Introduction to Taxonomy

Taxonomy, the branch of biology that deals with the classification of living and extinct organisms. It is concerned with distinguishing individual organisms, placing them in groups according to kinship, and arranging the groups in an orderly, meaningful, and useful system. Taxonomy includes not only the study of the structure, function, and development of an individual organism but also the study of the evolutionary development of the group to which it belongs.

Scientific classification is the method scientists have developed to arrange all of the world's organisms both extinct and living things into groups. They base their classification system on the biological similarities that exist among species (kinds) of organisms. Classification creates a method for organizing facts about organisms and groups of organisms.

Scientists keep making changes to classification. Since the mid-1900’s, biologists have gathered a lot of information on classification from studying the sequence (order) of compounds called bases within genes in the genomes of different species. A gene is the part of a cell that determines which traits an organism inherits from its parents. The term genome refers to all the genes on a cell’s chromosomes, structures that consist mainly of the genetic material DNA (deoxyribonucleic acid). By comparing genomes of different species, biologists have more accurately determined how these species are related to one another.

Scientists also use a method of analysis called cladistics in which they evaluate anatomical traits of at least three related species. These evaluations help them construct a cladogram, a kind of family tree illustrating how the species are related to one another.

The Work of the Taxonomist

Taxonomists must know something about other branches of biology, such as genetics (the study of heredity), morphology (the study of the structure of organisms), physiology (the study of the functions and activities of organisms), ecology (the study of the relationship of organisms to their environment), and paleontology (the study of organisms that lived in past ages). Their knowledge of structure and function must include cytology (which deals with cells) and biochemistry (the chemistry of living organisms). They must also know statistical methods and the rules and conventions of the internationally accepted taxonomic system.

Taxonomists' activities include the following:

  • They study specimens they have collected or those in collections made by others and preserved in museums and other institutions. They often study living organisms in their natural habitats, or dig for fossils in rock strata.
  • They record their data and study data recorded by other researchers.
  • Often applying statistical methods, they place organisms they have studied in the group (called a taxon, plural taxa) to which they conclude the organisms belong. If the organisms belong to no recorded taxon, they place them in a new one, selecting or coining a name for the taxon in accordance with accepted rules of nomenclature (the system of names).
  • In the process of carrying out their other activities, they study, test, and, when necessary, help to modify existing theories and principles of taxonomy.

Taxonomists who concentrate on systematics attempt to reconstruct evolutionary histories of species. Those who concentrate on cladistics describe similarities and differences between groups of species. Those who concentrate on phenetics identify similarities and differences between individual species.

The Taxonomic System

The taxonomic system of classification is composed of a hierarchy (series of ranks) that shows the kinship of contemporary organisms and also, when possible, ancestor-descendant relationships. The lower the rank of a taxon, the more similarities are possessed by its members.

Each taxon has a Latin or latinized name. International bureaus and congresses in various fields of biology standardize 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. Common, or popular, names are confusing because they vary from place to place. For example, one large species of cat may be known in various parts of North and South America as a puma, cougar, mountain lion, panther, or leon. But this cat has only one scientific name, Felis concolor. Scientists can identify the cat by that name no matter what their language.

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 are as follows:

  • Species (plural, species), 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, 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.
  • Phylum (plural, phyla), or Division (in the case of plants and fungi), 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—prokaryotes, protists, fungi, plants, and 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: Archaea and 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.)

An example of animal classification is that of the domestic dog, Canis familiaris:










A typical plant classification is that of the cork oak tree, Quercus suber:









As the examples show, 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 names. 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:

Members of the genus Bufo include B. americanus, the American toad, and B. woodhousei, Woodhouse's toad. A subspecies of Woodhouse's toad is B. w. fowleri, Fowler's toad.

More than 1,700,000 species of organisms have been identified, and thousands of new ones are identified and classified every year. New techniques are constantly being developed which make the identification and classification of organisms more accurate. For these reasons, organisms often have to be reclassified, and the ranks and names of taxa occasionally have to be revised.

International commissions of scientists establish the rules, or codes, for adopting scientific names. Different sets of codes exist for botanists, zoologists, and microbiologists. These three groups are working to merge their codes into one standard set of rules covering all life.


