Biological classification

Biological classification is the arrangement of organisms into categories that express their PHYLOGENY, or line of descent, based on information such as structure, development, biochemical or physiological functions, and evolutionary history of organisms. The purpose of such a classification is to provide a clear and practical way to organize and communicate information about organisms. Classification can show relationships between different ancient and modern groups, indicate the evolutionary pathways along which present-day organisms may have developed, and provide a basis for comparing experimental data about different plant and animal groups. Organisms included in a group 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 groups 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, and microorganisms into a single, completely satisfactory scheme. For example, some taxonomists classify algae with the protista or consider them plants. Recently, biotechnological techniques have enabled researchers to compare the DNA of various organisms to decipher the phylogeny of some organisms and helped to distinguish some closely related species with similar appearance.

HISTORY

ARISTOTLE (384-322 BC) is often called the father of biological classification. His classification scheme referred to readily apparent groups, such as birds, fishes, whales, and bats, and he recognized the need for groups and group names in the study of the animal kingdom. John RAY (1627-1705) used anatomical differences as the prime criterion for classification, bringing out both the resemblances and 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 (1707-1778). He applied it consistently to plants in Species Plantarum (1753), and to animals in Systema Naturae (10th ed., 1757). Linnaeus' system was readily applicable to the new concept of evolution of Charles DARWIN, 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.

LINNAEAN SYSTEM

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 the 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 groups of organisms for the complex 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 monkeys are most nearly like humans, and as a logical consequence of strictly biological classification, humans would be grouped not only in the class Mammalia but in the same ordinal division with the monkeys and apes.

HIGHER GROUPINGS

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 usually divided into many local populations, with limited interbreeding occurring among members of different local populations to maintain genetic continuity among the species. The genetic differences between species may be differences in anatomy, behavior, ecology, physiology, and cellular chemistry. 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. No rank above the species level can be defined in absolute terms.

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 that follows this article, a classification of modern humans, illustrates the major categories and some subdivisions.

MAJOR KINGDOMS

Originally, organisms were divided between two kingdoms, the Plantae (including bacteria, fungi, and algae) and the Animalia (including protozoa). The wealth of new data generated by the new technologies of molecular biology and electron microscopy led to the five-kingdom system proposed by R. H. Whittaker in the 1950s. In Whittaker's system, organisms are classified according to whether they are prokaryotic--single-celled, like bacteria, with neither internal membranes nor organelles--or eukaryotic--composed of one or more cells containing membrane- bound nuclei and organelles. The five kingdoms are Monera, Fungi, Protista, Plantae, and Animalia.

The Monera include the various kinds of bacteria and the photosynthetic cyanobacteria. These are prokaryotes; their single cells are surrounded by a noncellulose wall and lack membranous internal organelles. 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. The Protista, which include Amoeba, Paramecium, and Euglena, are primarily unicellular and aquatic. Most 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 protista species is not known. Algae may be placed here or in the kingdome 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. Most fungi excrete powerful enzymes to break down food into molecules that are absorbed. They are tough and resist drying out.

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 chlorophylls. 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 Animalia kingdom 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.

HOMOLOGY AND ANALOGY

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.

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 wings of birds and bats. 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. Similarities may also indicate anatomical or physiological convergence through adaptation to the same environment. For this reason, kangaroos and jerboas were originally united in a single taxon, though the former are marsupials and the latter placental rodents.

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