Sequence Homology and Protein Evolution

Hemoglobin, the oxygen-carrying protein of vertebrate blood, is similar in structure to myoglobin. However, hemoglobin is composed of four chains i.e., it has a quaternary structure. The four chains 

Comparison of the amino acid sequences of hemoglobin and myoglobin chains from different species of animals shows that the chains from related species are similar. The number of differences increases with phylogenetically more separated species. On the assumption that proteins evolve at a constant rate, the number of differences between two homologous proteins will be proportional to the time of divergence in evolution of the species. 

Draw an evolutionary tree for the human, rabbit, silkworm, and Neurospora (fungus) by using the data in the following table for differences in the respective cytochrome c sequence. 

These data allow us to construct an evolutionary tree, with branch lengths approximately proportional to the number of differences between the species (see Fig. 4-10). The human and rabbit show most similarity and therefore are connected by short branches. The silkworm cytochrome c is closer to both mammalian forms than it is to Neurospora, and so should be connected to the mammalian junction. The length of the silkworm branch will be approximately three times the length of the rabbit and human branches. Finally, Neurospora shows approximately the same number of differences with all the animal species, and therefore can be joined to their common branch. 

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