An informal group in

taxonomy Taxonomy is the practice and science of categorization or classification. A taxonomy (or taxonomical classification) is a scheme of classification, especially a hierarchical classification, in which things are organized into groups or types. ...

is a

taxonomic rank In biological classification, taxonomic rank is the relative level of a group of organisms (a taxon) in an ancestral or hereditary hierarchy. A common system consists of species, genus, family, order, class, phylum, kingdom, domain. While ol ...

that is not well defined. This type of group can be

paraphyletic In taxonomy (general), taxonomy, a group is paraphyletic if it consists of the group's most recent common ancestor, last common ancestor and most of its descendants, excluding a few Monophyly, monophyletic subgroups. The group is said to be pa ...

polyphyletic A polyphyletic group is an assemblage of organisms or other evolving elements that is of mixed evolutionary origin. The term is often applied to groups that share similar features known as homoplasies, which are explained as a result of conver ...

but is kept for ease, pending new systems of classification.

In zoology

Examples can be found in the classification of gastropods:

Opisthobranchia Opisthobranchs () is now an informal name for a large and diverse group of specialized complex gastropods which used to be united in the subclass Opisthobranchia. That taxon is no longer considered to represent a monophyletic grouping. Euopisth ...

Sorbeoconcha Sorbeoconcha is a taxonomic clade of snails, i.e. gastropods, mainly marine species with gills and opercula, within the clade Caenogastropoda. The taxon Sorbeoconcha was named by Winston Ponder and David R. Lindberg in 1997. Taxonomy 1997 taxo ...

Hypsogastropoda Hypsogastropoda is a clade containing marine gastropods within the clade Caenogastropoda. MolluscaBase eds. (2021). MolluscaBase. Hypsogastropoda. Accessed through: World Register of Marine Species at: http://www.marinespecies.org/aphia.php? ...

, and

Ptenoglossa The Ptenoglossa is an informal taxonomic group of sea snails. This group was considered paraphyletic or polyphyletic by Ponder and Lindberg (1997) in their classification of gastropod molluscs. The database WoRMS considers this suborder as bel ...

are informal groups nearby the level of the

order Order, ORDER or Orders may refer to: * Categorization, the process in which ideas and objects are recognized, differentiated, and understood * Heterarchy, a system of organization wherein the elements have the potential to be ranked a number of ...

. In human taxonomy, the informal taxonomic rank of race is variously considered equivalent or subordinate to the rank of subspecies.

In botany

alliance An alliance is a relationship among people, groups, or states that have joined together for mutual benefit or to achieve some common purpose, whether or not explicit agreement has been worked out among them. Members of an alliance are called ...

is also considered an informal grouping of species,A taxonomic revision of Hebe informal group “ Connatae” (Plantaginaceae∗), based on morphology and flavonoid chemistry. Alison V. Kellow, Michael J. Bayly, Kevin A. Mitchell, Kenneth R. Markham and Patrick J. Brownsey, New Zealand Journal of Botany, Volume 41, 2003, Issue 4, pages 613-635, genera or tribes to which authors wish to refer, that have at some time provisionally been considered to be closely related.

References

Grouping Gastropod taxonomy {{botany-stub