DICHOTOMOUS KEY: WHAT IT IS FOR, TYPES AND CHARACTERISTICS - BIOLOGY - 2025

A dichotomous key is a tool used in taxonomy to determine the identity of biological samples. Although often used for level-specific determinations, dichotomous keys can be developed for any required taxonomic level.

Dichotomous keys are also known as unique access keys or analytical keys. In addition to being used in biology, dichotomous keys are also used in geology, mineralogy, medicine, archeology, among others.

Plant diversity. To achieve the identification of these organisms, dichotomous keys are generally required. Taken and edited from Alverson, William Surprison; Field Museum of Natural History.

They are so named because each step of the key consists of two options, each of which can give the answer to the name sought or lead to a new step with two other options. There are keys that present more than two options in each step, when this is the case they are called polytomic or polyhotomic keys.

Dichotomous key types

Preliminary considerations

Originally, taxonomists considered that the degree of similarities shared by different groups of organisms indicated the level of kinship between them, but this is not always the case.

Similar lifestyles can lead to organisms that do not share common ancestors developing similar body shapes or structures; This is what is known as evolutionary convergence.

Because of this, scientists have turned to studying organisms during the early stages of their development, when environmental pressures and lifestyle have not yet affected their shape, to better understand the possible kinship that exists between different groups.

For this, some characteristics that are difficult or impossible to observe in the field must then be used frequently, because highly specialized equipment is required, or because they are characters that are lost in the adult state.

Adult starfish

For example, adult starfish have radial symmetry, as occurs with cnidarians (corals, jellyfish, among others), however they belong to the group of bilaterial organisms (such as vertebrates, for example), because in their early stages of development have bilateral symmetry and radial symmetry is acquired as adults.

Another example is the human being, which is evolutionarily related to sea squirts, sessile invertebrates that superficially seem to be more related to sponges than to vertebrates in general.

However, both groups share common characteristics at some stage of their development, such as the presence of a notochord, a hollow dorsal nerve cord and pharyngeal branchial clefts, characteristics that are lost or strongly modified in the adult stage.

Due to this, two types of dichotomous keys arise, whether or not they try to reflect phylogenetic relationships: the diagnostic and the synoptic.

Diagnostic key

It uses characteristics present in the organisms to be taxonomically identified, regardless of whether or not these characteristics have importance from the phylogenetic point of view.

They generally contrast one or a few characters in each of the key steps.

They are useful and relatively easier to use, but can create artificial groups. For example, if we want to create a dichotomous key to work with mammals, a characteristic that would allow us to group them into two groups (both artificial) is whether they are aquatic organisms (dolphins, manatees, seals, among others) or terrestrial (cows, monkeys).

Example of a diagnostic key (simplified) for decapod crustaceans

1A.- Abdomen as or larger than the cephalothorax, ending in a caudal fan composed of telsons and uropods …………………………………………………………………….. two

1B.- Abdomen smaller than the cephalothorax, without uropods ………..…. …… crabs

2A.- Laterally compressed abdomen ……………………………………………….. …………… 3

2B.-Dorso-ventrally depressed abdomen ………………………….. …………..… locusts

3A.- Pleura of the second abdominal somite not superimposed on that of the first ……….….. …………………………………………………………………..… …………………… penaeid shrimp

3B.- Pleura of the second abdominal somite superimposed on that of the first …………………… ………………………………………………………………………… ……………. carid shrimp

In this example, the first step of the key grouped penaeid shrimp, carid shrimp and also lobsters into a single group and left the crabs in a separate group. However, carid shrimp and lobsters are more closely related to crabs than to penaeid shrimp.

Indeed, penaeid shrimp belong to the infraorder Dendrobranchiata, while carids, lobsters and crabs belong to the infraorder Pleyocemata.

Synoptic key

This attempts to adapt to the taxonomic classification, creating groups that reflect phylogenetic relationships.

They generally contrast several characters simultaneously in each of the key steps. They are more difficult to use and may be impractical for field work, however they better reflect the degrees of kinship.

