I have just finished generating a 4DTv plot (example) for paralog and ortholog genes in a project I'm working on. While reading some papers where they also have made such a plot (listed in the introduction here), I see that many times they estimate divergence times between species using 4DTv sites. However, I haven't yet found any of these papers that says clearly what is done to compute divergence times from 4DTv.
4DTv plots show the ratio of transversions at fourfold degenerate sites in a set of pairwise alignments. As they show a ratio between the number of transversions and the total number of variants, this measure is used as a relative time measure to date back genome hybridization / duplication events.
Is there a way I could convert this to an absolute time measure? More in particular, I am interested in finding a way to convert the ratio of transversions into millions of years. What would I need to do that? What other variables should I have to make such calculation?
I am currently reading literature and books about time estimation models, but since that could take forever, I thought I'd might ask here as well :)