Taxonomy of Life
The taxonomic organization of species is hierarchical. Each species belongs to a genus, each genus belongs to a family, and so on through order, class, phylum, and kingdom. Associations within the hierarchy reflect evolutionary relationships, which are deduced typically from morphological and physiological similarities between species. So, for example, species in the same genus are more closely related and more alike than species that are in different genera within the same family. Carolus Linnaeus, an 18th-century Swedish botanist, devised the system of binomial nomenclature used for naming species. In this system, each species is given a two-part Latin name, formed by appending a specific epithet to the genus name. By convention, the genus name is capitalized, and both the genus name and specific epithet are italicized, for Canis familiaris or simply C. familiaris. Modern taxonomy is currently in flux, and certain aspects of classification are being refined. This table shows one traditional classification of five species of life out of the estimated five million species of the world.
|Sugar Maple||Plantae(plants)||Magnoliophyta||Rosidae||Sapindales||Aceraceae||Acer||A. saccharum|
|Bread Mold||Fungi(fungi)||Zygomycota||Zygomycetes||Mucoralis||Mucoraceae||Rhizopus||R. stolonifer|
|Pond Alga||Protista(algae,diatoms)||Chlorophyta||Euconjugatae||Zygnematalis||Zygnemataceae||Spirogyra||S. crassa|
Copyright © 2000 by Houghton Mifflin Company
Taxonomy is the area of the biological sciences devoted to the identification, naming, and classification of living things according to apparent common characteristics. It is far from a simple subject, particularly owing to many disputes over the rules for classifying plants and animals. In terms of real-life application, taxonomy, on the one hand, is related to the entire world of life on Earth, but on the other hand, it might seem an ivory-tower discipline that it has nothing to do with the lives of ordinary people. Nonetheless, to understand the very science of life, which is biology, it is essential to understand taxonomy. Each discipline has its own form of taxonomy: people cannot really grasp politics, for instance, without knowing such basics of political classification as the difference between a dictatorship and a democracy or a representative government and one with an absolute ruler. In the biological sciences, before one can begin to appreciate the many varieties of organisms on Earth, it is essential to comprehend the fundamental ideas about how those organisms are related—or, in areas of dispute, may be related—to one another.
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 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."
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.
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.
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.
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.
The Urge to Classify
One might ask what all the fuss is about. Why is classification so important? We attempt to answer that question from a few angles, including a brief look at the lengthy historical quest to develop a workable taxonomic system. But what was the original impulse that motivated that quest? One clue can be found in the Greek roots of the word taxonomy: taxis, or "arrangement," and nomos, or "law." The search for a taxonomic system represents humankind's desire to make order out of the complexities with which nature presents us. When it comes to the organization of ideas (including ideas about the varieties of life-forms), this desire for order is more than a mere preference. It is a necessity.
The Analogy to a Library
Imagine a library without any organizational system, with books simply crammed willy-nilly on the shelves. Such a place would be totally chaotic, and if one happened to find a book one was looking for, it would be a case of pure luck. The odds would be weighted heavily against such luck, especially in a university library or a large municipal or regional one. Just as a good-size university library has upward of a million volumes, and many large university libraries have several million, so there are at least a couple of million identified species, and the total may be much larger. Some entomologists (scientists who study insects) speculate that there may be ten million species of insect alone.
The Lure of a New Species
When a zoologist or botanist discovers what he or she believes to be a new species, the taxonomic system provides a standard against which to check it—rather as you would do if you thought you had discovered a book that was not in the library. If the "new" species matches an established one, that may be the end of the story—unless the scientist has discovered a new aspect of the species or a new subspecies. And if there is no match in the taxonomic "library," the scientist has discovered an entirely new life-form, with all the grand and terrifying ramifications that may ensue.
