A 'key' in Biology is a modeling method used for categorizing species using logical choices. A single-access key (Dichotomous key also called "sequential key", "analytical key", or "pathway key") is a key where the sequence and structure of identification steps is fixed by the author of the key. At each point in the decision process, multiple alternatives are offered, each leading to a result or a further choice. The alternatives are commonly called "leads", the set of leads at a given point a "couplet".

Single access keys are closely related to decision tree

Decision tree

A decision tree is a decision support tool that uses a tree-like graph or model of decisions and their possible consequences, including chance event outcomes, resource costs, and utility. It is one way to display an algorithm. Decision trees are commonly used in operations research, specifically...

s or self-balancing binary search tree

Self-balancing binary search tree

In computer science, a self-balancing binary search tree is any node based binary search tree that automatically keeps its height small in the face of arbitrary item insertions and deletions....

s. However, to improve the usability and reliability of keys, many single-access keys incorporate reticulation

Reticulation (single-access key)

In biology, a reticulation of a single-access identification key connects different branches of the identification tree to improve error tolerance and identification success...

, changing the tree structure into a directed acyclic graph

Directed acyclic graph

In mathematics and computer science, a directed acyclic graph , is a directed graph with no directed cycles. That is, it is formed by a collection of vertices and directed edges, each edge connecting one vertex to another, such that there is no way to start at some vertex v and follow a sequence of...

. Single-access keys have been in use for several hundred years . They may be printed in various styles (e. g., linked, nested, indented, graphically branching

Branching identification key

A branching identification key is a presentation form of a single-access key where the structure of the decision tree is displayed graphically as a branching structure, involving lines between items...

) or used as interactive, computer-aided keys. In the latter case, either a longer part of the key may be displayed (optionally hyperlinked), or only a single question may be displayed at a time.

If the key has several choices it is described as polychotomous or polytomous. If the entire key consists of exactly two choices at each branching point, the key is called dichotomous. The majority of single-access keys are dichotomous.

Diagnostic ('artificial') versus synoptic ('natural') keys

Any single-access key organizes a large set of items into a structure that breaks them down into smaller, more accessible subsets, with many keys leading to the smallest available classification unit (a species

Species

In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

 or infraspecific taxon). However, a trade-off exists between keys that concentrate on making identification most convenient and reliable (diagnostic keys), and keys which aim to reflect the scientific classification of organisms (synoptic keys). The first type of keys limits the choice of characteristics to those most reliable, convenient, and available under certain conditions. Multiple diagnostic keys may be offered for the same group of organisms: Diagnostic keys may be designed for field (field guide

Field guide

A field guide is a book designed to help the reader identify wildlife or other objects of natural occurrence . It is generally designed to be brought into the 'field' or local area where such objects exist to help distinguish between similar objects...

s) or laboratory use, for summer or winter use, and they may use geographic distribution or habitat

Habitat (ecology)

A habitat is an ecological or environmental area that is inhabited by a particular species of animal, plant or other type of organism...

 preference of organisms as accessory characteristics. They do so at the expense of creating artificial groups in the key.

An example of a diagnostic key is shown below. It is not based on the taxonomic

Taxonomy

Taxonomy is the science of identifying and naming species, and arranging them into a classification. The field of taxonomy, sometimes referred to as "biological taxonomy", revolves around the description and use of taxonomic units, known as taxa...

 classification of the included species — compare with the botanical classification of oaks.

In contrast, synoptic keys follow the taxonomic

Taxonomy

Taxonomy is the science of identifying and naming species, and arranging them into a classification. The field of taxonomy, sometimes referred to as "biological taxonomy", revolves around the description and use of taxonomic units, known as taxa...

 classification as close as possible. Where the classification is already based on phylogenetic studies, the key represents the evolutionary relationships within the group. To achieve this, these keys often have to use more difficult characteristics, which may not always be available in the field, and which may require instruments like a hand lens or microscope. Because of convergent evolution

Convergent evolution

Convergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action. Although their last common ancestor did not have wings, both birds and bats do, and are capable of powered flight. The wings are...

, superficially similar species may be separated early in the key, with superficially different, but genetically closely related species being separated much later in the key. Synoptic keys are typically found in scientific treatments of a taxonomic group ("monographs"). Despite being less useful for field identification

An example of a synoptic key (corresponding to the diagnostic key shown below) is shown further below. In plant

Plant

Plants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...

s, flower

Flower

A flower, sometimes known as a bloom or blossom, is the reproductive structure found in flowering plants . The biological function of a flower is to effect reproduction, usually by providing a mechanism for the union of sperm with eggs...

 and fruit

Fruit

In broad terms, a fruit is a structure of a plant that contains its seeds.The term has different meanings dependent on context. In non-technical usage, such as food preparation, fruit normally means the fleshy seed-associated structures of certain plants that are sweet and edible in the raw state,...

