Incomplete lineage sorting, also termed deep coalescence, retention of ancestral polymorphism, or trans-species polymorphism, describes a phenomenon in population genetics when ancestral gene copies fail to coalesce (looking backwards in time) into a common ancestral copy until deeper than previous speciation events. In other words, the tree produced by a single gene differs from the population or species level tree, producing a discordant tree. Effects caused by lineage sorting of genetic polymorphisms that were retained across successive nodes in the species tree have been called hemiplasy. Whatever the mechanism, the result is that a generated species level tree may differ depending on the selected genes used for assessment. This is in contrast to complete lineage sorting, where the tre