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Biological Classification and Nomenclature


Classification allows us to organize data that otherwise would be overwhelming. Unfortunately, memorizing classifications is boring—but necessary. Classification groups together organisms that have things in common; knowing for example that (among other characteristics) mammals all have hair, feed their young on milk, have a single bone on each side of the lower jaws, and have three ear bones (auditory ossicles) in the middle ear means that you already know a lot about any animal if you know it's a mammal.

Taxonomy is the science of making classifications, while biological nomenclature concerns the legalistics of classification; that is, the rules involved in how organisms are named. Each major group of organisms has an international code of nomenclature that spells out the legalistics (for example, zoology has the "International Code of Zoological Nomenclature"). The purpose of such codes (and the organizations that produce them) is to promote stability within the discipline. Thus, ideally, each species is identified by a unique name shared with no other species; this is the reason biologists generally use the scientific name for an organism—it generally is recognized by all biologists as referring to a specific single organism or group of organisms. Common names, on the other hand, often are applied to more than one organism and often different common names are applied to the same organism.

The basic unit used in biology is the species. There are several concepts as to what a species is. Somewhat over-simplified, the majority of biologists use concepts that can be summarized as the biological species concept or the morphological species concept.

There are rules that apply at the level we'll be working. For example, the species name consists of two words, the first of which is the genus to which the species belongs; the second is an adjective modifying the first. Thus Homo sapiens is the species to which we belong; "homo" is Latin for man and "sapiens" for wise, and thus we're named "wise man" (modest, aren't we?). The name of a species (also, of a genus, subgenus, and subspecies) is set off from other parts of a text by putting it in a different print font (usually italics) to indicate that it is a formal scientific name. The old symbol to tell a type-setter to use the font for italics is underlining, and that still is used in hand-written material or when a writing instrument, such as an old fashioned typewriter, cannot handle italics.

Note some other points. The generic portion of the species name is always capitalized. It may be abbreviated in the species name if it has been spelled out earlier and there is not chance of confusion (thus H. sapiens if Homo sapiens has been used recently and there is no chance of confusion). In zoology, the second part of the species name is never capitalized; in botany, there are circumstances where capitalization is possible, but when it's allowable is not always easily determined; it is acceptable to not capitalize it even when allowable.

Biological classification is set up as a hierarchy, the only form of a classification intended to show genetic relationships that makes sense under evolutionary theory. In a hierarchy, units are successionally placed into more inclusive groupings; or, what really is the same thing, the whole is divided again and again into smaller and smaller groups have characteristics in common. In biology, the working from basic units to larger and larger makes the most sense because the basic and most objective unit is the species. Thus species may be grouped into genera, genera into families, etc.

The obligate taxonomic ranks. These are the ranks considered necessary (obligate) to minimally classify an organism, and are:

 Phylum (or, in plants, Division)

We commonly add to this minimal classification. For example, we often break down the phylum Chordata into several subphyla, including the subphylum Vertebrata (vertebrates, the subphylum to which we belong).

Just to give you a feel for the hierarchical setup, the following is the hierarchy of ranks as used recently in a major classification within the class Mammalia:


Most of these ranks are not necessary for anyone except specialists who need to finely dissect relationships, but some groups really need one or more ranks beyond the minimal. For example, the most common family of rodents in North America is subdivided into three subfamilies that show notably differences among them; considering them only at the family level would obscure the major differences within the family.

In plants, for our purposes, we need to focus primarily on the family, genus, and species; in animals, primarily on orders, families, genera, and species.

All of the plants we'll consider belong to the kingdom Plantae; most are in the subkingdom Tracheobionta (vascular plants); and most in the superdivision Spermatophyta (seed plants). Conifers, such as pines, junipers, spruces, and firs, go into the division Coniferophyta (conifers); most other plants we'll be concerned with are in the division Magnoliophyta (or Angiospermopsida), commonly called angiosperms or, more often, flowering plants.

Traditionally, the flowering plants have fallen into two major groups, informally given as dicot and monocots. The names come from the so-called seed leaves, the first leaves to appear in the germinating seed. These leaves are called cotyledons, thus the names. Generally, the dicots have two cotyledons appearing at the same time (di: two), whereas the monocots have one (mono: one). Other features are important. Monocot flowers usually have parts in threes or multiples of three, whereas dicots have flower parts in fours or fives or multiples of these. Generally, the veins of the leaves are parallel in monocots and reticulate (forming a network) in dicots. The vascular bundles that carry water, minerals, carbohydrates, etc., through the plant body are scattered through the stem of monocots, but occur cylindrically in dicots.

Most of the flowering plants are dicots, although it's estimated that there are about 69,000 monocots. Much of the economy of the world is based on monocots. All the true grains (wheat, maize, rice, oats, rye, etc.) are monocots (grasses actually, which belong to the Poaceae). Grasses form the basic foods of grazers such as cattle and bison. Other important monocots are the palms (Palmaceae), with about 225 genera, and the orchids (Orchidaceae) with about 20,000 species.

Among animals, there are three phyla that we will concentrate on: Chordata (in the form of subphylum Vertebrata), Mollusca (snails and clams as far as we're concerned here), and Arthropoda (subphylum Chelicerata: spiders and scorpions and relatives from our point of view; subphylum Crustacea: mostly crayfish and relatives as far as the desert is concerned); and subphylum Uniramia (millipedes, centipedes, and insects). There are a few others we'll look at briefly, but this is mostly it. Again, we'll look at lower classification when we get to the animals. If you're entirely unfamiliar with the phyla, go to the University of Michigan Animal Diversity Web site.


Last Update: 25 Jun 2006