 characteristics often are important for primary taxonomic classification:

Example of a diagnostic dichotomous key for some eastern United States United States The United States of America is a federal constitutional republic comprising fifty states and a federal district...  oak Oak An oak is a tree or shrub in the genus Quercus , of which about 600 species exist. "Oak" may also appear in the names of species in related genera, notably Lithocarpus... s based on leaf Leaf A leaf is an organ of a vascular plant, as defined in botanical terms, and in particular in plant morphology. Foliage is a mass noun that refers to leaves as a feature of plants....  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 Southern live oak Quercus virginiana, also known as the southern live oak, is a normally evergreen oak tree native to the southeastern United States. Though many other species are loosely called live oak, the southern live oak is particularly iconic of the Old South....  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 Willow Oak Quercus phellos is a deciduous tree in the red oak group of oaks. It is native to eastern North America from southern New York south to northern Florida, and west to southernmost Illinois and eastern Texas...  Quercus phellos 4. Leaf broad, about 2-3 times as long as broad — Shingle oak Shingle Oak Quercus imbricaria, the Shingle Oak, is a deciduous tree in the red oak group of oaks. It is native primarily to the Midwestern and Upper South regions of North America, from southern New York west to northern Illinois and eastern Kansas, and south to central Alabama and Arkansas...  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 Blackjack oak Quercus marilandica is a small oak, one of the red oak group Quercus sect. Lobatae, but fairly isolated from the others...  Quercus marilandica 6. Leaves mostly with 7-9 lobes — Northern red oak Northern Red Oak Quercus rubra, commonly called northern red oak or champion oak, , is an oak in the red oak group . It is a native of North America, in the northeastern United States and southeast Canada...  Quercus rubra 7. Leaves with 5-9 deep lobes — White oak White oak Quercus alba, the white oak, is one of the pre-eminent hardwoods of eastern North America. It is a long-lived oak of the Fagaceae family, native to eastern North America and found from southern Quebec west to eastern Minnesota and south to northern Florida and eastern Texas. Specimens have been...  Quercus alba 7. Leaves with 21-27 shallow lobes — Swamp chestnut oak Swamp Chestnut Oak Quercus michauxii, the swamp chestnut oak, is a species of oak in the white oak section Quercus section Quercus, native to bottomlands and wetlands in the southern and central United States, from New Jersey south to northern Florida, and west to Missouri and eastern Texas; it is rare north of the...  Quercus prinus This key first differentiates between oaks with entire leaves with normally smooth margins (live oak Live oak Live oak , also known as the southern live oak, is a normally evergreen oak tree native to the southeastern United States... s, Willow oak Willow Oak Quercus phellos is a deciduous tree in the red oak group of oaks. It is native to eastern North America from southern New York south to northern Florida, and west to southernmost Illinois and eastern Texas... , Shingle oak Shingle Oak Quercus imbricaria, the Shingle Oak, is a deciduous tree in the red oak group of oaks. It is native primarily to the Midwestern and Upper South regions of North America, from southern New York west to northern Illinois and eastern Kansas, and south to central Alabama and Arkansas... ), and other oaks with lobed or toothed leaves. The following steps created smaller and smaller groups (e. g., red oak, white oak), until the species has been keyed out.

Example of a synoptic (taxonomic) dichotomous key for some eastern United States United States The United States of America is a federal constitutional republic comprising fifty states and a federal district...  oak Oak An oak is a tree or shrub in the genus Quercus , of which about 600 species exist. "Oak" may also appear in the names of species in related genera, notably Lithocarpus... s, reflecting taxonomic classification 1. Styles short; acorn Acorn The acorn, or oak nut, is the nut of the oaks and their close relatives . It usually contains a single seed , enclosed in a tough, leathery shell, and borne in a cup-shaped cupule. Acorns vary from 1–6 cm long and 0.8–4 cm broad... s 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 Southern live oak Quercus virginiana, also known as the southern live oak, is a normally evergreen oak tree native to the southeastern United States. Though many other species are loosely called live oak, the southern live oak is particularly iconic of the Old South....  Quercus virginiana 3. Mature plant a small shrub — Dwarf live oak Quercus minima 4. Leaves with 5-9 deep lobes — White oak White oak Quercus alba, the white oak, is one of the pre-eminent hardwoods of eastern North America. It is a long-lived oak of the Fagaceae family, native to eastern North America and found from southern Quebec west to eastern Minnesota and south to northern Florida and eastern Texas. Specimens have been...  Quercus alba 4. Leaves with 21-27 shallow lobes — Swamp chestnut oak Swamp Chestnut Oak Quercus michauxii, the swamp chestnut oak, is a species of oak in the white oak section Quercus section Quercus, native to bottomlands and wetlands in the southern and central United States, from New Jersey south to northern Florida, and west to Missouri and eastern Texas; it is rare north of the...  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 Willow Oak Quercus phellos is a deciduous tree in the red oak group of oaks. It is native to eastern North America from southern New York south to northern Florida, and west to southernmost Illinois and eastern Texas...  Quercus phellos 6. Leaf broad, about 2-3 times as long as broad — Shingle oak Shingle Oak Quercus imbricaria, the Shingle Oak, is a deciduous tree in the red oak group of oaks. It is native primarily to the Midwestern and Upper South regions of North America, from southern New York west to northern Illinois and eastern Kansas, and south to central Alabama and Arkansas...  Quercus imbricaria 7. Leaves mostly with 3 lobes — Blackjack oak Blackjack oak Quercus marilandica is a small oak, one of the red oak group Quercus sect. Lobatae, but fairly isolated from the others...  Quercus marilandica 7. Leaves mostly with 7-9 lobes — Northern red oak Northern Red Oak Quercus rubra, commonly called northern red oak or champion oak, , is an oak in the red oak group . It is a native of North America, in the northeastern United States and southeast Canada...  Quercus rubra

