Homology

The ability to compare protein and DNA sequences not only shows us where evolution has occurred but provides insight into its mechanisms. By comparing genomes, we find that mutations can occur on a small scale: Even a single nucleotide change is a mutation. They can also occur on a large scale, as happens when sequences are inserted, deleted, duplicated, or moved between chromosomes.

Many mutations that replace single nucleotides have no effect because of the "degeneracy" or redundancy of the genetic code. The genetic code has more codons (sixty-four) than amino acids (twenty). As a consequence, most amino acids are specified by two to four different codons. Because of this, some mutations can be "silent," with one nucleotide replacing another but without changing the specified amino acid. Other mutations are said to be "conservative." This occurs when a mutation replaces one amino acid with another that has similar properties: They may be chemically similar sharing the same charge, shape, or polarity.

If, however, the mutation affects the function of an important protein, that mutation may result in an evolutionary dead end, because it is less likely to be passed on to a future generation. As a result, important sequences show fewer mutations, whereas less important sequences show more change. Such properties can be deduced by comparing sequences from different organisms. Proteins that interact with other molecules, such as DNA or RNA, tolerate fewer changes in structure, and show little change through evolution. The histone proteins that form the backbone of the eukaryotic chromosome are important examples.