Before you read this, check out Razib’s excellent 10 questions with Jim Crow, which I am unfortunately knocking off the top of the blog.
So a little while back, I wrote about a paper describing the evolution of a bacteria over the course of an infection (Aetiology also had a post–a better one, truth be told– on the same paper). I speculated that different individuals might present different environments for the bacteria, thus presenting slightly different selective pressures.
Moving up one level, one could imagine two populations being different genetically, so a pathogen could in theory adapt differently to life being passed around in the two. But does this happen?
A new paper suggests that the answer is yes. The authors analyse HIV sequences from a couple genes isolated from individuals in two different African populations. From the sequences, they determine which codons are under selection in the different populations, and come up with seven amino acid sites under differential selection. They link this to the different genetics (especially genetics of the immune response) of the two groups.
I certainly find that plausible, but I think there’s certainly another possibility here: essentially, there’s more than one way to skin a cat. We’re talking about differential selection at the level of a single codon and assuming that the differential selection reflects differential selective pressures in the populations. But maybe the different virus populations just came across different, yet equivalent, ways to adapt to the same selective pressures. That is, maybe we’re just seeing an example of the stochasticity of evolution.