Editing Polyphyly (section)

== Avoidance ==
{{See also|Phylogenetic nomenclature#Philosophy}}

In many schools of [[Taxonomy (biology)|taxonomy]], the recognition of polyphyletic groups in a classification is discouraged. [[Monophyletic]] groups (that is, [[clades]]) are considered by these schools of thought to be the only valid groupings of [[organism]]s because they are diagnosed ("defined", in common parlance) on the basis of [[synapomorphy|synapomorphies]], while paraphyletic or polyphyletic groups are not.  From the perspective of ancestry, clades are simple to define in purely [[Phylogenetics|phylogenetic]] terms without reference to clades previously introduced: a [[Phylogenetic nomenclature#Phylogenetic definitions of clade names|node-based clade definition]], for example, could be "All descendants of the last common ancestor of species X and Y". On the other hand, polyphyletic groups can be delimited as a conjunction of several clades, for example "the flying vertebrates consist of the bat, bird, and pterosaur clades".

From a practical perspective, grouping species monophyletically facilitates prediction far more than does polyphyletic grouping. For example, classifying a newly discovered grass in the monophyletic family [[Poaceae]], the true grasses, immediately results in numerous predictions about its structure and its developmental and reproductive characteristics, that are synapomorphies of this family. In contrast, Linnaeus' assignment of plants with two [[stamen]]s to the polyphyletic class Diandria, while practical for identification, turns out to be useless for prediction, since the presence of exactly two stamens has developed convergently in many groups.<ref>{{cite journal |first=Clive A. |last=Stace |author-link=Clive A. Stace |year=2010 |title=Classification by molecules: What's in it for field botanists? |journal=Watsonia |volume=28 |pages=103–122 |url=http://www.archive.bsbi.org.uk/Wats28p103.pdf |access-date=July 31, 2013 |archive-url=https://web.archive.org/web/20121015135726/http://archive.bsbi.org.uk/Wats28p103.pdf |archive-date=October 15, 2012 |url-status=dead |df=mdy-all }}</ref>