Sequence alignment evolutionary basis

The evolutionary basis of sequence alignment. The term, similarity is... 

At the level of protein molecules, selective pressure results from the need to maintain function, which in turn requires maintenance of the specific 3D structure. This evolutionary history is the basis for the success in aligning protein (or nucleotide) sequences. Accordingly, conservation and mutation patterns observed in alignments contain very specific information about 3D structure. How much variation is tolerated without loss of structure Two naturally evolved proteins with more than 25% identical residues (length >80 residues) are very likely to be similar in 3D structure,... 

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