What is a monophyletic lineage
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As Taxon (Plural: taxa) is used in biology to describe a group of living beings recognized as a systematic unit. This system is usually expressed through a separate name for this group. The rules of the formation of scientific names for taxa is the content of the corresponding nomenclature code, the classification according to the degree of relationship in the corresponding hierarchical system is the task of the taxonomy, which in turn is based on the results of the biological system. Different systematic ideas lead to different taxonomic approaches and thus to alternative results in the scientific naming of taxa. For traditional reasons, these have been given names based on the Latin or Greek language to this day.
Monophyly, paraphyly and polyphyly
The aim of modern phylogenetic systematics is to build a natural system of the animal kingdom that reflects the relationships between taxa and their evolutionary history. While traditional systematists mainly relied on morphological data, phylogenetic systematics include molecular biological data such as sequence analyzes of ribosomal DNA in their analysis in order to be able to derive family trees on this basis.
According to this requirement, only a monophyletic taxon can be characterized as natural in the sense of phylogeny. A monophyletic taxon consists of all relatives who can be traced back to a common ancestor as well as these ancestors themselves. A monophyletic taxon is therefore self-contained. An example of such a monophylum is the taxon Mammalia: all mammals can be derived from a single common ancestor.
The monophyletic taxon is opposed to the non-natural para- and polyphyletic taxa, which are not related and have a purely descriptive character. From the point of view of the phylogenetic system, paraphyletic and polyphyletic taxa are to be rejected and avoided.
A paraphyletic taxon describes a taxon in which all members can be traced back to a common lineage, but it does not contain all descendants of this lineage. For example, the taxon "Reptilia" can be characterized as paraphyletic, because the crocodiles, traditionally counted as "reptiles", are more closely related to the birds than all other "reptiles" and the taxon "Reptilia" would have to include the birds in order for it to become monophyletic. One such monophyletic taxon that unites "reptiles" and birds is called Sauropsida. Another example of such a paraphyletic taxon is that of the "bony fish" ("Osteichthyes"), which from a phylogenetic point of view should also include the terrestrial vertebrates (Tetrapoda). A monophylum from "Osteichthyes" and Tetrapoda is called an osteognathostomata.
A polyphyletic taxon however, includes groups that cannot be traced back to a common ancestor. It thus combines several lines of development that have arisen independently of one another without a common evolution. Examples of a polyphyletic taxon are the "worms" ("Vermes"), which can be traced back to several independent stem lines and which do not reflect any common relationship. For example, the flatworms (platelets) belong to the primordial mouths (protostomia), while acorn worms (Enteropneusta) belong to the new mouths (deuterostomia).
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