Classification, Scientific, is the method scientists have developed to arrange all of the world’s organisms into groups. Scientists include extinct organisms as well as living things in this arrangement. 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. It also reflects how living things have developed and changed over time.
The classification of organisms is a science called taxonomy or systematics. Scientists called taxonomists or systematists update and refine classification. They base their decisions on the work and opinion of biologists.
Most taxonomists use the same basic framework for classification, but not all agree on how different groups of organisms fit into this framework. Thus, classification schemes commonly differ in details.
Scientists often learn new facts that require them to make changes to classification. Since the mid-1900’s, biologists have gathered much of this information 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 cladistics, researchers 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 language of classification
has always used Latin and Greek words. Early Western scholars gave organisms Latin and Greek names, and later scientists have kept them because their definitions do not change over time. Every known organism belongs to a particular species. Each species has a two-word scientific name known as a binomial, which should appear in italics. The first word is the genus name, and the second is the epithet, an adjective modifying the genus name. Scientists call this method of naming organisms the binomial system of nomenclature or binomial nomenclature.
People in different regions of the world refer to organisms by different common names. Yet each organism has only one correct scientific name. Scientists anywhere can recognize the organism by this name. 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, Puma concolor. Scientists can identify the cat by that name no matter what their language.
Many binomials consist of Greek or Latin terms describing the organism. For example, the scientific name of the spotted skunk, Spilogale putorius, means smelly, spotted weasel. Other binomials are derived from the name of a person or from a place where the organism is found. The little twinflower’s scientific name, Linnaea borealis, honors the Swedish naturalist Carolus Linnaeus. It also indicates that the flower comes from boreal (northern) forests.
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.
Groups in classification.
Eight groups called taxa (singular, taxon) make up the basic system in scientific classification. Additional taxa exist in some taxonomic schemes. The primary groups are (1) domain, (2) kingdom, (3) phylum or division, (4) class, (5) order, (6) family, (7) genus, and (8) species. Domain ranks as the highest level and largest group, while species is the lowest and smallest. 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.
Domain
represents a recent addition to scientific classification, being formally proposed only in 1990. Most taxonomists break up life into three domains: (1) Archaea, (2) Bacteria, and (3) Eukaryota, also called Eucarya or Eukarya. Archaea and Bacteria have prokaryotic cells, which lack a nucleus. Animals, plants, and all other living things have eukaryotic cells that include a nucleus.
Kingdom
formerly ranked as the highest level taxon in biological classification. From the 1970’s to the 1990’s, most scientific textbooks used a classification system with five kingdoms—those containing prokaryotes, protists, fungi, plants, and animals. However, scientists realized that the prokaryote kingdom actually consists of two different kinds of microbes. This realization led to the splitting of prokaryotes into two kingdoms: Archaea and Bacteria. Each of these two kingdoms makes up a domain of the same name. All other kingdoms belong to the domain Eukaryota.
The kingdom Archaea includes organisms that inhabit harsh environments. Some even live off sulfur emitted from deep-sea hydrothermal vents. Some scientists split the archaea into several kingdoms. The kingdom Bacteria consists of all bacteria and many other soil and water microbes. Some scientists divide the bacteria into as many as 15 different kingdoms. 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.
Phylum,
or division, 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.
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. Birds make up the class Aves. Feathers cover the bodies of all birds, and none of them feed milk to their young.
Order
consists of groups that are more alike than those in a class. Dogs, manatees, and raccoons are all mammals. But dogs and raccoons have well-developed canine teeth and eat flesh. They are grouped with similar animals in the order Carnivora. Manatees have only molars and eat mostly water plants. They are grouped with the dugong in order Sirenia.
Family
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.
Genus
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.
Species
represents the basic unit of scientific classification. Members of a species have many common characteristics, but they differ from all other forms of life in one or more ways. Species members can breed with one another, and the young grow up to look much like the parents. For many species, however, 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. Sometimes groups within a species differ so much from other groups in the species that they are called subspecies or varieties.
Development of classification.
People around the world have attempted to classify living things for thousands of years. In the 300’s B.C., the Greek philosopher and naturalist Aristotle made one of the first attempts to record information on classification. Only about 1,000 organisms were known in his time. Aristotle classified animals as those with red blood—animals with backbones—and those with no red blood—animals without backbones. He divided plants by size and appearance as herbs, shrubs, or trees. Aristotle’s scheme served as a basis of classification for almost 2,000 years.
Beginning in the late 1400’s, explorers and scientists began to travel more widely. People discovered new species around the world, and naturalists developed new classification schemes. The most famous of these naturalists, Carolus Linnaeus, lived during the 1700’s. Like modern taxonomists, Linnaeus classified organisms into taxa and used two-word names for each species. However, he often based his higher groupings on superficial physical resemblances, and later scientists revised many of his groupings.
Some scientists are developing an alternative set of rules for naming organisms called the PhyloCode. This new code would eliminate all taxa above species level. Instead, the naming of a new group would include information about where the group fits into the overall phylogeny (evolutionary history). Many biologists, however, are critical of this system.