Structural variants of single-access keys

The distinction between dichotomous (bifurcating) and polytomous (multifurcating) keys is a structural one, and identification key software may or may not support polytomous keys. This distinction is less arbitrary than it may appear. Allowing a variable number of choices is disadvantageous in the nested display style, where for each couplet in a polytomous key the entire key must be scanned to the end to determine whether more than a second lead may exist or not. Furthermore, if the alternative lead statements are complex (involving more than one characteristic and possibly "and", "or", or "not"), two alternative statements are significantly easier to understand than couplets with more alternatives. However, the latter consideration can easily be accommodated in a polytomous key where couplets based on a single characteristic may have more than two choices, and complex statements may be limited to two alternative leads.

Another structural distinction is whether only lead statements or question-answer pairs are supported. Most traditional single-access keys use the "lead-style", where each option consists of a statement, only one of which is correct. Especially computer-aided keys occasionally use the "question-answer-style" instead, where a question is presented with a choice of answers. The second style is well known from multiple choice

Multiple choice

Multiple choice is a form of assessment in which respondents are asked to select the best possible answer out of the choices from a list. The multiple choice format is most frequently used in educational testing, in market research, and in elections-- when a person chooses between multiple...

 testing and therefore more intuitive for beginners. However, it creates problems when multiple characteristics need to be combined in a single step (as in "Flower red and spines present" versus "Flowers yellow to reddish-orange, spines absent").

Lead style 1. Flowers red ... 2 Flowers white ... 3 Flowers blue ... 4

Question-answer-style 1. What is the flower color? - red ... 2 - white ... 3 - blue ... 4

Presentation styles

Single-access keys may be presented in different styles. The two most frequently encountered styles are the

• Nested style in which all couplets immediately follow their lead, at the expense of separating the leads within a couplet.
  • The most frequent subtype of nested keys are called "indented key", where indentation increases with each level. With a large key this can lead to much whitespace in print, and consequently little remaining room for lead text and illustrations. Although "indented key" is sometimes used as a synonym for nested key, the indentation itself is not an essential feature of a nested key. (Examples of non-hyperlinked, indented nested keys may be found at www.env.gov.bc.ca)

• Linked style: The leads within a couplet immediately follow each other, making polytomous keys easy to achieve. At the end of each lead some form of pointer (a numbering system, hyperlinks, etc.) create the connection to the couplets that follow this lead.

The nested style gives an excellent overview over the structure of the key. With a short key and moderate indentation it can be easy to follow and even backtrace an erroneous identification path. The nested style is problematic with polytomous keys, where each key must be scanned to the end to verify that no further leads exist within a couplet. It also does not easily support reticulation (which requires a link method similar to the one used in the linked style).

Advantages and disadvantages

A large amount of knowledge about reliable and efficient identification procedures may be incorporated in good single-access keys. Characteristics that are reliable and convenient to observe most of the time and for most species (or taxa), and which further provide a well-balanced key (the leads splitting number of species evenly) will be preferred at the start of the key. However, in practice it is difficult to achieve this goal for all taxa in all conditions. If the information for a given identification step is not available, several potential leads must be followed and identification becomes increasingly difficult.

Although software exists that helps in skipping questions in a single-access key, the more general solution to this problem is the construction and use of multi-access key

Multi-access key

In biology or medicine, a multi-access key is an identification key which overcomes the problem of the more traditional single-access keys of requiring a fixed sequence of identification steps. A multi-access key enables the user to freely choose the set and characteristics that are convenient to...

s, allowing a free choice of identification steps and are easily adaptable to different taxa (e.g., very small or very large) as well as different circumstances of identification (e. g., in the field or laboratory).