In the fourth century B.C., Aristotle devised a system of classification of organisms. He grouped organisms into divisions according to similar characteristics. Several of these divisions are still used. An example is the division of animals into vertebrates (animals with backbones) and invertebrates (animals without backbones). For 2,000 years no significant advances were made in classifying organisms.

Beginning in the late 1400’s, explorers and scientists began to travel extensively. People discovered new species around the world, and naturalists developed new classification schemes. In the 17th century, the English naturalist John Ray introduced the modern concept of the species as the basic unit of classification. In the 18th century, Carolus Linnaeus, a Swedish naturalist, developed a more complex classification based upon Ray's definition of a species and established the two-name system for species. Both Ray and Linnaeus believed that species were fixed at the time of creation and never changed. Toward the end of the 18th century, Antoine-Laurent de Jussieu, a French botanist, grouped plants into 100 families. His work influenced Augustin de Candolle, a Swiss botanist who later devised a plant classification system. De Candolle was the first to use the term taxonomy.

Since World War II, rapid advances in the science of biochemistry have revolutionized the study of the structure and functions of organisms. More recently, the use of such techniques as DNA sequencing has allowed scientists to analyze and compare the genetic material of different species. This research has led to many revisions in the classification of organisms.

Some scientists are developing an alternative set of rules for naming organisms called the PhyloCode which would eliminate all taxa above species level. The naming of a new group would include information about where the group fits into the overall phylogeny (evolutionary history). However, many biologists are critical of this system.

Classification of Organisms

The classification system given here divides living organisms into five kingdoms—Monera, Protista, Fungi, Plantae, and Animalia. Although the five-kingdom classification is widely accepted, some scientists disagree on the placement of certain groups of organisms. In the following scheme, for example, slime molds are placed in the protist kingdom, and algae are placed in the monera and protist kingdoms; some scientists, however, place slime molds in the fungus kingdom and place the red, brown, and green algae in the plant kingdom. Formerly, all organisms were considered to be either plants or animals and placed in the plant or animal kingdom.

Scientists also disagree on the number of phyla or divisions in each kingdom. For example, the system given in this article places organisms of the monera kingdom into 4 phyla and organisms of the protist kingdom into 11 phyla; some scientists, however, recognize as many as 17 phyla of monera and 27 of protists.

All organisms in the five kingdoms described here are composed of one or more cells. The members of the monera kingdom are prokaryotes. (A prokaryote is a single-celled organism having a nucleoid instead of a true nucleus; in a nucleoid, the hereditary material is not bound by a membrane.) The members of the other four kingdoms are eukaryotes. (A eukaryote is a single-celled or multicelled organism having cells with true nuclei—that is, nuclei that are bound by membranes.)

Viruses, which do not have a cell structure, are not considered living things by most scientists and are not listed in the classification system given in this article. Those scientists who consider viruses to be living things place them in the monera kingdom, even though they are not cellular.

The Kingdom Monera

This kingdom is called Prokaryotae by some biologists.

Phylum Archaea, or Archaebacteria

Bacteria-like organisms found in environments that lack oxygen, are very hot and acidic, or have high salt concentrations. Some use sulfur as an energy source and live in deep-sea hydrothermal vents. Others use carbon dioxide and hydrogen for energy, producing methane. Some taxonomists assign this phylum to a separate kingdom or subkingdom.

Phylum Schizophyta

Bacteria. Some contain chlorophyll. Most derive energy and carbon from dead organic matter; some derive energy from inorganic chemicals. Solitary or in colonies. Aquatic, terrestrial, or parasitic.

Phylum Cyanophyta

The cyanobacteria, or blue-green algae. Contain chlorophyll. Solitary or in colonies. Most are freshwater species.

Phylum Prochlorophyta

Contain chlorophyll; resemble the chloroplasts of plants.

The Kingdom Protista

This kingdom, sometimes called Protoctista, consists of single-celled microorganisms and simple multicelled organisms.

Phylum Protozoa

The protozoans. Single-celled; solitary or in colonies. Aquatic, terrestrial, or parasitic. Examples: amoeba, globigerina, paramecium.

Phylum Chrysophyta

Golden-brown algae, or diatoms, and yellow-green algae. Most are single-celled. Marine and freshwater. Contain chlorophyll.

Phylum Pyrrophyta, or Dinoflagellata

Fire algae, or dinoflagellates. Most are single-celled; marine. Contain chlorophyll.

