Figure 1.

Model for the independent evolution of β-like globin gene clusters in birds and mammals. The diagram shows major phyla as broad branches leading to eutherian mammals, marsupials, and birds, with the contemporary species at the top. Within the broad species branches are gene trees for the β-like globin gene clusters. The ancestral gene cluster (bottom of the diagram) has proto-β-globin genes (pβ1 and pβ2, each colored red and blue) and a regulatory element (yellow and orange rectangle for an LCR and/or enhancer). This gene cluster is proposed to have duplicated to generate the ancestor to the eutherian β-like globin genes (eβ1 and eβ2, colored in shades of red, with an LCR as a yellow rectangle) and the ancestor to the avian β-like globin genes (aβ1 and aβ2, colored in shades of blue, with an enhancer as an orange rectangle). The path of evolution to the genes related to the eutherian ɛ-γ-δ-β-globin gene cluster is drawn with red lines, and the path to the genes related to the avian ρ-β-ɛ-globin gene cluster is drawn with blue lines. Separations of genes by duplications are marked with yellow circles, and separations of genes by speciation events are marked with mauve diamonds. Note that the duplication to generate the ancestors to eutherian and avian β-like globin genes preceded the divergence of birds and mammals, hence both gene clusters were present in all three lineages depicted and still are found in contemporary marsupial mammals. The marsupial photographed is a pretty-faced wallaby.