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| Section: Genetics » Genetic Basis of Evolution and Speciation |
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| Evolution at molecular level |
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Content
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Evolution at Molecular Level
Evolution of proteins
The evolution at molecular level is reflected in protein differences. Proteins can be called 'chemical fingerprints' of evolutionary history, because they bear amino acid sequences that have changed as a result of genetic changes. Organisms that bear large number of common amino acid sequences may, therefore, be considered to be more closely related than to those with greatly different amino acid sequences. In comparisons between haemoglobins of. vertebrates, E. Zuckerkandl (1965), calculated an average of. 22 differences between human haemoglobin chains (α and β) and the similar haemoglobins of horse, pig, cattle and rabbit. It was, therefore, concluded that human haemoglobins diverged from those of these animals by about one amino acid change per 7 million years. The rate of change of amino acids is, however, different for different proteins. For instance, insulin, cytochrome and histones have changed at a much slower rate, where as fibrinopeptides at a much faster rate. This range in the rate of changes indicates that there are selective constraints limiting the amount of mutational changes in a protein and also that some amino acid changes may be neutral in their effect on the function of proteins. |
It is interesting to note that when α haemoglobin chains from different organisms (human, horse and fish) are compared with β chain of human beings, α chain differs by 75 amino acid residues in humans and by 77 in each horse and carp. One may ask why should organisms so divergent may have same level of divergence from human β chain. This has been explained on the basis of common rate of neutral mutations, and based on this, relative age of each haemoglobin chain (myoglobin, α chain, γ chain, δ chain and β chain) could be worked out. The myoglobin having 86 per cent difference must have (650 million years) differentiated from β earliest in the history and δ chain having only 7 per cent difference (35 million years), must have differentiated most recently (Figs. 48.1, 48.2). |
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| Fig. 48.1. Phylogenetic relationships among myoglobin and four haemoglobin chains, deduced from amino acid substitutions (redrawn from Zuckerkandl, Nature 192 : 1961) |
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| Fig. 48.2. Myoglobin differentiation from α and β chains from earliest times in history. |
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We also know that new haemoglobin types appear in an organism in the presence of old ones. These new types are believed to have originated by duplication of a gene, one of the two genes then differentiating through mutations. Evolution by gene duplications has been shown in many cases. While in some cases, the duplicated genes may preserve same functions, in other cases they may diverge to perform different functions. Their similar origin is, however, reflected in certain amount of similarity in amino acid sequences (consult Multigene Families in Eukaryotes on multigene families).
It should, however, be realized that a study of only one type of protein alone is insufficient to infer phylogenetic relationships. For instance, haemoglobins of man and gorilla, differ by a single amino acid and this difference is no greater than is observed between two mutants of same genotype in an organism. Therefore, for getting reliable information about phylogenetic relationships, more than one type of proteins should be studied.
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