Phylogenetic origins and adaptive evolution of avian and mammalian haemoglobin genes

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Abstract
Recent years have seen rapid growth in amino acid sequence data on globins and nucleotide sequence data on haemoglobin genes and pseudogenes, and cladistic analysis1 of these data continues to reveal new facets of globin evolution. Our present findings demonstrate: (1) avian and mammalian embryonic α genes (π and ξ, respectively) had a monophyletic origin involving an α locus duplication about 400 Myr ago soon after the duplication which separated α and β genes; (2) much later in phylogeny, independent β-gene duplications produced the embryonic ρ locus of birds and embryonic ɛ and fetal γ loci of mammals. This parallels the earlier finding2 that myoglobins evolved more than once from generalized globin ancestors. Here we support the view2 that such globin evolution resulted from natural selection acting on mutations in duplicated genes. Thus, our evidence contradicts the neutralist view3,4 in which almost all amino acid substitutions in descent to extant globins evaded positive selection.