In genetics, coalescent theory states that all genes in a population will coalesce to a single common ancestor. Using this theory, one can determine the probability that any two genetic sequences will coalesce in a given generation, which provides a prediction of how long ago the most recent common ancestor to those sequences lived. A recent book on coalescent theory was written by Jotun Hein, and initial chapters of another book by John Wakeley are available free online. For brief overviews, see Magnus Nordborg's introduction, Gil McVean's course notes, or Alan Rogers's course notes, all available online.