Phylum Euglenophyta

Most are single-celled flagellates containing chlorophyll. Freshwater, marine, or parasitic. Example: euglena.

Phylum Phaeophyta

Brown algae. Multicelled; marine. Contain chlorophyll. Example: kelp.

Phylum Rhodophyta

Red algae. Multicelled; most are marine. Contain chlorophyll. Example: Irish moss.

Phylum Chlorophyta

Green algae. Single- and multicelled; most are freshwater. Contain chlorophyll.

Phylum Myxomycota

The plasmodial slime molds. Body is a thin, flat mass of protoplasm with many nuclei. Certain kinds parasitic.

Phylum Acrasiomycota

The cellular slime molds. Body is multicelled and sluglike.

Phylum Chytridiomycota

The chytrids. Single-celled. Parasitic or absorb dead organic matter.

Phylum Oomycota

White rusts, downy mildews, and water molds. Parasitic or absorb dead organic matter. Some of these organisms have caused serious plant diseases, such as the potato blight in 19th-century Ireland.

The Kingdom Fungi

This kingdom consists of organisms with cell walls made of chitin. Fungi (except for yeasts) are multicelled but their cell walls are usually incompletely formed. Most fungi obtain nutrients by absorbing dead organic matter.

Division Zygomycota

Molds. Many live on decaying vegetation; some parasites. Example: black bread mold.

Division Ascomycota

Molds, morels, truffles, and yeasts. Form a reproductive structure called an ascus. Some parasitic. Example: brewer's yeast; ergot; powdery mildews.

Division Basidiomycota

Mushrooms, rusts, and smuts. Form a clublike reproductive structure called a basidium. Some parasitic. Example: common mushroom; stem rust.

Division Deuteromycota, or Fungi Imperfecti

Molds and yeasts. Lack structure for sexual reproduction. Many parasitic. One genus is the source of the antibiotic penicillin. Examples: athlete's foot fungus, damping-off fungus.

Division Mycophycophyta

Lichens. Symbiotic partnership between fungi and chlorophytes (green algae) or between fungi and cyanophytes (blue-green algae).

The Kingdom Plantae

This kingdom consists of multicelled organisms with cell walls made of cellulose. Most plants make their own food through photosynthesis.

Subkingdom Bryophyta

Division Bryophyta

Hornworts, liverworts, and mosses. Simple leaves and stems. No roots; vascular system absent or very simple. Contain chlorophyll. Mostly freshwater or terrestrial.

Subkingdom Tracheophyta

The tracheophytes are plants with well-developed vascular systems. Most contain chlorophyll. Though all plants produce spores, only the higher (more complex) plants also produce seeds, which develop in cones or in flowers.

Division Psilophyta

Whisk ferns. Most primitive of vascular plants. Branching, leafless stems; no true roots.

Division Lycophyta, or Lycopodophyta

Club mosses. Branching stems with numerous small leaves. Spores produced by club-shaped organ.

Division Sphenophyta

Horsetails. Jointed stems with whorls of leaves.

Division Pterophyta, or Filicinophyta

Ferns. Have feathery leaves called fronds.

Division Cycadophyta

Cycads. Palmlike. Produce seeds in cones.

Division Ginkgophyta

The ginkgo. Has fan-shaped leaves. Produces seeds in fleshy, roundish structures.

Division Gnetophyta

Gnetophytes, such as welwitschia. Produce seeds in cones.

Division Coniferophyta

Conifers. Have needlelike or scalelike leaves. Mostly evergreen. Produce seeds in cones.

Division Anthophyta, or Angiospermophyta

Flowering plants. Largest plant division. Produce seeds in flowers.

The Kingdom Animalia

This kingdom consists of multicelled organisms with cell membranes but no cell walls. Most animals obtain nutrients by digesting food in an internal cavity.

Subkingdom Parazoa

These animals have tissues that are not organized into organs.

Phylum Placozoa

A single marine species. Resembles an amoeba; about 0.1 inch (2.5 mm) in diameter.

Phylum Porifera

The sponges. Porous bodies with fibrous or brittle internal skeleton. Mainly marine and colonial.

Subkingdom Eumetazoa

These animals have tissues organized into organs; the organs are usually organized into systems.

