 When individual gene histories are incongruent with each other or with their containing species trees we have genealogical discordance. We have already seen how hybridization and horizontal gene transfer can cause discordance, but as systematics has become increasingly connected with population genetics, another source of discordance has loomed large—incomplete lineage sorting. Any two copies of a gene in a population will share a common ancestor in the past called their point of coalescence. If two gene lineages in the same species lineage fail to coalesce within that species branch (that is, their point of coalescence is earlier than than the most recent speciation or lineage branching) we have what is called incomplete lineage sorting (ILS). In a case of incomplete lineage sorting it is possible that one of the gene lineages first coalesces with a gene lineage from another species and thus is a case of genealogical discordance. This generates what Avise and Robinson (2008) have termed hemiplasies, i.e., homologous characters whose phylogenetic histories are topologically discordant with higher level phylogenies due to ILS. Maddison (1997) provides a very readable introduction to the relationship between gene trees and species trees and the problems of genealogical discordance while Degnan and Rosenberg (2009) is a slightly more advanced introduction to the biology of incomplete lineage sorting and its affects on phylogenetic inference.