The Linnaean Hierarchy
The Linnaean taxonomy is a formal system for classifying and naming living things based on a simple hierarchical structure, from most general to most similar The basic hierarchy as formulated by Linneus, is as follows:
- Imperium ("Empire") - the phenomenal world
- Regnum ("Kingdom") - the three great divisions of nature at the time - animal, vegetable, and mineral
- Classis ("Class") - subdivisions of the above, in the animal kingdom six were recognized (mammals, birds, amphibians, fish, insects, and worms)
- Ordo ("Order") - further subdivision of the above - the class Mammalia has eight
- Genus - further subdivisions of the order - in the mammalian order Primates there are four. e.g. Homo
- Species - subdivisions of genus, e.g. Homo sapiens.
- Varietas ("Variety") - species variant, e.g. Homo sapiens europaeus.
As can be seen, Linneus wrote in Latin, the standard intellectual language of the time. His hierarchical system still reflected the old medieval feudalistic worldview ("Order" for example referred to an order of monks). And concepts like evolution were alien to him. For Linneus and his contemporaries, the world and all it's creatures was created once and for all, by the Judaeo-Christian God. Nevertheless this basic formula, as set out in the 10th edition of his Systema Naturae, published in 1758, was and still is considered the foundation of all modern taxonomy (at least until the cladists came along!)
As time progressed changes were made. The rank of Empire is obviously superfluous, while Variety came to be used only by gardeners, insect collectors, etc. The use of Latin was replaced by the vernacular, although it is still retained in the actual generic and specific names. And two new ranks were erected - Phylum (or Division in the case of Plants) was added between Kingdom and Class, and Family between Order and Genus, giving seven hierarchical ranks in all. So, in this nested system of rankings, kingdoms are made up of phyla, phyla of classes, classes of orders, and so on; each higher rank including at least one and usually more subordinate members. This seven-layered hierarchy is the version still used today
This very versatile arrangement can be used to classify every living organism, living or extinct. The following table (below) illustrates this by means of the following examples: the fruit fly so familiar in genetics laboratories (Drosophila melanogaster), humans (Homo sapiens), the peas used by Gregor Mendel in his discovery of genetics (Pisum sativum), the "fly agaric" mushroom Amanita muscaria, and the bacterium Escherichia coli.
What's in a name?
The formal international agreement on names, ranks, and so on, is laid out in the International Code of Zoological Nomenclature, the International Code of Botanical Nomenclature, and the Bacteriological Code of Nomenclature. These set guidelines and publish a reports containing the rules of nomenclature. For example, the Law of Priority (Article 25) says that if a genus or species has been accidentally given two names, only the earlier one is valid. The later name becomes a "junior synonym". This is the case even when it is better known (or more evocative). To give a famous illustration, the Jurassic dinosaur Brontosaurus, named by the 19th century American paleontologist Othniel C. Marsh in 1879, was later found to be the same animal as Apatosaurus, which was actually named by the same guy two years previously (this was during the great dinosaur rush when Marsh and his rival Edward Drinker Cope were engaged in a bitter feud to see who could discover the most prehistoric animals!). Therefore Apatosaurus is the correct name, even though "thunder lizard" (Brontosaurus) would seem more appropriate than "deceptive lizard" (Apatosaurus), and even though the later name honors the same man (maverick paleontologist Dr Bob Bakker has suggested using Brontosaurus anyway!). A similar thing happened with Eohippus ("dawn horse") and Hyracotherium ("hyrax beast"). The better known, more appropriate name was later fond to be describe the same animal as had been previously named. In some cases things are not so clear cut, and a ruling from the Commission in charge of these things is necessary to decide which name to use.