The new species might be an herb from which a cure can be synthesized for a devastating disease, or it could be a parasite that carries a new and previously unknown malady. Whatever it is, it is better to know about it than not to know, and though the vast majority of "new" species are not nearly as exciting as the preceding paragraph would imply, each has its part to play in the overall balance of life. Discovery of new species is particularly important when those species are endangered or might be in the process of disappearing even as they are identified.
Without knowing anything about scientific taxonomy, almost anyone can begin to classify animals and perhaps plants. If we limit the discussion purely to animals, there are many basic parameters according to which we could classify them, just off the tops of our heads, as it were. For example, there are aquatic and terrestrial animals, and these general groupings can be broken down further according to biome or habitat (see Biomes). There are animals that walk, fly, swim, slither, or move by some other means. Animals can be divided according to their forms of reproduction, whether asexual or sexual, oviparous or viviparous (expelling or retaining a fertilized egg, respectively), and so on. As discussed in Food Webs, animals may be classified as herbivores, carnivores, omnivores, or detritivores or as primary, secondary, or tertiary consumers. They may be endothermic or ectothermic (warm-blooded or cold-blooded), and they may be covered with scales, feathers, fur, or skin. (In the last case, that skin may be protected by either mucus or hair.)
On and on go the categories, and if one is inclined toward a classifying mind, this kind of mental exercise can be fun. Certainly, little children enjoy it, and many educational programs and games call on the child to group animals thus. Although these kinds of groupings, and the efforts to place animals into one group or another, constitute a form of classification, there is a great difference between this and scientific taxonomy.
Science Versus "common Sense."
Taxonomy is tied closely to evolutionary study, and Darwin's theory of evolution was a turning point in the history of scientific classification. Thus, taxonomists are concerned more with the evolutionary patterns that link organisms than they are with what may be only superficial similarities. Habitat, for instance, is significant in studying biomes, but it seldom plays a role in taxonomy. Nor is the ability to fly, as we have noted, necessarily an indicator of taxonomic similarities.
A striking example of the difference between scientific taxonomy and "common sense" classification is the fact that whales and dolphins are grouped along with other mammals (class Mammalia) rather than with fish and other creatures that most readily come to mind when thinking of aquatic organisms. In fact, whales and dolphins share not only a wide array of primitive characteristics with mammals (for example, the pentadactyl limb described earlier) but also the derived characteristic that defines mammal : the secreting of milk from mammary glands, by which a mother feeds her young. Not only is it impossible to get milk from a fish (even family Chanidae, known by the common name "milkfish"), but fish lack even that primitive characteristic, the pentadactyl limb, that links mammals, at least distantly, with nonmammalian creatures, such as birds (class Aves).
Common Terms and Folk Taxonomy
For the sake of convenience, in many places throughout this book, common terms such as bird, horse, fish, and so forth are used. But common terms are far from adequate in a scientific context, because such terminology can be deceptive, as exemplified by the nonduck "ducks" mentioned earlier. Likewise, shellfish and starfish are not "fish" as that term is usually understood. But while common terminology can be misleading, sometimes correlations with scientific taxonomy can be found in what is known as folk taxonomy. The latter is a term for the taxonomic systems applied in relatively isolated non-Western societies. For example, the folk taxonomy of native peoples in New Guinea identified 136 bird species in the mountains of that island, a figure that came amazingly close to the 137 species identified by the German-born American evolutionary biologist Ernst Mayr (1904-) when he studied New Guinea's birds using scientific methods.
Aristotle, Linnaeus, Darwin, and Beyond
Among his many other accomplishments as a thinker, Aristotle is regarded as the father of the biological sciences and of taxonomy. Among the dominant ideas in his work as a philosopher are the concepts of hierarchy and classification, and thus he took readily to the idea of classifying things. At his school in Athens, he put his students to work on all sorts of taxonomic pursuits, from listing the champions at the Pythian Games (a festival like the Olympics) to classifying the constitutions of various Greek city-states to analyzing the body parts of animals. Aristotle himself dissected hundreds of animals to understand what made them tick, and he proved to be some 2,000 years ahead of his time in recognizing that the dolphin is a mammal and not a fish. His system of classification, however, was a far cry from the ideas that developed in nineteenth-century taxonomy; rather than searching for evolutionary lines of descent, he ranked animals in order of their physical complexity.
