4.2 Amino acid sequence homologies and why they occur
Consider two genes encoding proteins that have 50% of their amino acid sequence in common.
How can this sequence homology be explained in terms of evolution?
The most parsimonious explanation is that the similarities result from the fact that the two organisms share a common evolutionary past and that the genes encoding the proteins in each of the organisms arose from a common ancestral gene.
Through the process of natural selection, randomly occurring DNA mutations that alter amino acids within the protein and have a detrimental effect upon the organism are selected against, i.e. they are not transmitted to the next generation and are lost from the population. As a consequence of natural selection, differences can accumulate between the two proteins; some of these changes might enhance or modify function, some might have no effect, but critical amino acids will remain unaltered. Over many millions of years of evolution, this process results in proteins in which regions that are critical for function are highly conserved.