XXV.3 The basic requirement on biological taxa is their monophyly
If a taxonomic system is to take into account the progress of cladogenesis, it would seem, at first glance, quite natural to require that each taxon include only those species that are mutually more related than any of them is related to any species classified under a different taxon. However, evolutionary systematists do not consider this to be a serious requirement and consciously ignore it in some cases. According to the usual definition, two species are more closely related than is either of them to a third species if they share a common ancestor which is simultaneously not an ancestor of the third species. The requirement of maximum mutual relatedness of species classified in a single taxon is sometimes erroneously interpreted as being equivalent to prohibition of creation of polyphyletic taxa, i.e. taxa including the members of two or more independent phylogenetic lines. A polyphyletic taxon would include at least two species whose immediate ancestors were not members of this taxon. The opposite of a polyphyletic taxon is a monophyletic taxon, i.e. a taxon including the members of a single phylogenetic line. A monophyletic taxon contains, or in the past contained, only a single species whose immediate ancestor was not a member of this taxon. (The topic of the taxa of organisms whose ancestor originated by symbiogenesis will be discussed in XXV.4.2.) The proponents of both the currently most influential directions of systematic biology, evolutionary systematists and cladists, agree on the prohibition of creation of polyphyletic taxa. However, they certainly don’t agree on where the boundary lies between polyphyly and monophyly.