Important criteria used for classification in each taxon
The taxonomic system of classification is composed of a hierarchy that is the series of ranks. Hierarchy shows the blood relationship of present day organisms and also, when possible, ancestor-descendant relationships. The lower the rank of a taxon, the more similarities is possessed by its members. Each taxon has a Latin or latinized name. International bureaus and congresses have standardized the ranks and nomenclature of the taxa. Such standardization of names makes it possible for an organism to be identified by biologists all over the world. Eight groups called taxa (singular, taxon) make up the basic system in scientific classification. Every organism has a place in each group. Taxonomists do research to determine the placement of each group within the overall classification framework. Often, the lack of data makes this task difficult, especially for rare or extinct organisms known only from fossils. Additional taxa exist in some taxonomic schemes.

The basic ranks of the classification system
Species is a group of organisms that resemble each other closely in structure and function and whose characteristics remain fairly constant through many generations. Species represents the basic unit of scientific classification. Members of a species normally interbreed with each other but not with members of other species. For many species scientists know little about the organisms other than their appearance.

Taxonomists must classify these organisms into species based solely on their morphology (form and structure). No two species in a genus have the same scientific name. For example, the eastern bluebird is Sialia sialis, the mountain bluebird is Sialia currucoides and the western bluebird is Sialia mexicana.

Genus (plural, genera) is a group of related species, which consists of similar groups, but members of different groups usually cannot breed with one another. For example, three similar bluebirds make up the genus Sialia: the eastern bluebird, the mountain bluebird and the western bluebird. These birds generally do not breed with one another.

Family is a group of related genera which is made up of groups even more alike than those in an order. Wolves and tigers both belong to the order Carnivora. But wolves belong to the dog family Canidae. Most members of this family have long snouts and bushy tails. Tigers form part of the cat family Felidae. Most members of this family have short snouts and short-haired tails.

Order is a group of related families, which consists of groups that are more alike than those in a class. Dogs, moles, raccoons and shrews are all mammals. But dogs and raccoons eat flesh and they are grouped with other flesh-eating animals in the order Carnivora. Moles and shrews eat insects. They belong to the order Insectivora with other insect-eating animals.

Class is a group of related orders. Class members have more characteristics in common than do members of a division or phylum. For example, mammals, reptiles and birds all belong to the phylum Chordata.

But each belongs to a different class. Apes, bears and mice are in the class Mammalia. Mammals have hair on their bodies and feed milk to their young. Reptiles, which include lizards, snakes and turtles, make up the class Reptilia. Scales cover the bodies of all reptiles and none of them feed milk to their young ones.

Phylum (plural, phyla) or Division (in the case of plants and fungi) is a group of related classes. It is the third highest taxon. For the animal, bacteria and archaea kingdoms, taxonomists generally use the term phylum. For fungi, plants and protists, scientists mostly use the term division, but they sometimes accept phylum. Human beings and all other animals with backbones belong to the phylum Chordata.

Kingdom is a group of related phyla, which was formerly ranked as the highest-level taxon in biological classification.

From the 1970’s to the 20th century, most scientific textbooks used a classification system with five kingdoms:

  1. prokaryotes,
  2. protists,
  3. fungi,
  4. plants
  5. animals.

But later scientists realized that the prokaryote kingdom consists of two different kinds of microbes.

This led to the splitting of prokaryotes into two kingdoms:

  1. Archaea
  2. Bacteria.

The kingdom Protista contains a mixed group of simple, mostly one-celled animals. These organisms include algae, water molds, downy mildews and amoebas. Many scientists have proposed splitting the protists into two or more separate kingdoms.

The plant kingdom, Plantae, contains mosses, ferns, conifers and flowering plants. The Kingdom Fungi includes mushrooms, bread molds, powdery mildews, yeasts and lichens. Many scientists also include green algae in this kingdom.

The animal kingdom, Animalia, includes mammals, fish, insects and worms.
In addition to these basic ranks, other ranks are often used to make finer distinctions. Additional ranks usually have the prefix sub- or super-. For example, in a species there may be smaller groups that differ slightly from one another. These groups are called subspecies. (The term subspecies is used only to identify animals; the comparable term for plants is variety. A cultivar is a cultivated variety, one that has been developed by a horticulturist)

A species is identified by both the genus and the species name. This classification system is called the binomial (two-name) system. Both words are printed in italics (if handwritten or typed, both are underlined). The initial letter of the genus name is capitalized, but that of the species is not. A subspecies is identified by three names, with the subspecies name following the genus and species name.
A variety is identified by three names, with the abbreviation var. (for variety) between the second and third names. A cultivar is also identified by three names. The last word, however, is not in italics; it is capitalized and set off by single quotation marks. The name of a hybrid genus or species is preceded by a multiplication sign (X). When the genus or species name has been established by context, it may be abbreviated by using the initial letter only.