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Classification of Animals

To identify animals and learn more about them, it helps to become familiar with the way they are classified. Biologists arrange animals into groups on the basis of traits which they share with other animals and their genetic relationships with each other. This orderly way of classifying animals forms the basis of the field of study called taxonomy. Modern scientific taxonomy is based on physical characteristics (such as teeth, skin, fur, feather, or scale patterns, size, or the structure of body parts) and on genetic characteristics. Some key characteristics are basic to taxonomic descriptions. Others are not part of the basic description, but correspond to evolutionary relationships upon which taxonomic classifications are based.

The field of study called systematics focuses specifically on the evolutionary relationships between living organisms. A Swedish scientist named Carolus Linnaeus laid the foundation of modern systematics with a work called Systema Naturae, which he published in 1758. Linnaeus wrote in Latin, the international language at the time, and Latin continues to be the basis of most scientific names. Sometimes the names of the various taxonomic categories are converted into forms that are more comfortable to everyday English; for example, instead of Mammalia we usually just say mammal.

Linnaeus designed his system of classification so that each animal and plant that he described had one and only one correct name and this name would not be shared with any other organism. Then he presented a method for organizing all these named organisms into a series of nested groups, based on their similarities and differences. In essence, it became a type of filing system, with the top levels including many, many different kinds of organisms and the lowest levels containing but a single type of plant or animal. This hierarchical Linnaean system uses clearly defined shared characteristics to classify organisms into each group represented by these different levels.

The most important categories in this hierarchical system, from higher and more inclusive to lower and more specific, are kingdom, phylum, class, order, family, genus, and species. A kingdom is one of the highest primary divisions into which all objects are placed. All animals are part of the Animal Kingdom. Each kingdom is divided into smaller units called phyla (the plural form of phylum). For example, animals that have a nerve cord are classified as members of the Phylum Chordata. The chordates are further divided into classes such as Mammalia, Aves, Reptilia, and Amphibia. Members of each class have characteristics which they share with other members of their class, but which generally are not found in members of the other classes. Classes are divided into families. Families are subdivided into genera (the plural form of genus); and genera are subdivided into species. A family usually contains more than one genus, and each genus usually includes more than one species. Animals that share the same genus are very similar and probably evolved from a common ancestor. The species is the most fundamental unit and contains a single type of animal.

For an example of how this works, consider the taxonomy of the domestic dog. Dogs are classified as members of the Class Mammalia, Order Carnivore, Family Canidae, Genus Canis and Species familiaris. Usually when we ask for the name of a "species," we really mean we want to know the genus and species of that animal's scientific name.

As more is learned about a species, its classification may change. In order to manage changes in scientific names, there is an international committee which approves each proposed name change. They follow a set of rules outlined in the International Code of Zoological Nomenclature. No single classification is final, because additional studies may show new relationships among animals that were not clear using previous evidence. Scientific taxonomy is dynamic. Although some groups of animals are more subject to changes than others, new discoveries can lead to changes even among the best-known groups of animals.

To learn more about the animals included here, you may wish to consult the listed references about an animal. Four general types of references are included: higher taxonomies with world-wide coverage; regional or state checklists; regional handbooks and identification guides; and original taxonomic descriptions and revisions.

Higher taxonomies organize major taxonomic units in phylogenetic order, with the most primitive members of the group first and the most advanced last. Many such publications include a description of the distinguishing characteristics of the higher categories. Higher taxonomies provide guidance for arranging families and species in scientific publications and museum collections. Within each class, organisms are usually arranged in phylogenetic order. Classes are typically arranged either in ascending or descending hierarchical order; i.e. from mammals to amphibians or the reverse.

Regional checklists include species of a taxonomic group, such as a class or phylum, usually listed in phylogenetic order. Some of these are simply lists of scientific names with authors and dates and the major locations where the organisms occur, such as Atlantic Ocean, freshwater, or terrestrial. Others include a full taxonomic history and range for each species, but do not include common names or descriptions.

Regional handbooks, biological surveys, and identification guides usually are arranged in taxonomic order and describe characteristics by which each species can be identified as well as biological characteristics, where and when the animal can be found, and other important information. As you make use of these references you will see that in many cases the names have changed.

Scientific names represent biological relationships and are assigned to animals after careful study of each species. Many animals also have common names. Common names reveal what people think about animals and their ideas about how animals are related to each other. They also tell us about the importance of animals in our daily lives. Some common names match scientific taxonomy very closely. Other common names divide animals into more groups than scientists do, especially when the animal is very familiar to us or important in our lives. For example, there are many different names for breeds of household pets such as dogs, even though scientific taxonomy recognizes only one domestic dog species, Canis familiaris.

Other animals are lumped together using similar common names in spite of very different biological histories. For example, many people use a single common name to refer to very different snakes and lizards because they do not know much about them. In Georgia, the name "gopher" refers to both a mammal and a turtle, even though it is clear that the Pocket Gopher is very different from the Gopher Tortoise . To make things even more complicated, the Pocket Gopher is sometimes called a "salamander," perhaps because of the sand mounds it creates. Thus, using only common names, the Pocket Gopher can be confused with a turtle as well as with the amphibians known as salamanders (Order Caudata).

Despite the possibility for confusion, common names are a widely recognized way of referring to animals. However, they lack the universal recognition needed for accurate identifications and scientific research. To reduce this confusion, lists of scientific and common names and field guides such as those sponsored by The Audubon Society or published in the Peterson Field Guides series include scientific names to indicate the animal's scientific classification as well as common names.