Beetle species classified in a showcase.
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From Grolier's New Book of Knowledge

Biological classification is the arrangement of organisms into categories that express their phylogeny, or line of descent. Organisms are categorized based on information such as similarity in anatomy, development, and biochemical processes and physiological functions, and on fossil evidence. The comparison of molecular structures, such as DNA sequences, also aids in classification.

The purpose of such a classification is to provide a clear and practical way to organize and communicate information about organisms. Classifications can show relationships between different kinds of ancient and modern taxa (distinct groups of organisms). They can indicate the evolutionary pathways along which present-day organisms may have developed. And they can provide a basis for comparing experimental data about different plant, animal, and other groups.

Organisms included in a taxon share a common genetic heritage in their DNA, and they must be more closely related to each other than they are to the members of other taxa of the same rank. However, classifications of organisms are modified as ideas of their phylogeny change.

Taxonomy is the theory and practice of classifying organisms. It is a branch of systematics, the study of the diversity of organisms. The first scheme for classifying animals into logical groupings may have been proposed by Aristotle more than 2,000 years ago. Since that time, many new classification systems have been proposed. None, however, has succeeded in fitting all plants, animals, fungi, and microorganisms into a single, completely satisfactory scheme. For example, some taxonomists classify algae with the Protista. Others consider them plants. Biotechnological techniques have enabled researchers to compare the DNA of various organisms to decipher the phylogeny of some organisms and helped to distinguish closely related species with similar appearance.


Aristotle (384–322 B.C.) is often called the father of biological classification. He recognized the need for groups and group names in the study of the animal kingdom. His classification scheme referred to readily apparent groups, such as birds, fishes, whales, and bats. John Ray (1627–1705) used anatomical differences as the prime criterion for classification, bringing out both the resemblances and the differences between groups—for example, lung breathing or gill breathing. This is still a preferred method for identification of organisms.

The standard and universal binomial nomenclature for species is attributed to Carolus Linnaeus (1707–78). He applied it consistently to plants in Species Plantarum (1753) and to animals in Systema Naturae (10th ed., 1757). Linnaeus's system was readily applicable to Charles Darwin's new concept of evolution, which was published in On the Origin of Species (1859). Darwin proposed the theory that organisms evolve by the process of natural selection. The theory had no immediate effect on existing classifications themselves. But it provided a new explanation, nearness of descent, for the natural grouping of organisms. This approach is fundamental to modern classification schemes.


Linnaeus arranged classification categories as a series of nested sets. His sequence from broadest to smallest category is: kingdom, class, order, genus, and species. Related groups of organisms were determined by their many shared characteristics. He stressed especially those having to do with sustenance, feeding, and digestion.

The basic Linnaean unit in the classification of living forms is the species (plural, species). Each species is given a unique, two-part name in Latin. The name is always underlined or italicized in print. The name consists of the genus, which is a group of species more closely related to one another than to any other group, followed by the specific name, which identifies a particular species within a genus. The first letter of the genus is capitalized, while the specific name is in lowercase, as in Homo sapiens (human) and Sciurus carolinensis (gray squirrel). The binomial species name replaced the much longer descriptive phrases of earlier classifications.

Linnaeus named taxa of organisms based on their complexes of defining characters. For example, he gave the name Mammalia to the group of animals that possess mammary glands and secrete milk to nourish their young. He also recognized that apes are most nearly like humans, an observation that leads to a logical consequence of strictly biological classification. That is, humans are grouped not only in the class Mammalia but in the same ordinal division with the monkeys and apes.


The smallest unit of classification is usually the species, the only taxonomic unit with clear biological meaning to the organisms. A species includes all organisms that can interbreed and produce fertile offspring in nature. Thus the genes of one species cannot be transferred to another through sexual reproduction.

A species is normally divided into many local populations. Occasional interbreeding between members of different local populations maintains genetic continuity within the species. The genetic differences between species may be expressed as differences in anatomy, behavior, ecology, physiology, and cellular chemistry.

Ranks above the species level cannot be defined in absolute terms but only in relationship to categories of the next higher or next lower rank. Categories above the species level indicate nearness of relationship and thus common descent, but they are not biologically equivalent. That is, a family of rodents may not be comparable to a family of flowering plants.

As the number and diversity of known organisms increased, the classification levels of phylum and family were added to Linnaeus's original five. Other categories were formed by adding the prefixes super-, sub-, and infra- to the names of main categories. Species that are closely related are grouped together into a genus (plural, genera). Genera with similar characteristics and origins are grouped into families. Families are grouped into orders, orders into classes, and classes into phyla in animals and into divisions in plants. Related phyla or divisions are placed together into kingdoms.

The table associated with this article, a classification of modern humans, illustrates the major categories and some of their subdivisions.


