As was mentioned in the introduction to this series of blogposts, both species and languages are often presented in a tree model. In biology, trees of each individual gene are created in order to account for horizontal transmission and other processes in which the history of a gene differs from the general history of its genome. From the sum of these trees, the species trees are then derived, a method called gene tree reconciliation (Nakhleh 2013). In linguistics on the other hand, phylogenetic trees normally are built on cognate sets of related words, from which the most likely tree of the languages is calculated. A closer equivalent however would be to describe the history of each individual word or word form, including regular sound change, irregular changes to its form, semantic changes, borrowings, and processes of word formation, and to derive the language tree based on the sum of the word histories (Gray, Greenhill, and Ross 2007, 15). Unlike its biological equivalent, this is normally done manually.