Charles Darwin was born Feb. 12, 1809, so this is his, um, 197th birthday. (Do check out the link, a picture of a young Charles as imagined by 'Olduvai George'. )

Though I live in the US, home of the biggest crusade for ignorance (now packaged as Intelligent Design), here in Massachusetts the anti-evolution bozos haven't made much headway, and I've never had personal contact with anyone denying evolution. The magnificent legal decision in Dover that I mentioned earlier was a great kick in the teeth for the ID people, and I have to say that I have a real sense of optimism that sense will prevail.

My own work isn't directly related to evolution, but of course as Theodosius Dobzhansky said, "nothing in biology makes sense except in the light of evolution", and that applies to my work as much as anything. I work on antigen presentation, the process by which immune responses are triggered. In particular, I'm interested in antiviral immunity. You can think of antigen presentation as the process by which cells turn themselves inside out. VIruses live inside cells, of course, whereas the T cells that do the heavy lifting of immunity are outside those cells. There has to be a process by which cells signal what's going on inside them, and transmit that info to the outside where T cells can evaluate it, and decide whether there's a problem.

I won't go into the process in any detail, but it involves binding of intracellular peptides (including viral peptides) to a set of proteins called MHC class I (short for major histocompatibility complex class I). That gives an easy out to the viruses (which of course want to avoid this whole process): Any viral mutants that fail to bind to the MHC class I, will be invisible to the immune system.

This happens in the short term, with some viruses that persistently infect their hosts. For example, HIV and hepatitis C virus regularly undergo mutations as they infect a person, and you can correlate those changes to the prevalent immune response within an individual -- over the years, you can see the major viral population in the person changing, while the T cell response straggles behind it, like twin motorboat wakes.

But those changes, in the one infected person, don't help the virus in the long run -- that is, in its quest to infect a new host. (In fact, mostly those changes are detrimental to the virus. To the extent a virus is well adapted at the outset, any changes away from that set of DNA sequences are going to harm its ability to replicate, even if it's now able to avoid the immune response.) The reason those changes that avoid Person A's immune response don't help in Person B, is that almost everyone has different MHC alleles. MHC is by far the most variable gene set in the human (and other species) population; there are around a thousand different alleles. So though the virus has evolved to evade Person A's particular set of MHC class I alleles, that doesn't help it at all against Person B's alleles.

That should tell you why, in fact, there are so many MHC alleles. There's a strong benefit, evolutionarily speaking, to you having a different set of MHC alleles than your neighbour's. If everyone has the same set of alleles, then the person who evolves a new allele, will be less likely to get sick, and therefore will enjoy a significant reproductive advantage, than his neighbour. (There are also reasons that MHC alleles can appear more readily than many other genes that mainly change by point mutations.)

So you'd expect that, if you start with a small population, then over time you would see new MHC alleles popping up in the population. For example, the nomadic hunters that populated North America 10000-odd years ago were a fairly small population. They crossed the Bering Strait and their children, and grandchildren, and great-to-the-nth-grandchildren, headed south, eventually ending up in Tierra del Fuego. You would expect to see families of MHC alleles heading North to South, with new alleles appearing on the trip (as well as some of the original ones, hanging on); and you'd expect the MHC alleles to form in a family as well, with the new ones furthest south looking like derivatives of the ones that were present further north).

That's pretty much what happens. The picture here is a scan from a review, nearly ten years old now, by Parham and Ohta (Science. 1996 Apr 5;272(5258):67-74). It shows HLA (another term for MHC) alleles geographically in the new world, and they follow the predicted pattern very nicely.

Nothing in biology makes sense except in the light of evolution.