In most aspects of his other work, Aristotle established sharp distinctions between his own ideas and those of his teacher, Plato (427?-347 B.C.). For example, Aristotle rejected Plato's position that every idea we can conceive is but a dim reflection of an essential concept—for example, that our idea of "red" is only a shadowy copy of the perfect notion of "redness." Yet in his taxonomy, Aristotle seemed to hark back to his days as Plato's star pupil. The Aristotelian principles of classification were governed by the idea that there are constant, unchanging "essences" that unite classes of organisms. This idea of essences is completely at odds with the empirical (experience-based) mentality that governs taxonomy today. Nonetheless, for two millennia, Aristotelian ideas represented the cutting edge in taxonomy and much else.
The Middle Ages and Renaissance
After Aristotle and his brilliant student Theophrastus (371?-287? B.C.), the father of botany, there would be no Western biological theorists of remotely comparable stature until the time of the Renaissance. In the meantime, taxonomy, as with so many other areas of learning in Europe, declined badly. During the Middle Ages, what passed for taxonomic writings consisted primarily of bestiaries, books full of fanciful and imaginary creatures, such as the unicorn. The first signs of scientific reawakening in the biological sciences in general, and taxonomy in particular, came with plant and animal catalogues by such great medieval scholars as Peter Abelard (1079-1142) and Albertus Magnus (ca. 1200-1280). Even so, their work consisted primarily of summations of existing Aristotelian knowledge rather than new contributions.
In the sixteenth century, the Swiss scientist Konrad von Gessner (1516-1565) wrote Historia animalium (1551-1558), a groundbreaking work that included descriptions of many animals never before seen by most Europeans. Gesner also denounced the practice of including fictitious animals in bestiaries. Around the same time, the discoveries of new plant and animal species in the New World began to point up the need for a taxonomy that went beyond Aristotle's. The first scholar of the modern era to attack this problem was the Italian botanist Andrea Cesalpino (1519-1603), but nearly two centuries would pass before the development of a workable classification system.
Linnaeus and Others
The man who revolutionized taxonomy was born Carl von Linné but adopted the Latinized name Carolus Linnaeus. Even that late in scientific history, scholars still wrote chiefly in Latin, not because they were trying to adhere to tradition but because it remained a common language between educated people of different countries. Thus, Linnaeus's great work, which he first published in 1737 but revised numerous times, was named Systema naturae, or "The Natural System." Thanks to Linnaeus, Latin became enshrined permanently as the language of taxonomy the world over, but this was far from his only accomplishment.
It was Linnaeus who introduced binomial nomenclature, in a 1758 revision of his Systema, and also Linnaeus who established several of the obligatory rankings. Moreover, he instituted the first taxonomic keys, and his system, first applied in botany, became accepted in the zoological community as well. Others, including Baron Georges Cuvier (1769-1832), Michel Adanson (1727-1806), and Comte Georges Buffon (1707-1788), refined Linnaeus's system, but he stands as a towering figure in the discipline.
Later, the French natural philosopher Jean Baptiste de Lamarck (1744-1829) proposed a distinction between vertebrates, or animals with spinal columns, and invertebrates. Today this distinction is not considered as useful as it once was, since it is lopsided—that is, there are nine times as many invertebrates as vertebrates in the animal kingdom—but at the time, it represented an advancement. Less questionable were the distinctions introduced in 1866 by the German biologist Ernst Haeckel (1834-1919) between plants, animals, and single-cell organisms. As Haeckel reasoned, at the level of unicellular organisms, distinctions between plant and animal really make no sense.
