 Coalescent theory is all about tracing lines of ancestry between genes in a (diploid) population. Ordinarily we think of ancestor-descendant lineages of organisms, but we can equally (indeed more easily) think in terms of lineages of genes at a locus, while simply ignoring the organisms that the genes are housed within. The starting point of coalescent theory is the observation that all the genes at a locus in a current population must ultimately stem from a single ancestral gene copy in the past (the “most recent common ancestor” or MRCA). This is in effect the flip-side of genetic drift. If we go back far enough in a population’s ancestry, we must arrive at a point at which all of the genes bar one have left no descendants in the current population. This is because, at every round of reproduction, a given gene copy has a certain chance of not leaving any descendants in the next generation, i.e., being eliminated by drift. This implies that, as we trace back in time, the gene lineages will join up, or “coalesce”.