Editing Common descent (section)

===Selectively neutral similarities===

Similarities which have no adaptive relevance cannot be explained by [[convergent evolution]], and therefore they provide compelling support for universal common descent. Such evidence has come from two areas: [[amino acid]] sequences and DNA sequences. Proteins with the same three-dimensional structure need not have identical amino acid sequences; any irrelevant similarity between the sequences is evidence for common descent. In certain cases, there are several [[Genetic code|codon]]s (DNA triplets) that code redundantly for the same amino acid. Since many species use the same codon at the same place to specify an amino acid that can be represented by more than one codon, that is evidence for their sharing a recent common ancestor. Had the amino acid sequences come from different ancestors, they would have been coded for by any of the redundant codons, and since the correct amino acids would already have been in place, [[natural selection]] would not have driven any change in the codons, however much time was available. [[Genetic drift]] could change the codons, but it would be extremely unlikely to make all the redundant codons in a whole sequence match exactly across multiple lineages. Similarly, shared nucleotide sequences, especially where these are apparently neutral such as the positioning of [[intron]]s and [[pseudogene]]s, provide strong evidence of common ancestry.<ref name="Sharma2005">{{cite book |last=Sharma |first=N. S. |title=Continuity And Evolution Of Animals |url=https://books.google.com/books?id=x7Q47fvCfLIC&pg=PA32 |year=2005 |publisher=Mittal Publications |isbn=978-81-8293-018-6 |pages=32–}}</ref>