Linnaean Hierarchy

Scientific names are strange and rather intimidating to the layman. Why in the world would a scientist prefer to use the name Felis domes-ticus, when there is a perfectly good common name: "domestic cat"? The reason lies in the need to have a universal system of names that can be understood by all scientists regardless of their native language. In fact, the reason that all scientific names are either Latin (felis means "cat" in Latin) or latinized (frequently classical Greek) is a historical remnant of the way European scientists communicated with each other across national borders. Latin was the universal language of the Western educated world until the nineteenth century. By continuing to use Latin, scientists have adopted a "neutral language." Perhaps it is inconvenient to modern scientists who lack training in classical Latin and Greek, but at least we do not have to fight over whether Felis domesti-cus should be called cat (English), chat (French), or neko (Japanese Romanji).

Some curious features of the name for the domestic cat might strike you. Why is the name in italics, and why is one part of the name capitalized and not the other? That is another convention. The names of species (Homo sapiens) and the names of genera (Homo, Felis) are always "set apart" from the other words, and the first part of the name is always capitalized while the second part is not, even when named after a person. Why are there two words for a species? A bit of background is needed to understand this and other mysteries of scientific nomenclature.

The scientific classification of living organisms has a long history that began with Aristotle (384-322 B.C.E.) in such works as the History of Animals (350 B.C.E.). Carl Linnaeus (1707-1778), a Swedish botanist/naturalist/physician, devised the major features of the classification hierarchy that we use today. Working at first alone and then with colleagues, Linnaeus attempted to organize all known biological diversity, both plant and animal, into "natural" systems of classifica tion. He produced a groups-within-groups system, organized using more or less inclusive categorical ranks that still characterize biological classification today.

One reason for the success of Linnaeus's system was his way of naming species. Taking his clue from Aristotle, Linnaeus proposed that each species of plant or animal should be given a two-part name, called a binomial. All species belong to a genus, so the first part of the name (Felis) was formed from the name of the genus; the second part was the specific epitaph (domes-ticus). The name of the cat species became Felis domesticus, while that of the chimpanzee became Homo troglodytes (since changed to Pan troglodytes). This idea became popular, principally because it replaced the long and rather cumbersome descriptive names attached to species with a short and highly functional name. Another reason for success was the fact that Linnaeus produced a flexible and expandable system that utilized categorical ranks to organize larger and more inclusive group of species. Many species could be grouped into a genus; many genera could be grouped into a family; and so forth. What is more, these ranks could be reused. For example, there are more than 400 taxa of fishes classified at the family rank. Some of the basic categories, and an example of a group at each rank, are shown below:

Kingdom Animalia Phylum Chordata (lancets, fishes, amphibians, etc.)

Class Mammalia (mammals)

Order Primates (lemurs, monkeys, apes, humans)

Family Hominidae (great apes and humans)

Genus Homo (fossils and recent humans) Species Homo sapiens (us)

Note that each line contains the name of a group (Animalia, Primates, and so forth). Such groups are called "taxa" (singular, taxon). Second, each taxon is associated with a rank category (kingdom, order, and so on). The taxon name refers to an actual group of organisms, while the categorical rank refers to the position of the taxon in the classification relative to other taxa. So, the Order Primates has a position in the hierarchy that is equivalent to the position of other orders, such as Carnivora (bears and lions), or Coleoptera (beetles). Third, each level, from lowest to highest, contains more of these groups of organisms. Mammalia contains many orders (Ungulata for hooved mammals, Carvinova for bears and cats, and so forth). From the layperson's point of view, taxa seem to have impossible names. That is because all of the names, not just the species names, are either Latin names or latinized Greek or modern names. Sometimes we recognize the Latin root (such as "carnivore"), but usually we are stuck. Do you know what the name for the monkey family—Cerco-pithecidae—is derived from? Answer: It is a compound name derived from the classical Greek for "tail" (cerco) and "monkey" (pithe-cus), along with the ending used for families of animals (idae).

