Scientific classification
From Wikinfo
Scientific classification is a term used to describe how biologists organize all life on Earth. The way that forms of life are classified are, based primarily on evolutionary similarity.
The earliest known system of classifying forms of life comes from the Greek philosopher Aristotle. The next major advance in developing systems for scientific classification of living beings was by the Swiss professor, Conrad Gessner (1516�1565). He was one of the most voluminous writers on systematic zoology, and was so highly esteemed that his Historia animalium was republished a hundred years after his death.
Gesner�s work, like that of John Johnstone (b. 1603), who was of Scottish descent and studied at St Andrews, and like that of Ulysses Aldrovandi of Bologna (b. 1522), was essentially a compilation, more or less critical, of all such records, pictures and relations concerning beasts, birds, reptiles, fishes and monsters as could be gathered together by one reading in the great libraries of Europe, travelling from city to city, and frequenting the company of those who either had themselves passed into distant lands or possessed the letters written and sometimes the specimens brought home by adventurous persons.
The exploration of parts of the New World next brought to hand descriptions and specimens of many novel forms of animal life. In the latter part of the 16th century and the beginning of the 17th careful study of animals commenced, which, directed first to familiar kinds, was gradually extended until it formed a sufficient body of knowledge to serve as an anatomical basis for classification. Advances in using this knowledge to classify living beings bears a debt to the research of medical anatomists, such as Fabricius (1537�1619), Severinus (1580�1656), Harvey (1578�1657), and Tyson (1649�1708). Advances in classification due to the work of entomologists and the first microscopists is due to the research of people lile Malpighi (1628�1694), Swammerdam (1637�1680), and Hook (1635�1702).
John Ray (1627-1705) was an English naturalist who published important works on plants, animals, and natural theology. His classification of plants in his Historia Plantarum was an important step towards modern taxonomy. Ray rejected the system of dichotomous division by which species were classified according to a pre-conceived, either/or type system, and instead classified plants according to similarities and differences that emerged from observation.
The commencement of anatomical investigations deserves notice here as influencing the general accuracy and minuteness with which zoological work was prosecuted, but it was not until a late date that their full influence was brought to bear upon systematic zoology by Georges Cuvier (1769�1832).
Two years after John Ray�s death Carolus Linnaeus (1707�1778) was born. His great work, the Systeina natisrae, ran through twelve editions during his lifetime (1st ed. 1735). He is best known for his introduction of a method of modern classification; he created systematic zoology and botany in their present form. Linnaeus adopted Ray�s conception of species, but he made species a practical reality by insisting that every species shall have a double Latin name � the first half to be the name of the genus common to several species, and the second half to be the specific name.
Previously to Linnaeus long many-worded names had been used, sometimes with one additional adjective, sometimes with another, so that no true names were fixed and accepted. Linnaeus by his binomial system made it possible to write and speak with accuracy of any given species of plant or animal. He proceeded further to introduce into his enumeration of animals and plants a series of groups, viz, genus, order, class, which he compared to the subdivisions of an army or the subdivisions of a territory.
Linnaeus' System has been the standard method classifying all forms of life on Earth until very recently. However, a recent trend in biology since the 1960s, called cladism or cladistic taxonomy, has slowly been becoming more useful than the Linneaean system, and in recent years has become very popular. It is now expected that the cladistics system will overtake the older system. Cladistics requires taxa (named groups in a taxonomy) to be clades. In other words, cladists argue the classification system should be reformed to eliminate all non-clades (paraphyletic and polyphyletic groups). In fact, some cladists have argued for entirely abandoning the Linnaean system of ranked taxa in favor of clades. A formal code of phylogenetic nomenclature, the Phylocode[1], is currently under development for a cladistic taxonomy that abandons the Linnaean structure.
Contents |
Linnaeus' System
This system was devised by Carolus Linnaeus (1707-1778).
The system works by placing each organism into a layered hierarchy of groups. Each group at a given layer is composed of a set of groups from the layer directly below. Therefore, in theory, one needs know only the lowest layer (species) of a particular organism in order to uniquely determine the other six layers. In practice, however, many species actually have the same species designation, so when specifiying a species, scientists use the bottom two layers - a system called binomial nomenclature.