Originally, organisms were divided between two kingdoms, the Plantae (including bacteria, fungi, and algae) and the Animalia (including Protozoa).

Ernst Haeckel — He was also the first to divide the Animal Kingdom into unicellular (protozoan) and multicellular (metazoan) animals.

The wealth of new data generated by the new technologies of molecular biology and electron microscopy led to the five-kingdom system proposed by Robert Whittaker in the 1950s. In Whittaker's system, which is used in this encyclopedia, organisms are classified according to whether they are prokaryotic or eukaryotic. Prokaryotic organisms are single-celled, like bacteria, with neither a nucleus nor membrane-bound organelles. Eukaryotic organisms are composed of one or more cells containing membrane-bound nuclei and organelles .

The five kingdoms are Monera, Fungi, Protista, Plantae, and Animalia. (Other arrangements and numbers of kingdoms have also been proposed, each with its own valid reasons of classification. For example, an effort now known as Deep Green was initiated in the early 1990s at the Missouri Botanical Garden to establish more clearly the evolutionary relationships among the green plants. The result of this effort may be to realign some branches of presumed family trees for living organisms and divide a familiar grouping such as Whittaker's Plantae into further kingdoms.) Photosynthetic organisms—plants and cyanobacteria—are autotrophs; that is, they synthesize organic molecules from inorganic resources. With few exceptions, other organisms are heterotrophs; they must consume other organisms that then serve as resources for synthesizing organic molecules.

The Monera include two major subdivisions: eubacteria, the familiar bacteria and the cyanobacteria; and the archaebacteria. These are prokaryotes. Their single cells are surrounded by a noncellulose wall and lack membranous internal organelles, and they have a single main loop of DNA with little associated protein. Most species of archaebacteria inhabit extreme environments such as deep-sea hydrothermal vents. Chemical evidence suggests that they are more closely related to eukaryotes than to the eubacteria, and some taxonomists create a separate kingdom for them. In the remaining four kingdoms, the cells of organisms are eukaryotic. The DNA is combined with proteins in chromosomes and surrounded by a double nuclear membrane; and the cells contain energy powerhouses called mitochondria and a variety of other organelles.

The Protista, which include amoeba, Paramecium, and Euglena, are primarily unicellular and aquatic. Many Protista live in marine and freshwater environments, although some live in the tissue fluids of other organisms. Their variety is immense, and the true number of protistan species is not known. Algae may be placed here or in the kingdom Plantae.

The kingdom Fungi, which includes mushrooms, yeast, and the fungi that cause athlete's foot, are characterized by cell walls of chitin and other noncellulose polysaccharides. They are tough and resist drying out. Most fungi excrete powerful enzymes to break down food into molecules that are absorbed.

The kingdom Plantae consists of many-celled organisms that live by photosynthesis. Plants are characterized by cells that are surrounded by a wall of cellulose and other polysaccharides. The cells in photosynthetic parts contain chloroplasts with light-absorbing pigments called cholorophylls. Some plants, such as mosses and liverworts, lack vascular transport tissues and cling to the ground. Vascular plants, such as ferns, conifers, and flowering plants, have tubular systems of xylem and phloem cells that transport water up from the ground and circulate nutrients dissolved in water.

The defining characteristic of all members of the kingdom Animalia is their development from a blastula, the hollow ball of cells that arises from mitosis of a fertilized egg. Animal cells have no surrounding wall and are usually organized into multicellular tissues. Most animals ingest food. The kingdom encompasses the greatest diversity of forms, including sponges, mollusks, insects, and humans.


An outcome of the Darwinian revolution is that classifications of organisms are based on their phylogeny. The creation of a phylogeny for organisms involves identification of species followed by assessment of their similarities and differences to determine probable relationship. However, similarity alone is not an adequate basis for assessing relatedness. Characteristics are said to be homologous if they are inherited through common descent, no matter what their form and use—for example, finger bones in primates and bats. Within a group, unique homologous similarities may indicate close relationship through common ancestry, as in the single pair of gnawing incisors in the upper dentition of all rodents.

Similar characteristics may indicate relationship, or they may simply reflect a primitive common heritage seen also outside the group under study. For example, all humans have backbones, but so do fish, frogs, and birds. Sometimes characteristics that appear homologous are found to have independent origins. Such homoplasy does not imply relationship. Characteristics are analogous if they serve the same function but cannot be traced back to the same feature in the common ancestor, as in the flight membranes and their supports in birds and bats. Convergence in design has led to errors in classification. For example, kangaroos and jerboas were originally united in a single taxon, although the former are marsupials and the latter placental rodents. Analogous characteristics reflect similar design solutions for the same environmental conditions, but they do not indicate common ancestry.

John H. Wahlert