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Encyclopedia > Dichotomous key

An identification key, also known as a dichotomous key, is a method of deducing the correct species assigment 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--68.32.19.33 21:20, 13 Jan 2005 (UTC)--68.32.19.33 21:20, 13 Jan 2005 (UTC)--68.32.19.33 21:20, 13 Jan 2005 (UTC)

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.


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, 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.


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  Results from FactBites:
 
Dichotomous Keys (528 words)
: A dichotomous key is a tool that allows the user to determine the identity of items in the natural world, such as trees, wildflowers, mammals, reptiles, rocks, and fish.
Keys consist of a series of choices that lead the user to the correct name of a given item.
Use characteristics that are generally available to the user of the key rather than seasonal characteristics or those seen only in the field.
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