The principles used to recognize and to rank taxonomic groups are established by a theory of taxonomy, which means a theory of classification of life beings acoording to a criterion. It was an ideia of Linnaeus, in the eighteenth century, creating an scheme of arranging organisms in groups and given them taxonomic ranks, to indicate the degree of inclusiveness of the groups. The two major teories of taxonomy are:
Traditional Evolutionary Taxonomy
It had a high influence of the paleontologist George Gaylord Simpson. He used five ranks: class, order, genus, species, and variety. It establishes two principles for recognizing and ranking:
1) common descent; and
2) amount of adaptive evolutionary change.
Therefore, evolutionary taxa needs to have a single evolutionary origin, showing unique characteristics, providing both monophyletic and paraphyletic groups.
This theory has been challenged, mostly because phylogenetic trees can be really difficult to obtain, becoming very hard to base a classification system only on common descent and adaptie evolution.
Phylogenetic Systematics (Cladistics)
It was first proposed by Willi Hennig, being called "Hennigian systematics". He used 7 ranks: kingdom, phylum or division, class, order, family, genus, species. This is a stronger theory, emphasizing the common descent and the cladogram of tge group that's being classified. A cladogram is a diagram showing the relationship between terminal taxa, forming groups and indicating a shared history between species, but not necessarily a direct ancestry. All taxa recognized by this system must be monophyletic, avoiding paraphyly.
Today we use the principles of evolutionary systematics, which has been revised recently (using the principles of cladistics, among others).