Taxonomy: Reflecting Order in Nature
We humans are inveterate classifiers. We classify just about everything, dividing our world into a wide variety of arbitrary categories. Different people will often classify things in different ways. For example, some might put all their socks in one drawer and all their underwear in another; others might put thermal longjohns and thick wool socks together in one drawer, bikini briefs and ankle socks together in another. No classificatory system is inherently 'better' than another. The only criterion for the success of a classification system is its utility. Because different people have different needs, what 'works' for one individual may not for another. But this lack of universality doesn't dissuade each of us from working out a system that is right for our purposes. Try as we might, we cannot seem to resist grouping, organizing, and classifying everything we come across.
Taxonomy is the scientific art of classifying living things. Biological classification has two basic objectives: 1) to serve as a basis for generalization in comparative studies and, 2) to serve as an information storage system. The system of classification used by biologists today is based on a hierarchical scheme devised by Swedish naturalist Carl von Linnι (often Latinized to Linnaeus). In the 10th edition of his Systema Naturae (published in 1758), Linnι listed every type of animal known to him, organizing them into groups based on overall similarity. The Linnaean system comprises seven major categories, called taxa (singular: taxon, meaning "rank"). Arranged from the broadest, most inclusive category, to the narrowest, most exclusive category, these taxa are:
KINGDOM, PHYLUM, CLASS, ORDER, FAMILY, GENUS, SPECIES.
According to Linnaeus' system, each species is given a unique, two-part name called a binomen. Each binomen consists of a genus and species epithet. The species is the basic unit of classification and the only 'natural' one. All other taxa are arbitrary, and therefore subject to changes due to new data or interpretations. It is therefore surprising that experts differ in their definition of what, exactly, a species is. In simplest terms, a species is a population or series of populations of closely related and similar organisms. In theory, members of a given species are supposed to resemble one another more than they do members of any other species. In reality, variation within species is sometimes greater than variation between species. In sexually reproducing organisms, species is therefore often more narrowly defined by the 'biological species concept': a population or series of populations that freely interbreed with one another under 'natural conditions' to produce viable offspring (that is, offspring that are themselves capable of breeding). It is important to bear in mind, however, that members of a species are not like copies of a book, identical and unchanging over time. Variation within species provides the raw material on which natural selection operates. As a result, species as a whole are rarely stagnant - more like temporary eddies in a stream of evolutionary change.
In general, the lowest and highest ranking taxa are fairly stable. In contrast, most of the in-between taxa are in a near-constant state of flux. Species get switched from one genus to another, bumped from one family or class into another or even have new ones erected to contain only them. Such single-species taxa such as family Rhincodontidae which contains only the Whale Shark (Rhincodon typus) are termed "monotypic". Sometimes, formerly separate families or classes are "lumped" together, while at other times, single taxa get "split" into two or more. This kind of name-swapping often frustrates non-biologists, making it difficult to keep up and giving the impression that taxonomists haven't quite got their act together. But such name changes are never made capriciously or for trivial reasons. They are always the result of sound scientific reasoning based on new discoveries and how various experts interpret their meaning. One can take heart that, with the refinement of new tools such as molecular genetics, criteria for taxonomic changes are becoming more objective and better defined. Taxonomy is thus a dynamic field, continually changing to keep pace with new discoveries about species and new ideas about how they relate to one another.
The rules governing scientific names of animals are codified by an organization called the International Commission on Zoological Nomenclature, or ICZN. In most cases, these rules are pretty straight-forward. No two species can have the same binomen. Formal descriptions of proposed new species must be published in the scientific literature, available to all scholars for reference and if necessary refutation. In addition, a representative specimen (called a "type") should be deposited in the permanent collections of a museum or university, as the ultimate standard for comparison with other species. The oldest valid name usually has priority. In the case of a dispute such as if there is more than one contender for the oldest valid name of a species the ICZN makes a formal ruling on which name is to be used. Similarly, if instating a name would cause more confusion that it would clear up - as when the oldest valid name for a species has been unused and forgotten for many decades the ICZN may rule to suppress it in favor of one that has been widely used and stable during that period. The tangled taxonomic history of the Sandtiger Shark from Carcharias, to Triglochis, to Odontaspis, to Eugomphodus, and back to Carcharias provides an excellent case in point.