₂ cascade evolution and calls for cross-disciplinary studies to decode the genetic and physiological architecture underlying adaptive canalization in extreme environments.