I've mentioned several times that population geneticists often look at the world backwards. Sometimes when they do, the result is very useful. Consider genetic drift, for example. So far we've been trying to predict what will happen in a population given a particular effective population size. But when we collect data we are often more interested in understanding the processes that produced any pattern we find than in predicting what will happen in the future. So let's take a backward look at drift and see what we find.