Twenty-four alpha globin chain sequences were analyzed for degrees of mutational change during the descent of therian mammals. A new iterative computer procedure is described for constructing trees which depict plausible phylogenetic relationships for homologous polypeptide chains of different species. The procedure uses a least-squares coefficient defined on a network topology to search for that topology in which the mutation distance between two species most closely approximates the sum of mutation lengths for the link path connecting the two species. Next, a method, based on three theorems, is described for constructing maximum parsimony ancestral codon residues, i.e. residues which minimize mutational change over the network. Using this maximum parsimony method, a number of topologies of alpha chain sequences were analyzed. The findings suggest that during the initial radiation of eutherian mammals alpha globin genes evolved more rapidly than later in mammalian phylogeny, with the slowing rate of change being more apparent in higher primates than in other mammals.
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