Darwin and the Twentieth Century
By far the most influential figure in taxonomy during the nineteenth century was the man also recognized as the most influential figure in all of biology during that era: Darwin. Whereas Linnaeus had retained the Aristotelian focus on the "essence" of the animal's features, Darwin swept away such notions and, in his Origin of Species (1859), proposed that the "community of descent" is "the one known cause of close similarity in organic beings" and therefore the only reasonable basis for taxonomic classification systems. As result of Darwin's work, taxonomists became much more oriented toward the representation of phylogeny in their classification systems. Therefore, instead of simply naming and cataloguing species, modern taxonomists also try to construct evolutionary trees showing the relationships between different species.
Since Darwin's time, taxonomy has seen numerous innovations, including the introduction of cladistics by Hennig and of numerical taxonomy by Sokal and Sneath. Taxonomists today make use of something unknown at the time of Darwin: DNA (deoxyribonucleic acid, a molecule that contains genetic codes for inheritance), which provides a wealth of evidence showing relationships between creatures. For example, a comparison of human and chimpanzee DNA reveals that we share more than 98% of the same genetic material, indicating that the two lines of descent are related more closely than either is to apes.
The Five Kingdoms
There are several taxonomic systems, distinguished in part by the number of different kingdoms that each system recognizes. The system used in this book is that of five kingdoms, listed here, which is the result of modifications by the American biologists Lynn Margulis (1938-) and Karlene V. Schwartz (1936-) to the work of earlier taxonomists. (It should be noted that biologists are increasingly using a system of six kingdoms under three domains: eubacteria, arachaea, and eukaryotes. For the sake of simplicity, however, the five-kingdom system is used here.) These five kingdoms are as follows:
Monera : bacteria, blue-green algae, and spirochetes (spiral-shaped, undulating bacteria). Members of this kingdom, consisting of some 10,000 or more known species, are single-cell prokaryotes, meaning that the cell has no distinct nucleus. Some researchers have divided Monera into Eubacteria, or "true" bacteria, and Archae-bacteria, which are bacteria-like organisms capable of living in extremely harsh and sometimes anaerobic (oxygen-lacking) environments, such as in acids, saltwater, or sewage.
Protista (or Protoctista) : protozoans, slime molds (which resemble fungi), and algae other than the blue-green variety. Made up of more than 250,000 species, this kingdom is distinguished by the fact that its members are single-cell organisms, like the Monera. These organisms, however, are eukaryotes, or cells with a nucleus as well as organelles (sections of the cell that perform specific functions).
Fungi : fungi, molds, mushrooms, yeasts, mildews, and smuts (a type of fungus that afflicts certain plants). Fungi are multicellular, consisting of specialized eukaryotic cells arranged in a filamentous form (that is, a long, thin series of cells attached either to one another or to a long, thin cylindrical cell). There are some 100,000 varieties of fungi.
Plantae : plants, of which there are upward of 250,000 species. Although plant is a common term, there is no universally accepted definition that includes all plants and excludes all nonplants. One of the most important characteristics of plants is the fact that they receive their nutrition almost purely through photosynthesis. Beyond the plant kingdom, this is true only of a few protests and bacteria. (For the most part, the three lower kingdoms obtain nutrition through absorption.) Other characteristics of plants include the fact that they are incapable of locomotion; have cells that contain a form of carbohydrate called cellulose, making their cell walls more or less rigid; are capable of nearly unlimited growth at certain localized regions (unlike most animals, which have set numbers of limbs and so forth); and have no sensory or nervous system.
Animalia : animals, of which there are more than 1,000,000 species. Like plants, animals are characterized by specialized eukaryotic cells, but also like plants, the comprehensive definition of animal is not as obvious as one might imagine. Mobility, or a means of locomotion, is not a defining characteristic, since sponges and corals are considered animals. The principal difference between animals and plants is at the cellular level: animals either lack cells walls entirely or have highly permeable walls, unlike the cellulose cell walls in plants. Another defining characteristic of animal is that they obtain nutrition by feeding on other organisms. Additionally, animals usually have more or less fixed morphological characteristics and possess a nervous system. The fact that most animals are mobile helps account for the large number of animal species compared with those of other kingdoms; over the course of evolutionary history, mobility brought about the introduction of animals to a wide range of environments, which required a wide range of adaptations.