The complete scientific name includes genus and species, the name of the scientist who first described the species in a scientific journal deemed valid for taxonomic purposes, and the year that the paper was published. By convention that the genus and species are written in italics (or, where that is not possible, underlined, or even _underlined ascii wise_). The generic name is always capitalized, the trivial or species name is not. So we have (to use the above illustration) Apatosaurus ajax Marsh, 1877
When a species is placed in a genus different to the one originally named, then the discoverer's name is placed in brackets, even when it is the same guy who named both. So Brontosaurus excelsus Marsh, 1879 becomes Apatosaurus excelsus (Marsh, 1879)
The generic name can be abbreviated to a single capital letter, as in A. ajax. However just using the generic name alone refers to all species included in that genus, in this case Apatosaurus includes the species A. ajax, A. excelsus and A. louisae.
When a new genus is described, it is based on a particular species (i.e. nomenclature-wise the taxonomic hierarchy works from species up, not from kingdom down) which becomes the type species of that genus. So A. ajax is the type species of Apatosaurus.
Sometimes a species is deemed too different to belong in the genus it was formally placed in, and so becomes the type species of a new genus. So Apatosaurus alenquerensis de Lapparent & Zbyszewski, 1957 was recently made the type species for the genus Lourinhasaurus, hence Lourinhasaurus alenquerensis (de Lapparent & Zbyszewski, 1957). Of course whether a species should be retained in a former genus or placed in a new one is often an arbitrary choice, which brings us to the battle between the splitters and the lumpers.
Infra-orders and superfamilies
Even the seven-fold hierarchical system, with it's multiple ranks, was ultimately not sufficiently detailed. As knowledge of the natural world progressed and the number of groups of organisms identified became larger and larger, it became necessary to create further subcategories. These include Tribe between Family and Genus; and Division and Cohort between Class and Order. Moreover, each category can also have prefixes to create a higher grouping (super-), or lower (sub-, infra-) subdivisions So now there is also superorder, suborder, infraorder, subgenus, and subspecies. Again, each is arranged in nested ranks, e.g. there may be a number of superfamilies in each infra-order, and so on. This is illustrated by the tree-like diagram at the right (showing man's position in the Order Primates - from top to bottom we have class, order, suborder, infraorder, superfamily, family, genus, and species. Note: only a few of the many ramifications of the other branches are shown)
When we look at the preceding five species (see above table) in this light we see straight away that things have become more complex (see table below). Here the eight major ranks are given in bold; a selection of minor ranks are given as well.
- Higher taxa and especially intermediate taxa are prone to revision as new information about relationships is discovered. For example, the traditional classification of primates (class Mammalia — subclass Theria — infraclass Eutheria — order Primates) is challenged by new classifications such as McKenna and Bell (class Mammalia — subclass Theriformes — infraclass Holotheria — order Primates). See Mammalia classification for a discussion. These differences arise because there are only a small number of ranks available and a large number of branching points in the fossil record.
- Within species further units may be recognised. Animals may be classified into subspecies (for example, Homo sapiens sapiens, modern humans) or morphs (for example Corvus corax varius morpha leucophaeus, the Pied Raven). Plants may be classified into subspecies (for example, Pisum sativum subsp. sativum, the garden pea) or varieties (for example, Pisum sativum var. macrocarpon, snow pea), with cultivated plants getting a cultivar name (for example, Pisum sativum var. macrocarpon 'Snowbird'). Bacteria may be classified by strain (for example Escherichia coli O157:H7, a strain that can cause food poisoning.
Terminations of names
Taxa above the genus level are often given names based on the type genus, with a standard termination. The terminations used in forming these names depend on the kingdom, and sometimes the phylum and class, as set out in the table below.
- In botany and mycology names at the rank of family and below are based on the name of a genus, sometimes called the type genus of that taxon, with a standard ending. For example, the rose family Rosaceae is named after the genus Rosa, with the standard ending "-aceae" for a family. Names above the rank of family are formed from a family name, or are descriptive (like Gymnospermae or Fungi).
- For animals, there are standard suffixes for taxa only up to the rank of superfamily (ICZN article 27.2).
- Forming a name based on a generic name may be not straightforward. For example, the Latin "homo" has the genitive "hominis", thus the genus "Homo" (human) is in the Hominidae, not "Homidae".
