taxonomy: human classification systems,
using the example of classification of
‘living organisms’

When a new life form is found, it is assigned a formal scientific name, which describes how closely it is related to other creatures. Each name has seven components: Kingdom, Phylum, Class, Order, Family, Genus and Species. Kingdom is the broadest category; species is the most specific.

Animals within each of these categories share characteristics. All domestic cats are members of the same species. Lions and tigers each belong to different species, but share enough similarities that they belong to the same genus: Panthera.

Next, all those regarded as cats—lions, tigers, cheetahs, leopards, panthers and domestic pussies—belong to the family Felidae; while ’dogs’, being regarded as sufficiently similar one to another, belong to the family Canidae.

A newly discovered creature may be regarded as part of an already established family, or be put into an entirely new category.

People once divided organisms into two simple categories, or taxa—the kingdoms Plantae and Animalia. A third kingdom, Protista, was later proposed for those organisms that are not clearly plant or animal, such as protozoa, bacteria, and algae. Improvements in biochemical and genetic research, and advances in electron microscopy, enabled further classification. Micro-organisms without a distinct nucleus, called prokaryotes, are placed in the kingdom Monera; bacteria are the principal organisms in this kingdom. The classification of eukaryotes (micro-organisms having a distinct nucleus) remains controversial.

At present, most involved people classify all living and extinct organisms into either a five-kingdom system, or into a four-kingdom system. The five-kingdom system recognises Animalia, Plantae, Monera, Fungi, and Protista (the last being basically all eukaryotic micro-organisms or algae, protozoans, and slime molds), while the four-kingdom system recognises only the kingdoms Animalia, Plantae, Monera, and Virus. That is, while recognising the separation of the eukaryotes and prokaryotes, the four-kingdom system places the protozoans with the animals, and the fungi and algae with the plants.

Notice that the classification system is under constant revision and subject to various disputes/discussion. It is not some fixed rule on stone tablets, but just a convenient filing system.

The systematic means of classifying life was established by Carolus Linnaeus (1707 – 1778) in the mid-18th century. He gave ‘each’ creature in the scheme a two-part name, consisting of generic and specific names in Latin. For example, the name for the domestic dog is Canis familiaris, the first name indicating the genus, to which dogs belong; the second word is the name of the species within the genus.

The next level up is the order. For example, cats (Felidae) and dogs (Canidae) form two different families in the order Carnivora. This particular order includes 10 families of living mammals, so finding a new order is a major deal!

Here are the ten families of the Order Carnivora:
• Canidae (dogs, wolves, jackals, and foxes),
• Ursidae (bears),
• Procyonidae (raccoons),
• Mustelidae (skunks, mink, weasels, badgers, and otters),
• Viverridae (civets and mongooses),
• Hyaenidae (hyenas),
• Felidae (cats),
• Otariidae (eared seals),
• Odobenidae (walrus), and
• Phocidae (earless seals)
  the Carnivora are in their turn members of the class Mammalia.

Then the class of mammals come under the phylum chordata in the sub-phylum Vertebrata, which in turn is part of the Kingdom Animalia—see above which is where we came in!

Oh, you noticed—yes, we have sub-phyla and sub-species, and so on, as the system continues to adapt to new understanding and new finds. Just like any human filing system, things are not always tidy and keeping up with the filing, as new information comes in, takes a great deal of discussion and effort!

The classification system has developed as new understanding grows. Linnaeus sought a natural method of arrangement, but he actually defined types of species on the basis of idealised morphology (shape). The greatest change from Linnaeus’ outlook is reflected in the phrase “the new systematics”, which was introduced in the 20th century and through which an explicit effort is made to have taxonomic schemes reflect evolutionary history. The basic unit of classification, the species, is also the basic unit of evolution; that is, a population of actually or potentially interbreeding individuals. Such a population shares, through interbreeding, its genetic resources. In so doing, it creates the gene pool—its total genetic material—that determines the biological resources of the species and on which natural selection continuously acts. This approach has guided work on classifying animals away from somewhat arbitrary categorisation of new species, to that of recreating evolutionary history (phylogeny) and incorporating it in the system of classification.

This study is now advancing rapidly as the DNA (deoxyribonucleic acid) genome structures are being picked apart at laboratories around the world.

