Saturation with mutations by Transition/Transversion analysis

Transition mutations (G - A, C - T) are more frequent than transversions ( C to A or G, G to C or T, T to A or G, etc) for several biochemical reasons. However there are 2X more possible transversions than transitions, so any two random DNA sequences will have 2X more apparent transversions than transitions. As organisms evolve from a common ancestor (100% sequence identity) to different lineages with less and less identity, the ratio of transisitons:transversions changes to give an indication of how much saturation of silent sites has occurred. This plot show data from complete genomes of Primate Immunodeficiency Viruses (HIVs and SIVs) in comparison to data from Vertebrate (fish, reptiles, mammals, amphibians, birds) complete mitochondrial genomes. Vertebrates shared a common ancestor some 500 million years ago. The primate immunodeficiency viruses are of unknown age, but perhaps as much as 10 million years has passed since their origin.