Taxonomic Keys: Meaning, Suggestions and Types

The below mentioned article provides an overview on Taxonomic Keys.

Meaning of Taxonomic Key:

A key is a device, which when properly constructed and used, enables a user to identify an organism. Keys are devices consisting of a series of contrasting or contradictory statements or propositions requiring the identifier to make comparisons and decisions based on statements in the key as related to the material to be identified.

Thus, a taxonomic key is a device for quickly and easily identifying to which species an unknown plant belongs.

The key consists of a series of choices, based on observed features of the plant specimen. It provides a choice between two contradictory statements resulting in the acceptance of one and the rejection of the other.

A single pair of contradictory statements is called a couplet and each statement of a couplet is termed a lead. By making the correct choice at each level of the key, one can eventually arrive at the name of the unknown plant.

Suggestions for the Use of Taxonomic Keys:

(a) Appropriate keys should be selected for the materials to be identified. The keys may be in a flora, manual, guide handbook, monograph, or revision. If the locality of an unknown plant is known, a flora, guide, or manual treating the plants of that geographic area may be selected.

If the family or genus is recognized, one may choose to use a monograph or revision. However, if the locality is unknown a general work may be selected. If materials to be identified are cultivated, one of the manuals treating such plants should be selected, since most floras do not include cultivated plants unless naturalized.

(b) The introductory comments on format details, abbreviations, etc. should be read before using the key.

(c) Both the leads of a couplet should be read before making a choice. Even though the first lead may seem to describe the unknown material, the second lead may be even more appropriate.

(d) A glossary should be used to check the meaning of terms, which one does not understand.

(e) Several similar structures should be measured, when measurements are used in the key, e.g. several leaves and not a single leaf should be measured. No decision should be made on a single observation. Rather it is often desirable to examine several specimens.

(f) The results should be verified by reading a description, comparing the specimen with an illustration or an authentically named herbarium specimen.

Suggestion for Construction of Taxonomic Keys:

(a) Constant characteristics rather than variable ones should be used.

(b) Proper measurements rather than terms like “large” and “small” should be used.

(c) Characteristics that are generally available to the user of the key rather than seasonal characteristics or those seen only in the field should be used.

(d) A positive choice should be made. The term “is” instead of “is not” should be used.

(e) If possible one should start both choices of a pair with the same word.

(f) If possible, different pairs of choices should be started with different words.

(g) The descriptive terms should be preceded with the name of the part to which they apply.

Types of Taxonomic Keys:

There are two types of keys:

(a) Dichotomous and

(b) Poly clave (also called Multiple Access or Synoptic).

i. Dichotomous Keys:

Keys in which the choices allow only two (mutually exclusive) alternative couplets are known as dichotomous keys. In constructing a key, contrasting characters are chosen that divide the full set of possible species into smaller and smaller groups i.e. the statements typically begin with broad characteristics and become narrower as more choices are required.

Each time a choice is made, a number of species are eliminated from consideration and the range of possible species to which the unknown specimen may belong is narrowed. Eventually, after sufficient choices have been made, their range reduces to a single species and the identity of the unknown plant is revealed. Dichotomous comes from the Greek root dich meaning “two” and temnein meaning “to cut“.

Couplets can be organized in several forms. The couplets can be presented using numbers (numeric) or using letters (alphabetical). The couplets can be presented together or grouped by relationships. There is no apparent uniformity in presentation of dichotomous keys.

(a) Types of Dichotomous Keys:

There are two types of dichotomous keys. They differ in the method by which the couplets are organized and how the user is directed to successive choices.

(i) Indented Keys (also called yoked):

Indents the choices (leads) of the couplet an equal distance from the left margin. The two choices of the couplet are usually labelled 1 and 1′ or la and lb. It is not necessary that the choices are numbered, but it helps. The user goes to the next indented couplet following the lead that was selected.

(ii) Bracketed Keys:

Provides both choices side-by-side. The choices of the couplet must be numbered (or lettered). It is very helpful if the previous couplet is given. This key has exactly the same choices as the first example. The choices are separated, but it is easy to see the relationships. While this key might be more difficult to construct, it gives more information to the user.

