Taxonomic Categories

Taxonomic Categories

Introduction

  • In Classification the category is a part of overall taxonomic arrangement, it is called the taxonomic category and all categories together constitute the taxonomic hierarchy.
  • Each category, referred to as a unit of classification, in fact, represents a rank and is commonly termed as taxon.
  • Taxonomic categories and hierarchy can be illustrated by an example.
  • Insects represent a group of organisms sharing common features like three pairs of jointed legs. It means insects are recognizable concrete objects which can be classified, and thus were given a rank or category.
  • Groups represent category. Category further denotes rank.
  • Each rank or taxon, in fact, represents a unit of classification.
  • These taxonomic groups/ categories are distinct biological entities and not merely morphological aggregates.
  • Taxonomical studies of all known organisms have led to the development of  common categories such as kingdom, phylum or division (for plants), class, order, family,genus and species.
  • All organisms, including those in the plant and animal kingdoms have species as the lowest category.
  • The basic requirement is the knowledgeof characters of an individual or group of organisms.
  • This helps in identifying similarities and dissimilarities among the individuals of the same kind of organisms as well as of other kinds of organisms.

Definition

One of a hierarchy of levels in the biological classification of organisms; the seven major categories are kingdom, phylum, class, order, family, genus, species.”

Taxonomic Categories

  • The main aim of a taxonomic study is to assign organism an appropriate place in a  systematic framework of classification.
  • This framework is called taxonomic hierarchy by which the taxonomic groups are arranged in definite order, from higher to lower categories.
  • Any one of a number of formal ranks used for organisms in a traditional Linnaean  classification.
  • Biological classifications are orderly arrangements of organisms in which the order specifies some relationship.
  • Taxonomic classifications are usually hierarchical and comprise nested groups of organisms. The actual groups are termed taxa.
  • In the hierarchy, a higher taxon may include one or more lower taxa, and as a result the relationships among taxa are expressed as a divergent hierarchy that is formally represented by tree diagrams.
  • In Linnaean classifications, taxonomic categories are devices that provide structure to the hierarchy of taxa without the use of tree diagrams.
  • By agreement, there is a hierarchy of categorical ranks for each major group of  organisms, beginning with the categories of highest rank and ending with categories of  lowest rank, and while it is not necessary to use all the available categories, they must be used in the correct order.
  • Both in animal and plant kingdom, the lowest category is the species and the highest is the kingdom; rest categories above the species form higher categories.
  • The categories in the hierarchy are in ascending order.
  • As we go from the lowest rank the species towards the kingdom, the number of similar characters decreases.

               

 

Categories commonly used in botanical and zoological classifications, from highest to  lowest rank

Botanical categories

Zoological categories

Division

Phylum

Classis

Class

Ordo

Order

Familia

Family

Genus

Genus

  • Conceptually, the hierarchy of categories is different than the hierarchy of taxa.
  • For example, the taxon Cnidaria, which is ranked as a phylum, includes the classes Anthozoa (anemones), Scyphozoa (jellyfishes), and Hydrozoa (hydras).
  • Cnidaria is a particular and concrete group that is composed of parts.
  • Anthozoa is part of, and included in, Cnidaria. However, categorical ranks are quite different.
  • The category “class” is not part of, nor included in, the category “phylum.”
  • Rather, the category “class” is a shelf in the hierarchy, a road mark of relative position.
  • There are many animal taxa ranked as classes, but there is only one “class” in the Linnaean hierarchy.
  • This is an important strength of the system because it provides a way to navigate through a classification while keeping track of relative hierarchical levels with only a few ranks for a great number of organisms.
  • When Linnaeus invented his categories, there were only class, order, family, genus, and species.
  • These were sufficient to serve the needs of biological diversity in the late eighteenth  century, but were quite insufficient to classify the increasing number of species discovered since 1758.
  • As a result, additional categorical levels have been created.
  • These categories may use prefixes, such as super- and sub-, as well as new basic levels such as tribe.
  • Linnaean categories are the traditional devices used to navigate the hierarchy of taxa. But categories are only conventions, and alternative logical systems, such as those used by phylogenetic systematists (cladists), are frequently used. See also Animal systematics; Classification, biological; Phylogeny; Plant taxonomy; Taxonomic categories; Zoological nomenclature.
  • An example of a modern expanded botanical hierarchy of ranks between family and species is- 

Familia

Subfamilia

Tribus

Subtribus

Genus

Subgenus

Section

Subsection

Series

Subseries

Species 

Linnaeus
17352 kingdoms

Haeckel
18663 kingdoms

Chatton
19252 empires

Copelad
19384

kingdoms

Whittaker19695 kingdoms

Woese et al.
19776 kingdoms

Woese et al.
19903 domains

 

(not treated)

 

Protista

Prokaryota

Monera

Monera

Eubacteria

Bacteria

 

Vegetabilia

 

Plantae

Eukaryota

 

 

Archaebacteria

Archaea

Animalia

 

Animalia

 

Protista

Protista

Protista

Eukarya

 

 

 

 

Fungi

Fungi

 

 

 

 

 

Plantae

Plantae

Plantae

 

 

Species

“A  species is a group of organisms capable of interbreeding and producing fertile offspring of both genders, and separated from other such groups with which interbreeding does not (normally) happen.”

  • Taxonomic studies consider a group of individual organisms with fundamental   similarities as a species.  
  • One species is distinguished from the other closely related species on the basis of the distinct morphological differences.
  • Let us consider Mangifera indica, Solanum tuberosum (potato) and Panthera leo (lion). All the three names, indica, tuberosum and leo, represent the specific epithets, while the first  words Mangifera, Solanum and Panthera are genera and represents another higher level of taxon or category.
  • Each genus may have one or more than one specific epithets representing different organisms, but having morphological similarities. For example, Panthera has another specific epithet called tigris and Solanum includes species like nigrum and melongena.
  • Human beings belong to the species sapiens which is grouped in genus Homo. The scientific name thus, for human being, is written as Homo sapiens.

