The coalescence process, or coalescence theory, is a way of reconstructing the relatedness, or genealogy, of a polymorphic DNA sequence. By constructing a genealogical tree of the different alleles of a DNA sequence by coalescence, the demographic history (whether the population grew or shrank or stayed the same size) can be determined. The age of a specific allele can also be estimated. It has been used in human genetics to infer the evolution and prehistory of the species, and is often used when studying the population genetics of other species, The mathematics behind coalescence is reasonably complex, and is performed by a computer program such as Genetree, but the principle of the process can be understood without it. If, for example, there were several different sequences of one DNA region from a certain population, then they would all be related and could all be placed at the tips...