Space does not permit a discussion of the various phyla, let alone the smaller divisions, inanything like the detail we have accorded to kingdoms. Furthermore, the distinctions among most phyla, apart from higher animals and some plants, makes for rather dry reading to a nonscientist. These divisions are discussed in furtherdetail, however, within the essays Species and Speciation. The latter essays also address the definition of species, a great and continuing challenge that faces taxonomists.
Taxonomy in Action
Two stories reported in National Geographic News online (see "Where to Learn More") in 2001 and 2002 illustrate the fact that scientific classification is an ongoing process, and that the world of taxonomy is frequently home to controversies and surprises. Lee R. Berger of the Geographic reported the first story, on December 17, 2001, under the heading "How Do You Miss a Whole Elephant Species?" As it turns out, there are not just two species of elephant, as had long been believed, but three.
In addition to the Asian elephant (Elephas maximus) scientists had long recognized the African savanna elephant, or Loxodonta africana, as a second species. However, DNA testing (see Genetics and Genetic Engineering) in 2001 revealed a second African variety, Loxodonta cyclotisare or the African forest elephant, formerly believed to constitute merely a subspecies.
The news was not entirely new: as early as a century prior to the announcement of the "new" species, zoologists had begun to suspect that the forest elephant was a separate grouping distinguished by a number of characteristics. For example, the forest elephant is physically smaller, with males seldom measuring more than 8 ft. (2.5 m) at the shoulder, as compared to 13 ft. (4 m) for a large savanna male. Additionally, ivory samples confiscated from poachers or illegal hunters have revealed that the material in the tusks of the forest variety is pinker and harder than that of its savanna counterpart.
Recognition of the third elephant species followed years of argument as to whether the two African varieties are capable of interbreeding, which would indicate that they are not separate species. That debate was rendered moot by the DNA studies, which showed that the African forest and savanna elephants are less closely related genetically than are lions and tigers, or horses and zebras.
A "new" Insect Order
The identification of the forest elephant in 2001 was a major taxonomic event, inasmuch as the elephant itself is a large and commonly known creature. However, it was still a matter only of identifying a new species, whereas in 2002, for the first time in 87 years, taxonomists identified an entirely new insect order. Actually, the order consists of a single known species, but this one is so different from others that it must be grouped separately. Discovered in Namibia, in southwestern Africa, the creature was given the nickname "the gladiator" in honor of the Academy Award-winning 2000 film of that name.
Entomologist Oliver Zompro of the Max Planck Institute of Limnology in Plön, Germany, described the creature as "a cross between a stick insect, a mantid, and a grasshopper," according to the Geographic. Because its first body segment is the largest, it is distinguished from a stick insect, whereas it differs from a mantid inasmuch as it uses both fore and mid-legs to capture prey. And while it looks like a grasshopper, "the gladiator" cannot jump.
Measuring as much as 1.6 in. (4 cm) long, the insect, whose order is designated as Mantophasmatodea, is a carnivorous, nocturnal creature. Its discovery raised the number of known insect orders to 31, a discovery that Piotr Naskrecki, director of the Conservation International Invertebrate Diversity Initiative, compared to finding a mastodon or saber-toothed tiger. Colorado State University ecologist Diana Wall described the discovery as "tremendously exciting" and told the Geographic, "This new order could be a missing link to determining relationships between insects and other groups. … Every textbook discussing the orders of insects will now need to be rewritten."
Where to Learn More
Classification—The Dinosaur FAQ (Web site). <http://www.miketaylor.org.uk/dino/faq/index.html#4>.
The Germplasm Resources Information Network (GRIN), Agricultural Research Service (Web site). <http://www.ars-grin.gov/npgs/tax/>.
