Taxonomy - How it works



Taxonomy in Context

The term taxonomy is actually just one of several related words describing various aspects of classification in the biological sciences. In keeping with the spirit of order and intellectual tidiness that governs all efforts to classify, let us start with the most general concept, which happens to be classification itself. Classification is a very broad term, with applications far beyond the biological sciences, that simply refers to the act of systematically arranging ideas or objects into categories according to specific criteria.

While its meaning is narrower than that of classification, even taxonomy still has broader applications than the way in which it is used in the biological sciences. In a general sense, taxonomy refers to the study of classification or to methods of classification—for example, "political taxonomy," as we used it in the introduction to this essay. Literary critics sometimes refer to a writer's taxonomy of characters. Within the biological sciences, however, the term designates specifically a subdiscipline involving the process and study of the identification, naming, and classification of organisms according to apparent common characteristics.

PHYLOGENY AND NOMENCLATURE.

Two other terms that one is likely to run across in the study of taxonomy are phylogeny and nomenclature. Phylogeny is the evolutionary history of organisms, particularly as that history refers to the relationships between life-forms and the broad lines of descent that unite them. Taxonomy is less fundamental a concept than phylogeny. Whereas taxonomy is a human effort to give order to all the data, phylogeny is the true evolutionary relationship between living organisms. Some scientists call phylogeny the tree of life, meaning that it represents the underlying hierarchical structure by which life-forms evolved and are related to one another.

The word naming was used earlier in the definition of taxonomy because it is a familiar, easily understandable word. However, a more accurate term, and one that helps illuminate the distinction between taxonomy and systematics, is

THE GREEK PHILOSOPHER ARISTOTLE IS REGARDED AS THE FATHER OF TAXONOMY. THE ARISTOTELIAN PRINCIPLES OF CLASSIFICATION WERE GOVERNED BY THE IDEA THAT THERE ARE CONSTANT, UNCHANGING "ESSENCES" THAT UNITE CLASSES OF ORGANISMS, WHICH IS COMPLETELY AT ODDS WITH THE EMPIRICAL MENTALITY THAT GOVERNS TAXONOMY TODAY. (The Library of Congress.)
T HE G REEK PHILOSOPHER A RISTOTLE IS REGARDED AS THE FATHER OF TAXONOMY . T HE A RISTOTELIAN PRINCIPLES OF CLASSIFICATION WERE GOVERNED BY THE IDEA THAT THERE ARE CONSTANT , UNCHANGING " ESSENCES " THAT UNITE CLASSES OF ORGANISMS , WHICH IS COMPLETELY AT ODDS WITH THE EMPIRICAL MENTALITY THAT GOVERNS TAXONOMY TODAY . (
The Library of Congress
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nomenclature. The latter can be defined as the act or process of naming as a well as a system of names, particularly one used in a specific science or discipline.

Homologous and Analogous Features

Before going on to discuss methods of classification, it is important to note just which characteristics of an organism's morphological aspect (i.e., structure or form) are important to scientists working in the field of taxonomy. In theorizing relationships between species, taxonomists are not interested in what are known as analogous features, those characteristics that are superficially similar but not as a result of any common evolutionary origin. Rather, they are interested in homologous features, or features that have a common evolutionary origin, even though they may differ in terms of morphological form.

One example of a shared evolutionary characteristic, discussed briefly in the essay Evolution, is the pentadactyl limb, a five-digit appendage common to mammals and found, in modified form, among birds. This is a homologous feature, indicating a common ancestor that likewise had limbs with five digits at the end. By contrast, there is no indication of a close evolutionary relationship in the fact that birds, butterflies, and bats all have wings that are similar in shape. Rather, the laws of physics require that a wing be of a certain shape in order to hold an object aloft, which is why the contour of an airplane wing, when viewed from the side, is remarkably like that of a bird's wing where it joins the animal's body.

Cladistics and Numerical Taxonomy

Cladistics is a system of taxonomy that distinguishes taxonomic groups or entities on the basis of shared derived characteristics, hypothesizing evolutionary relationships to arrange them in a tree like, branching hierarchy. The expression derived characteristics in this definition means that the characteristics that unite two types of organism are not necessarily present in a shared evolutionary ancestor. Rather, they have developed over the course of evolutionary history since the time of that shared ancestor.

In explaining cladistics to the ordinary human being, the vast majority of science writers seem to be at a loss as to how to make the topic comprehensible. Thus, such terms as derived characteristics and its opposite, primitive characteristics, usually are left undefined. A welcome exception is Paul Willis, who, in an on-line article for the Australian Broadcasting Corporation (see Where to Learn More) gave a wonderful illustration that was an attempt to analyze the relationships between a mouse, a lizard, and a fish.

"They've all got backbones," Willis wrote," so the feature 'backbone' is useless [as an indication of evolutionary branching]; it's a 'primitive' character that tells you nothing. But the [derived] feature 'four legs' is useful because it's an evolutionary novelty shared only between the lizard and the mouse. This implies that the lizard and mouse are more closely related to each other than either is to the fish. Put another way, the lizard and the mouse share a common ancestor that had four legs." Willis went on to note that "the more evolutionary novelties we can find that support a particular relationship, the greater our confidence that the relationship is correct. 'Air breathing,' 'neck' and 'amniotic egg' are another three evolutionary novelties that tie the lizard and the mouse together and leave the fish as a more distant relative."