Genus

Genus comprises a group of related species which has more characters in common in comparison to species of other genera.”

  • We can say that genera are aggregates of closely related species.
  • A genus term comes from Latin genus "descent, family, type, gender", cognate with Greek genos= race +stock= kin.
  • For example, potato, tomato and brinjal are three different species but all belong to the  genus Solanum. Lion (Panthera leo), leopard (P. pardus) and tiger (P. tigris) with several common features, are all species of the genus Panthera. This genus differs from another genus Felis which includes cats.
  • The composition of a genus is determined by a taxonomist.
  • The standards for genus classification are not strictly codified, and hence different authorities often produce different classifications for genera.
  • In the hierarchy of the binomial classification system, genus comes above species and below family.

Family

“Family has a group of related genera with still less number of similarities as compared to genus and species. “

The family (Latin: familia) is

  • A taxonomic unit, a taxon, in that rank. In that case the plural is families (Latin familiae).
  • An immediately lower rank, indicated by the prefix sub-: subfamily (Latin: subfamilia).
  • Families are characterised on the basis of both vegetative and reproductive features of plant species.
  • Among plants for example, three different genera Solanum, Petunia and Datura are placed in the family Solanaceae.
  • Among animals for example, genus Panthera, comprising lion, tiger, leopard is put along with genus, Felis (cats) in the family Felidae.
  • Similarly, if you observe the features of a cat and a dog, you will find some similarities and some differences as well. They are separated into two different families – Felidae and Cancidae, respectively.
  • Example- "Walnuts and Hickories belong to the Walnut family.

Order

“The Latin suffix -formes meaning "having the form of" is used for the scientific name of orders of birds and fishes, but not for those of mammals and invertebrates.”

The order (Latin: ordo) is

  • A taxonomic rank used in the classification of organisms.
  • An immediately higher rank, superorder, may be added directly above order, while suborder would be a lower rank.
  • Order and other higher taxonomic categories are identified based on the aggregates of characters.
  • Order being a higher category, is the assemblage of families which exhibit a few similar characters.
  • The similar characters are less in number as compared to different genera included in a family.
  • Plant families like Convolvulaceae, Solanaceae are included in the order Polymoniales mainly based on the floral characters.
  • The animal order, Carnivora, includes families like Felidae and Cancidae.

Class

“A taxonomic unit, a taxon, in that rank. In that case the plural is classes”

The class (Latin: classis) is

  • The composition of each class is determined by a taxonomist.
  • For example, dogs are usually assigned to the phylum Chordata (animals with notochords); in the class Mammalia; in the order Carnivora (mammals that eat meat).
  • This category includes related orders. For example, order Primata comprising monkey,gorilla and gibbon is placed in class Mammalia along with order Carnivora that includes animals like tiger, cat and dog.
  • Class Mammalia has other orders also.

Phylum

In biology, a phylum (plural: phyla) is a taxonomic rank below Kingdom and above Class. "Phylum" is equivalent to the botanical term division.

  • Classes comprising animals like fishes, amphibians, reptiles, birds along with mammals constitute the next higher category called Phylum.
  • All these, based on the common features like presence of notochord and dorsal hollow neural system, are included in phylum Chordata.
  • In case of plants, classes with a few similar characters are assigned to a higher category called Division.

Kingdom

  • The kingdom or regnum is a taxonomic rank in either (historically) the highest rank, or (in the new three-domain system) the rank below domain.
  • Each kingdom is divided into smaller groups called phyla (or in some contexts these are called "divisions").
  • A system of six kingdoms is used at many places-Animalia, Plantae, Fungi, Protista, Archaea, Bacteria.
  • While some describe five kingdoms as -Animalia, Plantae, Fungi, Protista, and Prokaryota or Monera.
  • The classifications of taxonomy are life, domain, kingdom, phylum, class, order, family, genus, and species.
  • All animals belonging to various phyla are assigned to the highest category called Kingdom Animalia in the classification system of animals.
  • The Kingdom Plantae, on the other hand, is distinct, and comprises all plants from various divisions.
  • Henceforth, we will refer to these two groups as animal and plant kingdoms. The taxonomic categories from species to kingdom have been shown in ascending order starting with species .These are broad categories.
  • However, taxonomists have also developed sub-categories in this hierarchy to facilitate more sound and scientific placement of various taxa.
  • For example, as we go higher from species to kingdom, the number of common characteristics goes on decreasing. Lower the taxa more are the characteristics that the members within the taxon share.
  • Higher the category, greater is the difficulty of determining the relationship to other taxa at the same level. Hence, the problem of classification becomes more complex.

DOMAIN

“A Domain (also super regnum, super kingdom, or empire) is the highest taxonomic rank of organisms, higher than a kingdom.

  • According to the three-domain system of Carl Woese, introduced in 1990, the Tree of Life  consists of three domains: Archaea, Bacteria and Eukarya. The arrangement of taxa reflects the fundamental differences in the genomes.
  • There are some alternative classifications of life: The two-empire system or super domain system, with top-level groupings of Prokaryota (or Monera), Eukaryota and the recently discovered Archaea empires.
  • The six-kingdom system with top-level groupings of Eubacteria, Archaebacteria, Protista, Fungi, Plantae, and Animalia.
  • As these groupings depend primarily on the analysis of genetic sequence data and cladistics, additional proposed arrangements are to be expected.
  • None of the three systems currently include non-cellular life.