Identification key

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For identification keys for life on Earth, see wikibooks:Dichotomous Key.

An identification key, also known as a dichotomous key, is a method of deducing the correct species assignment of a living thing. As the alternative name implies, it works by offering two (or sometimes more) alternatives at each juncture, and the choice of one of those alternatives determines the next step. For each level of distinction, there is normally a further indent. Some keys use different numbering systems.

1 Informal key
2 Taxonomic key
3 Keys versus descriptions
4 Characteristics of good keys
5 Common errors and problems
- 5.1 Errors
- 5.2 Problems
6 External links
7 See also

[edit] Informal key

The usual form is as follows, here the beginning of an informal key for some eastern United States oaks by their leaf characteristics:

1. Leaves usually without teeth or lobes: 2

1. Leaves usually with teeth or lobes: 5

2. Leaves evergreen: 3

2. Leaves not evergreen: 4

3. Mature plant a large tree — Southern live oak Quercus virginiana

3. Mature plant a small shrub — Dwarf live oak Quercus minima

4. Leaf narrow, about 4-6 times as long as broad — Willow oak Quercus phellos

4. Leaf broad, about 2-3 times as long as broad — Shingle oak Quercus imbricaria

5. Lobes or teeth bristle-tipped: 6

5. Lobes or teeth rounded or blunt-pointed, no bristles: 7

6. Leaves mostly with 3 lobes — Blackjack oak Quercus marilandica

6. Leaves mostly with 7-9 lobes — Northern red oak Quercus rubra

7. Leaves with 5-9 deep lobes — White oak Quercus alba

7. Leaves with 21-27 shallow lobes — Swamp chestnut oak Quercus prinus

This section of the key first differentiates between oaks with entire leaves with normally smooth margins (live oaks, Willow oak, Shingle oak), and other oaks with lobed or toothed leaves. In step two, the entire-leaved oaks are differentiated into two more groups. In step five, oaks with lobed leaves are differentiated between some of the red oak group and some of the white oak group.

Note that this is an informal key, i.e., not based on the taxonomic classification of the included species — compare with the botanical classification of oaks: Willow and Shingle oaks are in the red oak group, and Southern live oak in the white oak group. It is instead based on useful practical identification points. As such, it is typical of keys used for field identification of species, as found in field guides and other similar texts.

[edit] Taxonomic key

A stricter taxonomic key uses important characters for the distinction of major taxonomic subgroups within the group for the first leads, then gradually using lesser characters to break down the subgroups by their evolutionary relationships. Such keys are used in taxonomic studies. Because of convergent evolution^{[citation needed]}, superficially similar species may be separated apart early on, with superficially very different, but genetically closely related species being separated much later in the key. They may also require examination of characters not always available in the field, or characters requiring microscopic examination. Taxonomic keys are therefore not often used for field identification.

In plants, flower and fruit characters are very important for primary taxonomic classification; a taxonomic key for the above oak examples might therefore follow this format:

1. Styles short; acorns mature in 6 months, sweet or slightly bitter, inside of acorn shell hairless (Quercus sect. Quercus, white oaks): 2

1. Styles long, acorns mature in 18 months, very bitter, inside of acorn shell woolly (Quercus sect. Lobatae, red oaks): 5

2. Leaves evergreen: 3

2. Leaves not evergreen: 4

3. Mature plant a large tree — Southern live oak Quercus virginiana

3. Mature plant a small shrub — Dwarf live oak Quercus minima

4. Leaves with 5-9 deep lobes — White oak Quercus alba

4. Leaves with 21-27 shallow lobes — Swamp chestnut oak Quercus prinus

5. Leaves usually without teeth or lobes: 6

5. Leaves usually with teeth or lobes: 7

6. Leaf narrow, about 4-6 times as long as broad — Willow oak Quercus phellos

6. Leaf broad, about 2-3 times as long as broad — Shingle oak Quercus imbricaria

7. Leaves mostly with 3 lobes — Blackjack oak Quercus marilandica

7. Leaves mostly with 7-9 lobes — Northern red oak Quercus rubra

The concept here is that by concentrating on individual reliable characters, any form of life can ultimately be deduced as to its proper species. Sometimes keys can rely on obvious macroscopic criteria, as in the above example, but often they are forced to rely on obscure or even microscopic characteristics.

Other different keys may also be offered for the same group of organisms. When distinguishing between woody plants, a guide may offer separate keys for summer and winter use. Some other keys may use habitat preference to separate species.

[edit] Keys versus descriptions

An identification key is only a guide to what a plant, animal or fossil might be. Full identification requires comparison with a full and accurate description preferably in a monograph. This is often difficult in practise; many monographs being expensive, hard to obtain, and often out of print. Many are in foreign languages; Russian, German and Japanese being the most common and the most difficult for those with English as a first language. Many monographs are more than fifty years old and do not include all species.^{[citation needed]}

[edit] Characteristics of good keys

The description (especially of the species where determination may be of critical importance) should be diagnostic and differential.
A diagnostic character is one that is common to all members of the group and is unique to that group.
A differential character is one which separates one group from another.
Some characters which are neither differential nor diagnostic may be used in the description.

[edit] Common errors and problems

A high error rate in identification renders much ecological and both pure and applied biological work virtually worthless. This may be of great importance as in the case of medical, pest control or forensic work. Error rates in insect identification are discussed by Steve Marshall [1].

[edit] Errors

Distribution is not a character. Just because a species is not listed for a region does not mean that it does not occur there, and can be therefore be discounted. Very few regions have complete lists. Also, the organism may have been transported, particularly to locations near ports and airports. It may also have a new range due to global warming. Moreover, new indoor species are being imported all the time. For Europe and, probably, North Africa a Palaearctic key is advisable.
Rarity is not a character. An identification may be correct even though a species is very rare.
Many keys contain brief descriptions to allow more certain identification. These should not be used as an alternative to full descriptions.
Some keys identify only males (or, more rarely females). Keys to larvae usually work only for the final instar. There are important exceptions however, some larval Diptera of forensic relevance being notable exceptions.
Some keys exclude difficult groups. Such exclusions are usually mentioned in the title, but may only be mentioned in the text.

[edit] Problems

Lighting and magnification - very few keys give details of how the specimen was viewed (the magnification, lighting system, angle of view etc.).^{[citation needed]} This can cause problems. The author may, for instance refer to tiny bristles, hairs or chaetae--but how tiny?.
Language - almost no keys are multilingual and translations may be incorrect or misleading. Many keys contain vague words that do not translate.^{[citation needed]}
Hopeless keys - some keys are chaotic and do not work.^{[citation needed]}
Out of date keys present a problem since they do not include more recently described species. Also, in such works the nomenclature is out of date, causing problems of synonymy.