The Linnaean system provided European scholars with a flexible and powerful tool for organizing the ever-growing list of newly discovered species flowing into the universities and museums of Europe. However, it did not solve one major problem: the use of different names for the same taxon in different countries or by different scientists. A rose by any other name might smell as sweet, but how could a botanist in Sweden and another in England effectively communicate, if the same rose species had two different names? Beginning with A. P. Candolle in 1813 and extending to H. E. Strickland in 1843, taxonomists attempted to formulate rules that would govern the use of names internationally. This process evolved over the years to become the three major Codes of Nomenclature that now govern the names of plants, animals, and bacteria (there are other codes for viruses and cultivated plants).

The codes are an effort to formalize and standardize the naming of organisms in a manner that would ensure uniform use of names across international boundaries, languages, and cultures. The codes are simply sets of rules for naming and the use of names. As such, they do not speak to particular philosophies of classification, such as classifying strictly according to descent with modification, or exclusively by overall similarity, or even classifying by fives. They explicitly avoid setting rules for the actual practice of science, and that is why they have survived. In other words, the codes do not specify what you must name, only how you must form the name if you wish to name something, and what names you should use once they are formulated.

The general principles of Linnaean classification as embodied in the current Rules of Nomenclature are relatively simple, even if the practice strikes some seasoned scientists as arcane. Each specifies a beginning for the nomenclature of the group, and each is independent of the others. For example, all zoological nomenclature begins with the publication of Linnaeus's tenth edition of Systema Naturae, considered published on January 1, 1758. Earlier names (even those in Linnaeus's earlier editions of the Systema) are considered invalid. The rules differ among the three major groups of organisms, but all share some general principles.

1. Forming names. All taxa are given Latin or latinized names, a uninomial name for taxa ranked as genera and above and a binomial for species. Specific rules may also apply to taxa or certain ranks. For example, the names of taxa ranked as families in botanical classifications must end with the suffix "aceae." Thus the taxon Rosaceae requires no categorical rank to show its position in the hierarchy; it is immediately identifiable as a taxon ranked as a family, the Linnaean family of roses.

2. Taxa assigned to certain categories can have only one correct name, and that name is usually the earliest name applied to the taxon. The use of the earliest names is termed the Principle of Priority. For example, Homo sapiens is the only correct name for humans, and we use that name because it appears in the tenth edition of the Systema and thus is the oldest name available for our species. You can give humans another name if you wish, but no one will use it if they follow the rules. The Principle of Priority can be set aside, but only by submitting a petition to the appropriate International Commission of Nomenclature and only upon acceptance of the petition.

3. Two taxa cannot have the same name unless they are governed by different codes. It is possible for a plant species and an animal species to have the same name, but not for two plants species or two animal species. Two names for the same species of animals would be termed synonyms, and the older named is usually used.

4. Certain taxa are "objectively defined" in reference to actual specimens. For example, every new species named today is objectively defined in reference to a type specimen or series of type specimens. (In some cases it isn't a specimen of the actual organism, but some "work" done by a specimen, such as a fossil track or burrow made by an extinct worm.) These specimens are usually deposited in natural history museums where other scholars can examine them. Type specimens are not meant to be "typical" of the species. Rather, they document that the original systematist actually examined that particular specimen as he or she was formulating the name. Sometimes, especially in early works, a type specimen was not designated. It is common for a systematist who later studies the group to name a type specimen in such cases.

Each of the various codes goes on for pages, outlining how names should be formed, what names are to be used, and how names should be changed. The appropriate international commission, a body of scholars who can draft revisions and pass judgment on problems that arise, regularly reviews each code. Systematists who regularly name new species and other taxa are expected to know these rules. Reputable journals publish the names of new taxa only if the applicable code is followed. Most of this scholarship is uninteresting to those who use the names, but it is a necessary part of systematic scholarship and minimizes chaos.

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