The standard groupings (taxa) of taxonomy from most general to most specific are:
Several acronym mnemonics have been made for these, for instance King Phillip called out for good soup. Sometimes tribes, which lie between families and genera, and races, which lie below species, are also used. Intermediate ranks may be created by adding prefixes, for instance:
- Superorder
- Order
- Suborder
- Infraorder
The term varieties is sometimes used in place of subspecies. In horticulture, it refers to populations modified by selective breeding, for instance the Peace Rose, a hybrid Tea Rose. At the top of the scale, there has been a move towards the three domain system. The domains originally were replacements for the different kingdoms, but often count as a higher rank.
Could add a description of the difficulty in classifying microbes: their features are derived from direct visual observation, but include such procedural characteristics as Gram stain type, motility, ability to form spores, etc. However, given an unknown bacterium with a given set of characteristics, it is in general not possible to predict its phylogeny, toxicity, etc. Other methods, using genes, their DNA, and several types of RNA, are under development.
Examples Of Biological Classification
The fruit fly so familiar in genetics laboratories is Drosophila melanogaster. Its usual classification, as well as that of humans, is as follows
Fruit Fly (Drosophila)
| Kingdom | Animalia |
| Phylum | Arthropoda |
| Class | Insecta |
| Order | Diptera |
| Family | Drosophilidae |
| Genus | Drosophila |
| Species | melanogaster |
Human (Homo sapiens)
| Kingdom | Animalia |
| Phylum | Chordata |
| Subphylum | Vertebrata |
| Class | Mammalia |
| Subclass | Eutheria |
| Order | Primates |
| Suborder | Catarrhini |
| Family | Hominidae |
| Genus | Homo |
| Species | sapiens |
Cucumbertree (Magnolia acuminata)
| Kingdom | Plantae |
| Division | Magnoliophyta |
| Class | Magnoliopsida |
| Order | Magnoliales |
| Family | Magnoliaceae |
| Genus | Magnolia |
| Species | acuminata |
Note in this last example, that most of the taxa are named after the type genus, Magnolia.
Group Suffixes
Taxa above the genus level are often given names derived from the type genus. The suffixes used to form these names depend on the kingdom, and sometimes the phylum and class, as follows:
| Taxon | Plants | Algae | Fungi | Animals |
|---|---|---|---|---|
| Division/Phylum | -phyta | -phyta | -mycota | |
| Subdivision/Subphylum | -phytina | -phytina | -mycotina | |
| Class | -opsida | -phyceae | -mycetes | |
| Subclass | -idae | -phycidae | -mycetidae | |
| Order | -ales | -ales | -ales | |
| Suborder | -ineae | -ineae | -ineae | |
| Superfamily | -acea | -acea | -acea | -oidea |
| Family | -aceae | -aceae | -aceae | -idae |
| Subfamily | -oideae | -oideae | -oideae | -inae |
| Tribe | -eae | -eae | -eae | -ini |
| Subtribe | -inae | -inae | -inae | -ina |
Cladistics
Cladistics (or phylogenetic systematics) describes the evolutionary relationships between living things based on derived similarity. Cladistics differs from phenetics, which groups organisms based on overall similarity, and from more traditional approaches based on "key characters". Based on a wide variety of information, which includes genetic analysis, biochemical analysis, and analysis of morphology, relationship trees called "cladograms" are drawn up to show different possibilities.
Cladistics requires taxa (named groups in a taxonomy) to be clades. In other words, cladists argue the classification system should be reformed to eliminate all non-clades (paraphyletic and polyphyletic groups). In fact, some cladists have argued for entirely abandoning the Linnaean system of ranked taxa in favor of clades. A formal code of phylogenetic nomenclature, the Phylocode[2], is currently under development for a cladistic taxonomy that abandons the Linnaean structure.
See the article on cladistics for more information.
See also:
External Links:
References
- Adapted from the Wikipedia article, "Scientific classification" http://www.wikipedia.org/wiki/Scientific_classification August 21, 2003
