Phylogeny of hemoglobin

Evolutionary trees requiring in toto the fewest nucleotide replacements between ancestors and descendants were constructed from the amino acid sequences of heme-binding globin chains of 22 primates and a wide range of other multicellular organisms. These trees, which maximized the codon indentities due to common inheritance and minimized those due to parallel and back mutations, allowed an evolutionary dimension to be given to the knowledge on structure-function parameters in heme-binding globins. In the gene lineage specifying the monomeric globins out of which tetrameric hemoglobin evolved, a gene duplication produced the ancestral separation between the myoglobins and hemoglobins in jawed vertebrates. It then appears that at the stage of primitive fish in a common ancestor of teleosts and tetrapods a duplication in a gene coding for a pi type hemoglobin produced the separate a and p loci. By the time of basal amniotes sophisticated a2/38 type hemoglobins had evolved. The y divergence from typical p chains can be traced to a gene duplication in basal therian mammals and the separation of d type chains from ps to later duplications in proto-catarrhines and proto-platyr- rhines. From the mutational changes in the lineages emerging from these and other duplications, it was concluded that rates of molecular evolution increased with gene duplication and decreased after selection discovered useful specializations in the products of genes which had previously been silent and thus freer to accept mutations. Comparisons of mutational lengths of residue positions with strong functional roles to those without evident functional roles also suggested that the speed of molecular evolution decreases with the intensity of selection. The kinds of nucleotide replacements observed in the descent of the globin RNA messengers further pointed to the restrictive effects of natural selection by highlighting the fact that during protein evolution interchanges of amino acids with similar physical chemical properties were much more readily tolerated than dissimilar interchanges. Evidence was provided by the globin phylogenies for the hominoid affinities of the gibbon and the close phyletic relationship of the African apes to man. Moreover while rates of globin evolution were rapid in the early eutherian radiation, they were extremely slow in the later hominoid radiation. This was related to a more complicated internal organization evolving in human ancestors, thus increasing the possibility that mutations in structural genes would be harmful.