Goto, H. E. Animal Taxonomy. London: Arnold, 1982.
Lacey, Elizabeth A., and Robert Shetterley. What's the Difference?: A Guide to Some Familiar Animal Look-Alikes. New York: Clarion Books, 1993.
Margulis, Lynn, and Karlene V. Schwartz. Five Kingdoms: An Illustrated Guide to the Phyla of Life on Earth. New York: W. H. Freeman, 1988.
National Geographic News (Web site). <http://news.nationalgeographic.com/news/>.
O'Neil, Dennis. Classification of Living Things/Palomar College (Web site). <http://anthro.palomar.edu/animal/>.
Parker, Steve. Eyewitness Natural World. New York: Dorling Kindersley, 1994.
Simpson, George Gaylord. Principles of Animal Taxono my. New York: Columbia University Press, 1961.
Taxonomy Browser, National Center for Biotechnology Information, National Library of Medicine, National Institutes of Health (Web site). <http://www.ncbi.nlm.nih.gov/Taxonomy/>.
The Tree of Life Web Project (Web site). <http://beta.tolweb.org/tree/>.
Tudge, Colin. The Variety of Life: A Survey and a Celebration of All the Creatures That Have Ever Lived. London: Oxford University Press, 2000.
Whyman, Kathryn. The Animal Kingdom: A Guide to Vertebrate Classification and Biodiversity. Austin, TX: Raintree Steck-Vaughn, 1999.
Willis, Paul. "Dinosaurs and Birds: The Story." Australian Broadcasting Corporation (Web site). <http://www.abc.net.au/science/slab/dinobird/story.htm>.
The arrangement or classification of objects according to certain criteria. Systematics is a broader term applied to all comparative biology, including taxonomy. For classifying plants and animals, where the term taxonomy is most often applied, the criteria are characters of structure and function.
A given character usually has two or more states. These variations are used as the basis of biological classification, grouping together like species (in which the majority of the character states are alike) and separating unlike species (in which many of the character states are different). Since the acceptance by biologists of the concept of organic evolution, more and more effort has been made to produce systems of classification that conform to phylogenetic (that is, evolutionary) relationships. Taxonomy is thus concerned with classification, but ultimately classification itself depends upon phylogeny—the amount, direction, and sequence of genetic changes. Scientists try to classify lines, or clusters of lines, of descent. This has not always been the case, and in the past various other criteria have been used, such as whether organisms were edible (ancient times) and whether flowers had five stamens or four or some other number (Linnaean times). Modern taxonomists generally agree that the patterns or clusters of diversity they observe in nature, such as the groups of primates, the rodents, and the bats, are the objective results of purely biological processes acting at different times and places in the past. At the least, animal and plant taxonomy provides a method of communication, a system of naming; at the most, taxonomy provides a framework for the embodiment of all comparative biological knowledge. See also Animal systematics; Classification, biological; Numerical taxonomy; Organic evolution; Phylogeny; Plant taxonomy.
An ordered set of operations that results in the objective classification or labelling of objects, and other kinds of material culture, into discrete units or taxa. Such ordered classification is based on visible or scientifically etermined similarities and carefully defined traits. Taxonomy provides the basis for the organization of most archaeological materials. See also typology.
The classification of organisms in an ordered system that indicates natural relationships.
The classification of living things. (See Linnean classification.)
A specialist in taxonomy.
A system for classifying organisms on the basis of natural relationships and assigning them appropriate names.
Taxonomy (from ancient Greek τάξις taxis, arrangement, and νομία nomia, method) is the academic discipline of defining groups of biological organisms on the basis of shared characteristics and giving names to those groups. Each group is given a rank and groups of a given rank can be aggregated to form a super group of higher rank and thus create a hierarchical classification. The groups created through this process are referred to as taxa (singular taxon). An example of a modern classification is the one published in 2009 by the Angiosperm Phylogeny Group for all living flowering plant families (the APG III system).
