Genetic drift: the coalescent
You should be used to me saying that population geneticists take a backward view of the world. Today you'll see the most important example of that approach. Kingman introduced the coalescent about 30 years ago. It took a while before we realized how important and useful it is, but it has dominated work in empirical population genetics for the last 15-20 years. It is the basis of many powerful methods for analysis of genetic variation in populations. We'll barely scratch the surface, but I think you'll get enough of a flavor of what's going on to see why it can be so useful.