(b) Problems using Dichotomous Keys:

A key may be difficult to use at times because:

I. The key may not include all potential variations in the species;

II. The key may rely on features not present in that season;

III. The key may not include “all” species of interest;

IV. One may misinterpret a feature or make a mistake.

ii. Poly Clave Keys:

Another type of key, which is relatively a new alternative to dichotomous keys and becoming increasingly popular, especially because of the ease of computerizing them, is termed multiple access or poly clave or synoptic key. The advantage of these keys is that they allow the user to enter the key at any point.

This key is based on the identification of organisms by a process of elimination. In a written poly clave key there is a series of characters and character states. Each state is followed by a number or code for the species that possess that feature.

The user needs to select any character and then copy down the list of species that possess the feature. Then the user has to select another character and eliminate any species that is not common to both lists. This process has to be continued until the specimen is identified.

It’s easy to imagine how these keys are computerized. Consider a series of standard playing cards. Imagine each card has four holes punched into it along the top margin. If the card is a spade we cut the first hole through the margin; if a club, the second hole is notched to the margin; a heart the third hole is notched; and finally if it is a diamond the fourth hole is notched.

Further imagine that along the bottom of the card we punch 14 holes (2 – 10, J, K, Q, A) and cut a notch for the appropriate number. Thus, the Queen of Hearts will have a notch cut into the third hole on the top of the card, and the Queen notch on the bottom of the card.

Now, let’s use our punch card deck of cards to identify an unknown card. Shuffle another deck of cards and pick any card. Let’s assume that this “unknown” card is the Ace of Spades. To identify this unknown, we analyse the characters and two are obvious, suit and number.

Let’s start with suit – take a long needle and stick it through the “spades hole“. Since, only spades are notched, the other suits will remain on the needle and spades will drop out of the deck.

Now, collect the spades cards and put a needle through the next character, the Ace and, viola, the Ace of Spades falls out. This is the general principle of how the computerized version of poly clave keys work. The main difference is that a computer allows for countless holes (characters) and notches (states) to be included and does the needlework for us.

An example of a Poly clave key:

Poly clave Key to Pollination Type:

Pollination is the process of transferring pollen from one flower to another. Since plants can’t move, they utilize vectors such as wind, water and animals to accomplish this process for them. Flowers are specialized by shape, colour, door, nectar etc. in order to maximize the chance that a certain vector will accomplish pollination. These flower adaptations are collectively known as pollination syndromes or systems.

Plants differ in the degree of their specialization for a particular pollination system. For example, many orchids are pollinated by only a single type of bee.

Other flowers are not as specialized and may be pollinated by a variety of bees or perhaps beetles. In other cases, insects may visit flowers without actually transferring pollen. These factors make it difficult to determine with absolute certainty the pollination system by the poly clave key.

(a) Advantages of Poly clave Keys:

The advantages of a poly clave (multiple-access) key are:

I. They are easy to use.

II. They allow multi-entry i.e. the user can start anywhere. This is a significant advantage because the user can rely on characters that are most easy to observe, rather than having to deal with characters that may not be present in the specimen or are poorly developed.

III. They are order-free i.e. the user can work in any direction with any character.

IV. They are faster.

V. They are easily computerized. In fact, these keys are most commonly used in this form. Paper versions are typically large and unwieldy because each character needs to list all possible taxa.

(b) Suggestions to use a Poly clave Key:

I. Read through the list of characters to become familiar with the possibilities.

II. Scan the list to find a character with a state that you observe in your specimen. Start with a readily identifiable character that has only a few numbers (taxa) associated with it.

III. Write a brief description of the character and state and the numbers of the taxa that can be described by this state.

IV. Choose another character and state that describes your taxa. Write a brief description of this state below the name of the first state chosen. Then, scratch off the original list of any taxon that doesn’t appear in the second.

V. Continue this process until just one taxon remains for all of the states. If there is no single taxon described by the states chosen, and two or more remain, go back and check for errors.

VI. Read the name of the taxon after its number in the list of taxa. Check your identification with a description in a manual or the herbarium.