NUMERICAL TAXONOMY.

Cladistics, the most widely applied approach to taxonomy, has undergone considerable change since it was introduced by the German zoologist Willi Hennig (1913-1976) in the 1950s. Particularly important has been the marriage of cladistics with another taxonomic idea born in the mid-twentieth century, phenetics, or numerical taxonomy. Introduced by the Austrian biologist Robert Reuven Sokal (1926-) and the English microbiologist Peter Henry Andrews Sneath (1923-), numerical taxonomy is an approach in which specific morphological characteristics of an organism are measured and assigned numerical value, so that similarities between taxa (taxonomic groups or entities) can be compared mathematically. These mathematical comparisons are performed through the use of algorithms, or specific step-by-step mathematical procedures for computing the answer to a particular problem. The aim of numerical taxonomy is to remove all subjectivity (such as the taxonomist's "intuition") from the process of classification. Initially, many traditional taxonomists rejected numerical taxonomy, because its results sometimes contradicted their own decades-long studies of comparative morphological features. Nearly all modern taxonomists apply numerical methods in taxonomy, although there is often heated debate as to which particular algorithms should be used.

Identification, Classification, and Nomenclature

Earlier, taxonomy was defined in terms of its relationship to the identification, classification, and nomenclature of taxa. Let us now briefly consider each in turn, with the understanding that they are exceedingly complex, technical subjects that can be treated here in the most cursory fashion. The process of identification is a particularly complex one. When an apparently new taxon is discovered, a taxonomist prepares an organized written description of the characteristics of similar species, which are referred to as a taxonomic key. Instead of using pictures, which often poorly convey the natural variations in morphological features, taxonomists prefer to use a taxonomic key in written form, which provides much more detail and exactitude.

To put it in colloquial terms, by referring to a taxonomic key, a taxonomist may determine that if an organism "looks like a duck and quacks like a duck, it must be a duck"—only, in this instance, the taxa being compared are much more specific than the common term duck and the characteristics much more precisely described. (For one thing, there are several dozen species in the genus Anas, which includes all "proper" ducks, and many more species in the family Anatidae, or waterfowl, that are commonly called by "duck names"—including such amusingly named species as the ruddy duck, lack duck, freckled duck, and comb duck.) If there is no already established "duck" that the species in question resembles, the taxonomist may have discovered an entirely new genus, family, order, class, or even phylum.

A taxonomist may use what is called a dichotomous key, which presents series of alternatives much like a flow chart. For example, if the flowers of a sample in question are white and the stem is woody, then (depending on additional alternatives) it could be either species A or species B. If the flowers are not white and the stem is herbaceous (non-woody), then, presented with another set of additional alternatives, it is possible that the plant is either species C or species D.

CLASSIFICATION.

In discussing cladistics and phenetics, we touched briefly on the process of classification. Suffice it to say that this process is far more complex and technically elaborate than these few paragraphs can begin to suggest. We return later to specifics of classification as they relate to systems and innovations introduced by the Greek philosopher Aristotle (384-322 B.C. ), the Swedish botanist Carolus Linnaeus (1707-1778), and the English naturalist Charles Darwin (1809-1882), the three most important men in the history of taxonomy before the twentieth century. For the present, our focus is on the overall ranking system.

There are many possible ranks of classification but only seven that are part of what is known as the obligatory taxonomy, or obligatory hierarchy. These ranks are kingdom, phylum, class, order, family, genus, and species. Listed here are all possible ranks, with obligatory ranks in italics.

  • Kingdom
  • Subkingdom
  • Phylum
  • Subphylum
  • Superclass
  • Class
  • Subclass
  • Infraclass
  • Cohort
  • Superorder
  • Order
  • Suborder
  • Superfamily
  • Family
  • Subfamily
  • Tribe
  • Genus
  • Subgenus
  • Species
  • Subspecies

The reader occasionally may come across nonobligatory ranks, most notably subphylum, but for the most part the only ranks referred to in this book are the obligatory ones.

NOMENCLATURE.

In accordance with a tradition established by Linnaeus, all group names are in Latin, thus facilitating ease of communication. There are some rules concerning names of groups: for instance, those of families use the suffix- idae. In the world of taxonomy, however, few rules are accepted universally. Even as basic a term as phylum is not universal, since botanists prefer the word division.

The proper name of any ranking more general than species is capitalized (e.g., phylum Chordata), with species and subspecies names in lowercase. Genus, species, and subspecies names are rendered in italics (e.g., Homo sapiens, or "man the wise"), while proper names of the more general groupings are presented in ordinary type (e.g., class Mammalia). If the same name appears a second time in the same article, the genus name usually is abbreviated: thus, H. sapiens.

Just as most people (with such rare exceptions as Cher and Madonna) are identified by two names, a personal and a family name, taxonomy makes use of a system called binomial nomenclature, in which each type of plant or animal is given a two-word name, with the first name identifying the genus and the second the species. In binomial nomenclature, the genus name is analogous to the family name, inasmuch as there are many species within a genus, and the species name is like a personal name. The difference is that whereas there may be thousands of boys and men named John Smith, there is only one species called Homo sapiens. Beyond the species name, there may be subspecies names: humans are subspecies sapiens, so our full species name with subspecies is Homo sapiens sapiens. Additional rules govern the inclusion of a name or an abbreviation, at the end of the species or subspecies name, to recognize the individual who first identified it.



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