The exact definition of taxonomy varies slightly from source to source, but the core of the discipline remains: the conception, naming, and classification of organism groups. As points of reference, three recent textbook definitions are presented below:
Biological taxonomy is a sub-discipline of biology, and is generally practiced by biologists known as "taxonomists", though enthusiastic naturalists are also frequently involved in the publication of new taxa. The work carried out by taxonomists is crucial for the understanding of biology in general. Two fields of applied biology in which taxonomic work is of fundamental importance are the study of biodiversity and conservation. Without a working classification of the organisms in any given area, estimating the amount of diversity present is unrealistic, making informed conservation decisions impossible. As conservation becomes ever more politically important, taxonomic work impacts not only the scientific community, but society as a whole.
The 'definition' of a taxon is encapsulated by its description. There are no set rules governing the definition of taxa, but the naming and publication of new taxa is governed by sets of rules. In zoology, the nomenclature for the more commonly used ranks (superfamily to subspecies), is regulated by the International Code of Zoological Nomenclature. In the fields of botany, phycology, and mycology, the naming of taxa is governed by the International Code of Nomenclature for algae, fungi, and plants.
The initial description of a taxon involves five main requirements:
However, often much more information is included, like the geographic range of the taxon, ecological notes, chemistry, behavior, etc. How researchers arrive at their taxa varies; depending on the available data, and resources, methods vary from simple quantitative or qualitative comparisons of striking features, to elaborate computer analyses of large amounts of DNA sequence data.
Biological classification is a critical step in the taxonomic process, as it informs the user as to what the relatives of the taxon are hypothesized to be. Although the discipline of taxonomy itself does not deal with the investigations of how taxa are related to one another, it does serve to communicate these results to the user. To do this, it uses taxonomic ranks, including, among others (in order from most inclusive to least inclusive): Domain, Kingdom, Phylum, Class, Order, Family, Genus, and Species.[Note 1]
Today, traditional rank-based biological classifications persist in a structure largely unchanged since the 1700s; however, how the relationships of these taxa are investigated has changed drastically in recent decades. It is now common for biologists to devise a classification based on the results of phylogenetic analysis using DNA sequence data. Although phylogenetics itself is fundamental to modern-day systematics, its use for the description of new taxa, and placement within a classification scheme, is unrequired. As a result, phylogenetics tends to have a direct impact on taxonomic classifications, even though it is not a part of taxonomy.
In numerical taxonomy, the taxonomy is exclusively based on cluster analysis and neighbor joining to best-fit numerical equations that characterize measurable traits of a number of organisms. It results in a measure of evolutionary "distance" between species. This method has become quite rare in modern times, having been largely superseded by cladistic analyses, as numerical taxonomy is sensitive to being misled by plesiomorphic traits.
Taxonomy has been called "the world's oldest profession", and naming and classifying our surroundings has likely been taking place as long as mankind has been able to communicate. It would always have been important to know the names of poisonous and edible plants and animals in order to communicate this information to other members of the family or group.
In the East, one of the earliest recorded pharmacopoeias was written by Shen Nung, Emperor of China (c. 3000 BC). He wanted to spread information related to agriculture and medicine, and is said to have tasted hundreds of plants with the goal of learning their medicinal value. Records after this are difficult to interpret for some time, but medicinal plant illustrations show up in Egyptian wall paintings from c. 1500 BC. The paintings clearly show that these societies valued and communicated the uses of different species, and therefore had a basic taxonomy in place.
Historical records show that informally classifying organisms took place at least back to the days of Aristotle (Greece, 384-322 BC), who was the first to begin to classify all living things. Some of the terms he gave to animals, such as "invertebrates" and "vertebrates" are still commonly used today. His student Theophrastus (Greece, 370-285 BC) carried on this tradition, and wrote a classification of 480 plants called Historia Plantarum. Again, several plant groups currently still recognized can be traced back to Theophrastus, such as Cornus, Crocus, and Narcissus. The next major turn-of-the-millennia era taxonomist came in the form of Pliny the Elder (Rome, 23-79 AD). His elaborate 160-volume work Naturalis Historia described many plants, and even gave many of them Latin binomial names.
