Mutation as Driving Force of Evolution

Biomacromolecules, by C. Stan Tsai Copyright 2007 John Wiley Sons, Inc. 

The chemical bias by which certain amino acids change to varying extent and are replaced nonrandomly by certain other amino acids, does not arise from the nature of the genetic code or from the types of mutations that occur. The amino acid replacements that would be expected from random single nucleotide replacements are remarkably different from that actually observed among homologous proteins (Table 18.1). The numbers of observed replacements reflect the frequency with which each amino acid occurs in pro- 

TABLE 18.1 Relative frequencies of amino add replacement in a total of 1572 dosely related proteins 

Notes 1. Amino acid replacements expected for random single-nucleotide mutations (upper right) conpare with obsCTved rq)lacements (bottom left). The greatest discrepancies between the observed and random replacements are shown in boldface type. 

The evolutionary variations of a nucleic acid/protein also provide much information about the nucleic acid/protein itself. Evolutionary divergence into different species has resulted in many variants of the same functional nucleic acid/protein with different nucleotide/amino acid sequences. The differences and similarities of the biosequences of these variants reflect the constraints of structure and function for that nucleic acid/protein. 

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