Phylum Cnidaria, or Coelenterata

Bodies saclike. Tentacles with stinging cells. Solitary or colonial. Mainly marine. Examples: coral, jellyfish.

Phylum Ctenophora

The comb jellies. Similar to cnidarians, but with comblike rows of cilia. Marine, solitary. Example: Venus's girdle.

Phylum Mesozoa

Microscopic wormlike parasites. Have only one organ: a gonad.

Phylum Platyhelminthes

The flatworms. Flat, slender bodies. Aquatic, terrestrial, or parasitic. .

Phylum Nemertina, or Rhynchocoela

The ribbon worms. Flat, slender bodies. Proboscis may extend to several times the length of the body. Mainly marine or terrestrial.

Phylum Gnathostomulida

Microscopic marine worms. Hermaphrodites. Have comblike feeding structures.

Phylum Rotifera

The rotifers. Head with crown of cilia. A variety of body shapes, ranging from trumpetlike to spherical. Aquatic. Most free-living; some parasitic.

Phylum Gastrotricha

Bodies wormlike, covered with spines; two rows of cilia. Aquatic.

Phylum Kinorhyncha

Wormlike; appear segmented because of zonites (rings). Pull themselves along with head spines. Marine.

Phylum Nematoda

The roundworms. Slender, cylindrical bodies. Aquatic or terrestrial; mainly freeliving but some parasitic.

Phylum Nematomorpha

The hairworms. Extremely slender bodies. Adults aquatic or terrestrial. Larvae parasitic.

Phylum Loricifera

Microscopic; rotifer-like. Retractable spiny heads. Abdomen covered with spiny plates. Marine.

Phylum Acanthocephala

The spiny-headed worms. Bodies flat, with spiny proboscis. Parasitic.

Phylum Entoprocta

Bodies have tentacled, cupshaped part attached by stalk to rocks, shells, seaweed, or other animals. Usually marine and colonial.

Phylum Ectoprocta

Resemble entoprocts, except that ectoprocts have a coelum, or a body cavity lined with a membrane.

Phylum Phoronida

Marine worms with long tentacles. Some are hermaphrodites. Live in hard or leathery tubes formed from secretions. Mostly colonial.

Phylum Brachiopoda

The lamp shells. Soft body parts enclosed in bivalve shells. Most attach themselves firmly to rocks or other substrate.

Phylum Mollusca

The mollusks. Body parts usually soft; some enclosed in univalve or bivalve shell. Aquatic or terrestrial. Examples: snail, oyster, octopus.

Phylum Priapulida

Burrowing worms covered with spines. Retractable mouth. Marine.

Phylum Sipuncula

The peanut worms. Burrowing worms with retractable proboscis. Many have tentacles around mouth. Marine.

Phylum Echiura

The spoon worms. Burrowing worms with plump bodies; have proboscis that may extend to several times the body length. Marine.

Phylum Annelida

The segmented worms. Bodies segmented. Aquatic, terrestrial, or parasitic. Examples: earthworm, leech.

Phylum Tardigrada

The water bears. Microscopic. Have four pairs of stumpy legs. Most live in water around mosses or lichens.

Phylum Pentastoma

The tongue worms. Flat body with four claws. Parasites in lungs and nasal passages.

Phylum Onychophora

The velvet worms. Body with 14 to 43 pairs of clawed legs. Terrestrial.

Phylum Arthropoda

The arthropods. Largest animal phylum. Segmented bodies with external skeletons and jointed appendages. Aquatic, terrestrial, or parasitic. Examples: insects, spiders, millipedes, lobsters.

Phylum Pogonophora

The beard worms. Live on sea floor in hard, upright tubes formed from secretions. Long tentacles at front of body.

Phylum Chaetognatha

The arrowworms. Transparent, torpedo-shaped bodies. Marine.

Phylum Echinodermata

The echinoderms. Internal skeleton of calcite crystal plates. Unique circulatory system. Aquatic, mainly marine. Examples: sea urchin, starfish.

Phylum Hemichordata

The hemichordates. Soft-bodied; have gill-slits and proboscis. Marine, some kinds burrowing in sea bottoms. Solitary or in colonies. Example: acorn worm.

Phylum Chordata

The chordates. Large, varied phylum. Have nerve cord; most have bony internal skeleton. Aquatic or terrestrial. Examples: amphioxus, bird, fish, humans.