The molecular structure of HIV. Photo by PHYLOMON!.In the latest post at the group blog Nothing in Biology Makes Sense!, contributor Luke Swenson describes how biologists can reconstruct the evolutionary history of HIV to estimate when the virus make the jump from chimps to humans, or even when a single patient became infected.
Although HIV evolves rapidly, it does so at a fairly constant rate. In essense, you can use this constant rate to act like a clock to tell you roughly how many changes accumulate over a year. Then, by figuring out the number of changes it would take for both sequences to converge on a single identical sequence (their most recent common ancestor, “MRCA”), you can get an estimate of the date that the MRCA existed at.
This is one of the best cases I know about in which evolution directly informs medical practice and treatment, and it's well worth reading the whole thing. ◼