The name of any taxon may be followed by the "authority" for the name, that is, the name of the author who first published a valid description of it. These names are frequently abbreviated: the abbreviation "L." is universally accepted for Linnaeus, and in botany there is a regulated list of standard abbreviations. The system for assigning authorities is slightly different in different branches of biology: see author citation (botany) and author citation (zoology). However, it is standard that if a name or placement has been changed since the original description, the first authority's name is placed in parentheses and the authority for the new name or placement may be placed after it (usually only in botany).
The Splitters and the Lumpers
- "Splitters make very small units - their opponents say that if they can tell two animals apart, they place them in different genera, and if they cannot tell them apart, they place them in different species. Lumpers make large units - their opponents say that if a carnivore is neither a dog or a bear they call it a cat."
- G.G. Simpson, "The Principles of Classification and a Classification of Mammals", Bulletin of the American Museum of Natural History, vol.85, (New York, 1945) p.23
One thing the codes of nomenclature are unable to do anything about is personal preference as to how to divide up families, genera etc. Here we have the famous disagreement between the splitters and the lumpers, between those who prefer to lump together a large number of species in each genus, or genera in families, and those who would rather split genera among new families, and put species in new genera. Hence among, say, malacologists (those who study molluscs), there is on the one hand those who would lump all species of cone shells in the old traditional genus Conus, and those who would divide them up among a large number genera - Lithoconus, Floraconus, Parviconus, etc etc. This can be very annoying for amateur naturalists who would like to have the right name for their labels!
The situation becomes even more involved with the large degree of arbitrariness with these finer sub-rankings due to personal preference and bias. One man's superfamily may be another man's suborder!
When Linneus was around there were not really that many types of animals and plants known, so only a few classes, orders, families, and genera would suffice. As the natural world became better known with further voyages of discovery, as well as developments in biology, more and more new generic, family, and higher categories were required to handle it all
To a large extent this was completely justified, but in the last few decades there has developed a rather unfortunate tendency known as Taxonomic Inflation. This means that a previously considered ranking - e.g. a superfamily, is raised to a higher ranking - e.g. an order, without real justification. Here of course we have again the fact of the arbitrary nature of the Linnean ranking, as it could be argued either position is valid. And sometimes both versions co-exist (as with the splitters and the lumpers) and are found in different text books (and web pages). For example each of the major taxa of vascular plants may be considered as either Classes (e.g. Lycopsida) or the next higher rank, a Division (e.g. Lycophyta). In keeping with an on-going process of inflation, the latter is more often used now, but one still finds examples of the former approach quite frequently, even in recent books
In some cases however the degree of taxonomic inflation is completely ridiculous. Take the example of the brachiopod family Cranioidea (a type of marine shelled invertebrate). As these animals are quite distinct from other members of the phylum Brachiopoda they were given their own superfamily Cranioidea. This then became a distinct order - Craniida. Okay, fair enough. But then in a more recent classification they have been raised to the rank of class, the Craniata (containing the Craniida and two other orders, the Craniopsida and Trimerellida) and even a sub-phylum Craniiformea. Many other examples can be given, such as classes of micro-organisms (Protista) raised to kingdom and superkingdom rank! It is clear that this is taking things to excess, but part of the problem here is misguided attempt to combine the Linnean system with it's rival the Cladistic arrangement; this being impractical if not impossible due to the incompatibility between the two
Another thing to consider here is that there is also more than a little measure of anthropocentric chauvinism, because those organisms closer to us on the family tree (the old " chain of being") are generally given higher ranking. e.g. the orders of birds or mammals, if they were invertebrates, would never qualify higher than superfamily rank. Moreover the names often change as the classification does. Sometimes either name can be used; sometimes the older name is rendered invalid. In any case it can be seen that this sort of classification, no matter how useful, is not a fixed and absolutely objective system.
Palaeos com - The Linnean System