 Traditional population genetics models of the sort sketched above are “forward-looking”, in that their goal is to predict the future genetic composition of a population, or the fate of an allele, based on various assumptions about the evolutionary processes at work. Starting in the 1980s, a different approach to population genetics was developed known as “coalescent theory”, originally as a result of work in applied probability (Kingman 1982). Coalescent theory has a “backwards-looking” orientation: it aims to make inferences about a population’s history based on a sample of genes drawn from the current population (Wakeley 2008). Compared to the traditional forwards-looking models, coalescent theory allows a different set of evolutionary questions to be asked, and also yields simpler ways of calculating certain quantities of interest in the traditional models, such as fixation probabilities (Rice 2004, ch. 5). Also, coalescent theory yields predictions, for example about the amount of DNA sequence variation we should expect to find in a sample of genes from a natural population, that can be directly tested against data.