Example of a synoptic key (simplified) for decapod crustaceans

1A.- Crustaceans with an abdomen larger than the cephalothorax, laterally depressed. Pleura of the second abdominal somite not superimposed on that of the first. First three pairs of legs usually chelated ………………………. …………………………….. penaeid shrimp

1B.- Crustaceans with a variable size abdomen, if it is larger than the cephalothorax and laterally depressed, the pleura of the second abdominal somite is not superimposed on that of the first and the third pair of legs is not chelated ………….… …………………………………………………. two

2A.- Abdomen larger than the cephalothorax, laterally compressed ………….. ……………… …………………………………………………………………… …………………………………… carid shrimp

2B.- Abdomen of variable size, dorso-ventrally depressed …………….. ………………. 3

3A.- Abdomen larger than the cephalothorax, with well-developed pleurae….. locusts

3B.- Abdomen smaller than the cephalothorax, with reduced or absent pleurae …….. ……. ……………………………………………………………………………………………………………………. crabs

Characteristics that a good dichotomous key must present

For a dichotomous key to be really useful it must be well constructed and, if possible, easy to understand. For this, several aspects must be taken into account, including:

-Must use specific terms and uniformly in the key, avoiding the use of words or synonymous terms to refer to the same character.

-Avoid the use of ambiguous terms such as big or small. If necessary, make comparisons with other structures; for example "last anterolateral tooth of the carapace three or more times larger than the preceding tooth."

-If possible, characters that do not depend on sex or the age of the organism should be used. Otherwise, it must be indicated in what type of organisms the indicated characteristic is observed; for example "cheipeds of unequal size in adult males."

-Avoid the use of features that overlap; for example "androecium with six to eight stamens (species 1) vs androecium with four to six stamens (species 2)".

-In each pair of alternatives the same character must be contrasted, or if several characters are used, they must all be contrasted; for example «white flowers, gamopétalas (species 1) vs red flowers, dialipetalas (species 2) 2.

Dichotomous key of decapod crustaceans. Image of the crab, taken and edited by: Jonathan Vera Caripe. Image of the lobster, taken and edited from: NOAA FishWatch. Image of peneid shrimp, taken and edited from: Yale Peabody Museum of Natural History. Image of the carid shrimp, taken and edited by: Jonathan Vera Caripe.

Precautions When Using Dichotomous Keys

When using a dichotomous key it is advisable to keep the following in mind:

-Generally the keys do not include all species. Often the keys are limited to species that have been found in the study that presents them, or in the area where the study was carried out. But the fact that a species has not previously been found in a locality does not mean that eventually it cannot be located.

-Likewise, new species of different taxonomic groups are described daily or existing species are rearranged, so the keys may become obsolete.

-If you do not understand what the key is requesting, you must avoid continuing until you have fully completed it; a wrong decision will lead to a poor determination of the identity of the material under study.

-You have to be as thorough as possible in your observations, because the fact that you cannot see a character does not mean that it is not present; perhaps you are looking in the wrong place.

-It is highly recommended to confirm the determination made by comparing the material under study with detailed descriptions of the species or taxon that was reached in the key.

References

1. Identification key. On Wikipedia. Recovered from en.wikipedia.org.
2. Types of identification keys. Recovered from keytonature.eu.
3. A. Vilches, t. Legarralde & G. Berasain (2012). Elaboration and use of dichotomous keys in biology classes. Proceedings III Conference on Teaching and Educational Research in the field of Exact and Natural Sciences. Faculty of Humanities and Education Sciences. National University of La Plata.
4. Single-access key. On Wikipedia. Recovered from en.wikipedia.org.
5. Dichotomous key. On Wikipedia. Recovered from es.wikipedia.org.
6. LG Abele & W. Kim. 1986. An illustrated guide to the marine decapod crustaceans of Florida. State of Florida, Deparment of Enviromental Regulation Technical Series.