Taxonomic rank

In biological taxonomy, a taxonomic rank shows the level a group of living things holds in a system that organizes them based on how they are related through evolution. The highest ranks include big groups like the Eukarya and Animalia, while the lowest ranks are for specific types of organisms, such as Homo sapiens, Bufo bufo, Tyrannosaurus rex, and Vulpes vulpes.

Ranks can be described in two ways. One way uses set terms like species, genus, tribe, family, order, class, phylum, kingdom, and domain. Another way shows the rank by how far a group is indented in a list. This article focuses on the first method.

When we look at the red fox, Vulpes vulpes, we can see how these ranks work. The name vulpes tells us the specific type of fox, which belongs to the genus Vulpes. This genus is part of the family Canidae, which includes dogs and wolves. The family is in the order Carnivora, which also has bears and weasels. All these animals are part of the class Mammalia, which includes all warm-blooded animals that nurse their young. They are in the phylum Chordata, animals with a backbone, and finally, in the kingdom Animalia. At the very top, they belong to the domain taxon Eukarya, which includes all organisms with cells that have nuclei.

The way we name living things helps scientists talk about and study them clearly, even when different groups have similar names. This system makes it easier to understand how all living things are connected.

History

In the second half of the 20th century, two new ways of grouping living things changed how scientists classify them. The first is called cladistics, started by German entomologist Willi Hennig. Cladistics groups organisms based on how many traits they share, which helps show how recently they shared a common ancestor. The second method is molecular systematics, which uses genetic analysis to learn more about relationships between different kinds of life, especially when it’s hard to tell them apart just by looking. This helps with groups like viruses, bacteria, and archaea, or when many species evolved quickly from one ancestor a long time ago, such as placental mammals.

Main ranks

In his important books, like the Systema Naturae, Carl Linnaeus used a system to group living things into levels such as kingdom, class, order, genus, and species. Today, there are seven main levels: kingdom, phylum or division, class, order, family, genus, and species. There is also a level called domain, which was suggested by Carl Woese and is now commonly used.

When a living thing is given its official name, it is placed into one of these levels. The most basic levels are species and genus. For example, the name for humans is Homo sapiens. The first part, Homo, is the genus name, and the second part, sapiens, tells us the species. Sometimes, there can be an extra name to show more detail, like Homo sapiens sapiens for modern humans.

Ranks in zoology

There are special rules for naming groups of animals, as set out in the International Code of Zoological Nomenclature. These rules cover ranks like superfamily, family, subfamily, tribe, subtribe, genus, subgenus, species, and subspecies.

The Code groups names into three categories: family-group names, genus-group names, and species-group names. It lists specific ranks for each group, such as Family, Subfamily, Tribe, Genus, Species, and Subspecies. Sometimes scientists add extra unofficial ranks, like "species group" or "species complex", to help organize groups with many species, such as in the genus Drosophila.

For higher ranks like family and above, scientists can show a lower level by adding "infra" to the rank name, such as "infraorder" or "infrafamily".

Names for animal groups follow these patterns:

• Groups above the species level have one-part names.
• A species usually has a two-part name, like Canis lupus.
• A subspecies has a three-part name, like Canis lupus italicus.

Ranks in botany

Botanical ranks help us group plants based on their shared traits and history. These ranks start with Kingdom and go down to Division (or Phylum), Class, Order, Family, Genus, and finally Species. Each group at a certain rank usually shares special features and a common past.

Some special rules decide how plant names are given, especially for groups below the family rank. Names for groups at the genus level or higher have one part, while groups below genus but at the species level have two parts. Groups below species have three-part names and need a special word to show their rank. For example, Poa secunda subsp. juncifolia shows the rank of subspecies.

Main article: International Code of Nomenclature for Cultivated Plants

Examples

Here are classifications for five different species: the fruit fly called Drosophila melanogaster, humans (Homo sapiens), the peas used by Gregor Mendel to study genetics (Pisum sativum), the "fly agaric" mushroom (Amanita muscaria), and a bacterium called Escherichia coli. The eight main levels of classification are shown in bold, and some smaller levels are also included.

Table notes

• To keep the table simple, some middle levels are left out. For example, the mammals of Europe, Africa, and upper North America (except the Virginia opossum) belong to class Mammalia, but many smaller groups exist between this class and the order of mammals.
• The levels for bigger groups can change when scientists learn more about how living things are related. For example, the flowering plants used to be in a group called a division but are now in a smaller group. The way we classify mammals has also changed over time.
• Inside a species, there can be smaller groups. Animals can be split into subspecies (like modern humans, Homo sapiens sapiens) or special forms. Plants can be split into subspecies, varieties, or cultivated types with special names. Bacteria can be split into strains, like a kind of Escherichia coli that can cause food poisoning.

Terminations of names

Taxa above the group level often have names based on a type genus, with a special ending. The endings used depend on the kingdom (and sometimes the group or class), as shown in the table below.

Pronunciations given are the most Anglicized. More Latin-like ways of saying the words are also used.

Table notes

• In plants and fungi, names at the family level and below are based on the name of a genus, sometimes called the type genus of that group, with a standard ending. For example, the rose family, Rosaceae, is named after the genus Rosa, with the ending "-aceae" for a family. Names above the family level are also formed from a generic name, or are descriptive (like Gymnospermae or Fungi).
• For animals, there are standard endings for groups only up to the level of superfamily. A uniform ending has been suggested (but not recommended) in AAAS as -ida /ɪdə/ for orders, for example; some scientists who study tiny animals seem to use this system. Many higher animal orders also have such endings, e.g. Hyolithida and Nectaspida (Naraoiida).
• Creating a name based on a generic name can be tricky. For example, homo has the form hominis, so the genus Homo (human) is in the Hominidae, not "Homidae".
• The levels of epifamily, infrafamily and infratribe (in animals) are used when the branching of family trees needs more detailed distinctions. Although they fall below the level of superfamily, they are not regulated under the International Code of Zoological Nomenclature and so do not have formal standard endings. The endings listed here are regular, but informal.
• In virus studies, the formal endings for groups of viroids, of satellite nucleic acids, and of viriforms are similar to viruses, only -vir- is replaced by -viroid-, -satellit- and -viriform-. The extra levels of realm and subrealm end with -viria and -vira respectively.

All ranks

Scientists sometimes need to create new groups to organize living things based on how they are related. Different areas of study, like the study of viruses, bacteria, plants, fungi, animals, fish, and butterflies, each have their own special names for these groups.

Significance and problems

Taxonomic ranks are given based on how different organisms seem to be, but they don't perfectly show how nature changes smoothly. Some scientists, like Willi Hennig, thought these ranks should be dropped because they don't fit well with how we now understand evolution. These ranks were created before we knew about evolution, and they are still used because rules require them.

Some scientists believe the most important rank is "species," but others don't agree. A group of organisms called a taxon should ideally show their evolutionary relationships, but this isn't always required. There are no strict rules for how many species make up a genus or family. Scientists try to make these groups natural and not mixed-up, using all the information they have. Different groups of organisms aren't always equal in how old they are or how different they look, even if they have the same rank.

Enigmatic taxa

Enigmatic taxa are groups of living things whose place in the tree of life is not clear. We do not fully understand how they connect to other groups. (See Incertae sedis.)

Mnemonic

There are fun ways to help remember the groups in biological classification, like the phrase "King Phillip Came Over For Great Spaghetti". This stands for the groups: Kingdom(s), Phylum/Phyla, Class(es), Order(s), Family/Families, Genus, Species.

(See taxonomy mnemonic.)