It was not until c. 1500 years later that taxonomic works became ambitious enough to replace the ancient texts. This is often credited to the development of sophisticated optic lenses, which allowed for the morphology of organisms to be studied in much greater detail. One of the earliest authors to take advantage of this leap in technology was Andrea Cesalpino (Italy, 1519–1603), who is often referred to as "the first taxonomist". His magnum opus De Plantis came out in 1583, and described over 1500 plant species. Two large plant families that he first recognized are still in use today: the Asteraceae and Brassicaceae. Then in the seventeenth century John Ray (England, 1627–1705) wrote many important taxonomic works. Arguably his greatest accomplishment was Methodus Plantarum Nova (1682), where he published over 18,000 plant species. At the time his classifications were perhaps the most complex yet produced by any taxonomist, as he based his taxa on many combined characters. The next major taxonomic works were produced by Joseph Pitton de Tournefort (France, 1656–1708). His work from 1700, Institutiones Rei Herbariae, included over 9000 species in 698 genera, and directly influenced Linnaeus as it was the text he used as a young student.
The Swedish botanist Carl Linnaeus (1707-1778) ushered in a new era of taxonomy. With his major works Systema Naturae 1st Edition in 1735, Species Plantarum in 1753, and Systema Naturae 10th Edition, he revolutionized modern taxonomy. His works implemented a standardized binomial naming system for animal and plant species, which proved to be an elegant solution to a chaotic and disorganized taxonomic literature. As a result the Linnaean system was born, and is still used in essentially the same way today as it was in the eighteenth century. Currently, plant and animal taxonomists regard Linnaeus' work as the "starting point" for valid names (at 1753 and 1758 respectively). Names published before these dates are referred to as "pre-Linnaean", and not considered valid (with the exception of spiders published in Svenska Spindlar). Even taxonomic names published by Linnaeus himself before these dates are considered pre-Linnaean.
Modern taxonomy uses database technologies to search and catalog classifications and their documentation. While there is no commonly used database, there are comprehensive databases such as the Catalogue of Life, which attempts to list every documented species. The catalogue listed 1.4 million species for all kingdoms as of May 2012, claiming coverage of more than 74% of the estimated 1.9 million species known to modern science.
Almost anything—animate objects, inanimate objects, places, concepts, events, properties, and relationships—may be classified according to some taxonomic scheme. Taxonomies of the more generic kinds of things typically stem from philosophical investigations. Starting with the work of Aristotle in his work 'Categories' several philosophers, especially ontologists, arranged generic categories (also called types or classes) in a hierarchy that more or less satisfy the criteria for being a true taxonomy.
Taxonomy, or categorization, in human cognition has been a major area of research in psychology. Social psychologists have sought to model the manner in which the human mind categorizes social stimuli (Self-categorization theory is a prototypical example). Some have argued that the adult human mind naturally organizes its knowledge of the world into such systems. Anthropologists have observed that taxonomies are generally embedded in local cultural and social systems, and serve various social functions.
Other taxonomies, such as those analyzed by Durkheim and Lévi-Strauss, are sometimes called folk taxonomies to distinguish them from scientific taxonomies. Baraminology is a taxonomy used in creation science which in classifying form taxa resembles folk taxonomies. The phrase "enterprise taxonomy" is used in business (see economic taxonomy) to describe a very limited form of taxonomy used only within one organization. For example, a method of classifying boxes as "Type A", "Type B" and "Type C" used within a box company for categorizing box shipments. The military and health care/safety science fields also have their own taxonomies. In the field of modern computing, the semantic web requires formal XML extension taxonomies (like XBRL) often containing a collection of elements and attributes and qualified by an namespaces to help distinguish identically named elements.
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