Classification systems are entirely arbitrary human conveniences. For example, most people can hardly tell different types of conifers apart. To tell them apart requires close examination of the leaf structure, and much else. From the photo [thumbnail to left, click for larger notated image] of the maritime pine (pinus pinaster) you will see that the leaves come in pairs, whereas other pines have groups of 3 or 5 leaves. By such small differences are plants identifiable, especially by those who are, like myself, untutored in the esoterica![7]

Linnaeus also introduced a “sexual system” of plants, by which the numbers of flower parts—especially stamens, which produce male sex cells, and styles, which are prolongations of plant ovaries that receive pollen grains—became useful tools for easy identification of plants. This simple system, though effective, had many imperfections. Other classification systems, in which as many identification characters as possible were considered in order to determine the degree of relationship, were developed by other botanists; indeed, some appeared before the time of Linnaeus. The application of the concepts of Charles Darwin (on evolution) and Gregor Mendel (on genetics) to plant taxonomy has provided increasing insight into the process of evolution and the production of new species.

Systematic botany now uses information and techniques from all the subdisciplines of botany, incorporating them into one body of knowledge. Phytogeography (the biogeography of plants), plant ecology, population genetics and various techniques applicable to cells—cytotaxonomy and cytogenetics—have contributed widely to the current status of systematic biology and have, to some degree, become part of it. More recently, phytochemistry, computerised statistics, and fine-structure morphology have been added to the activities of systematic botany. Similar comments apply to other areas of life studies.

Note carefully here that the classification often refers to several points, in order to make a useful box into which to classify the item. (See also Categorisation 1—boxes.)

[This document was developed using material taken from the National Geographic and Enc. Brit.]

End notes

1. See Categorisation 1—boxes, particularly note [7].
   
   
2. The daisy [Bellis perennis – you will often see these species names written as follows: B. perennis] is in the family Asteracaeae or Compositae (yes, both family names are used, just to add to the confusion). It shares this family, I am told, with another 25,000 or so species, including the sunflower [Helianthus annus], the dandelion [taraxacum officianale] and a whole range of chrysanthemums. (Have a look sometime, and see whether you can detect the similarities. Here is further related information which may help you with the daisy.)
   
   Asteracaeae is one of the largest families, matched only by the Orchidaceae (orchids). As with much else, which family is regarded as the more various or larger depends on who is counting, and how. See also numbers or objects and categorisation 2 – contents of boxes.
   
   Remember that species do not exist in the real world, but in the heads of those people that study ‘them’. For example, in Costa Rica, there is a mountain where the ‘species’ of pine change as you go up the mountain. Now species 1 cannot breed with species 3, neither can either species breed with species 2. However, in reality, as you move from species 1 to 2, or from species 2 to 3, there is some cross-breeding. Such difficulties are the norm in human studies of the real world, which is a lot of what people spend their time discussing and trying to understand.
   
   
3. Other examples at the Order level are Marsupulia (marsupials), Insectivora (insects), Carnivora (carnivores).
   
   
4. Other groups at the Family level are Pongidae (apes), Hylobatidae (gibbons), Cebidae (new world monkeys), Lemuridae (lemurs).
   
   
5. Other groups at the Genus level are Pan (chimpanzee) and Gorilla (gorilla).
   
   
6. The ancient Greek root ‘homo-’ means “self” or “same”.
   The Latin noun ‘homo’ means “human”, or “man” in the general sense.
   In Latin, ‘sapiens’ means “wise”.
   Therefore, Homo sapiens means “Wise man”; perhaps some exaggeration here! There are other, extinct species divisions under the genus Homo now used in anthropology; such as neanderthalensis (H. neanderthalensis), erectus (H. erectus) and others. Of course, we do not know (yet?) whether neanderthalensis or erectus are true species, or whether they could have interbred between themselves or with humans (sapiens).
   
   
7. In the larger annotated image of pine needles, water-conserving leaf curl is noted. This leaf curl results in evaporation being much reduced from the hidden, inner side of the leaves. There is another species that has evolved this characteristic independently, that is in parallel evolution. This is erica, commonly known as heather. Erica has curling under leaves, while the species similar to it, calluna (English name: ling) does not.
   
   Both these plants (erica and calluna) also have waxy coatings on their leaves, again to reduce moisture loss. [A third method for plants to conserve water is by having a light down, or fur, on the leaves. This traps water, slowing down its evaporation.]
   
   This linked article describes another interesting example of adaptation for water conservation, this time the plant is rhubarb.

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