Dichotomous Key: Principle, Guidelines, Types, Examples, Uses

Within the vast expense of biological diversity, an estimated 100 million species await discovery alongside the 1.5 million already cataloged. To accurately identify these organisms and classify them, various tools have been developed.

Among these tools, the dichotomous key also called descriptive key or descriptive table, is a widely used method, which was pioneered by the renowned French naturalist Jean-Baptiste Lamarck in the late 18th century.

Derived from the Greek word “di” for “two” and “tome” for “cutting instrument”, a dichotomous key means dividing into two parts.

It operates on the principle of offering two contrasting choices at each step to progressively narrow down possible identification. Since the choice is made from two options, it is termed as dichotomous, if more it is called polychotomous. A polychotomous key uses similar question answers in a branching format but with more than two choices.

Dichotomous Key
Dichotomous Key

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Definition of Dichotomous Key

A dichotomous key is a systematic method of identifying patterns through contrasting statements, primarily focusing on physical characteristics. It aids researchers in determining the identity of organisms by presenting a series of choices leading to correct classifications, often emphasizing scientific names of closely related species. 

In the field of science, the dichotomous key serves as a crucial instrument in the classification of organisms, offering a structured approach to identification. By presenting paired statements or questions, this tool directs researchers toward accurate identification of various life forms, ranging from trees and wildflowers to mammas and rocks.

Principle of Dichotomous Key

The dichotomous key operates in binary principle, presenting users with two mutually exclusive characteristics at each step, guiding them through successive choices until final identification.

Why use the dichotomous key:

Purpose:

  • A dichotomous key serves as an essential scientific tool for identifying organisms based on their observable traits.
  • It allows for identifying and categorizing organisms, facilitating comprehension of complex scientific concepts, and organizing vast amounts of information to streamline organism identification.
  • Beyond the identification of plant and animal species, it can also be applied to mineral identification.
  • Their versatility and applicability across various disciplines make them an invaluable tool in scientific studies.

Guidelines for creating a Dichotomous key

1. Gather information

Begin by collecting data about organisms or items you wish to identify. Understand their key characteristics and how they differ from each other.

2. Note down the characteristics

Pay close attention to the specimens you need to identify with your dichotomous key. Record the observable characteristics that can be used for differentiation.

3. Arrange characteristics in order

Organize the characteristics from the most general to the most specific. This helps in narrowing down options as you progress through the key.

4. Divide specimens

Utilize binary choices, such as statements or questions, to divide your specimens into two groups based on the most general characteristic.

5. Try for subdivision

Based on contrasting characteristics, further divide the specimen into more specific categories. Continuously ask questions to refine the classification.

6. Depict a Dichotomous Key diagram

Choose between a text-based or graphical representation for your key. You can include images of the specimens for clarity.

7. Create binary choices

Ensure that each step of your dichotomous key offers two mutually exclusive choices. Clearly state these choices based on the identified characters.

8. Test your key

Use your dichotomous key to identify known organisms or objects to identify their effectiveness. Adjust as necessary to improve accuracy.

9. Refine and expand

Continuously refine and expand your key as you encounter new species or different organisms. Ensure that it remains comprehensive and adaptable to accommodate additions.

By following these steps one can create effective dichotomous key for identifying various organisms based on their distinct characteristics.

Types of Dichotomous Keys

Dichotomous keys, utilized for identifying organisms, come in various types, each with a distinct way of presenting information:

Branched Style

This type employs a tree diagram to graphically display statements, with each offering two choices leading to different answers. It is a “branching” or “tree” layout of a dichotomous key. It is suited for identifying organisms one at a time, as the tree cannot branch downwards from both sides simultaneously. Each question begins a new branch of a tree with the subsequent question being a sub-branch.

Nested Style

It is a simple and easy method where the two organisms are classified simultaneously. In the nested display, the question or each statement is nested under the answer they are leading to often achieved by using indentations for clarity.

Computer Program

As handheld computers become more prevalent, identification keys are increasingly being converted into the form of computer programs. This method provides users with the optimal opportunity to answer questions one at a time and automatically prompts the correct follow-up question. Like popular online quizzes, users can achieve accurate species identification by responding to a series of either/or questions.

Linked Style

In this style of a dichotomous key, questions and their corresponding answers are presented in list form. Each answer guides the user to the next relevant question, necessitating users to locate the correct question for accurate organism identification.

How to Use a Dichotomous Key

  • A dichotomous key is a tool for identifying unknown items or organisms, with each step offering two choices based on characteristics.
  • Exercise caution and consider both choices in each step to avoid misclassification.
  • Take time to understand the terms used in the key by referring to dictionaries or other resources.
  • Familiarize yourself with examples of characteristics mentioned in the key to making informed decisions. 
  • Avoid guessing and ensure accuracy by using appropriate measuring tools for items with provided measurements.
  • Compare the descriptions of identified items carefully, especially if left with two possible answers, to ensure a proper match.

Best Practices:

  • Focus on evaluating one characteristic at a time for clarity.
  • Prioritize observable characteristics, especially major ones, for initial divisions and use less obvious characteristics for further subdivisions.
  • Maintain consistency in the format of contrasting statements and be specific while avoiding repetition.
  • Frame questions to elicit yes or no answers rather than statements for clarity and precision.

Examples of Dichotomous Key

Example objects to identify apple tree, water-lily, fir tree, dandelion, astroturf, seaweed.

• 1. a. found in water …………………………………………………………. go to 2

• 1. b. found on land ………………………………………………………… go to 3

• 2. a. grows in salt water ………………………………………………… seaweed

• 2. b. does not grow in salt water ……………………………………… water-lily

• 3. a. a real plant ………………………………………………………….. go to 4

• 3. b. not a real plant ……………………………………………………. astroturf

• 4. a. grows more than 50 m tall ………………………………………… fir tree

• 4. b. grows less than 50 m tall …………………………………………….. go to 5

• 5. a. produces yellow flowers ………………………………………. dandelion

• 5. b. does not produce yellow flowers ……………………………… apple tree

Or the couplets may be grouped like this:

• 1. a. found in water ………………………………………………………… 2

• 2. a. grows in salt water ………………………………………….. seaweed

• 2. b. does not grow in salt water …………………………….. water-lily

• 1. b. found on land ………………………………………………………….. 3

• 3. a. real plant ……………………………………………………………… 4

• 4. a. grows more than 50 m tall ………………………………………. fir tree

• 4. b. grows less than 50 m tall ………………………………………………… 5

• 5. a. produces yellow flowers …………………………………………. dandelion

• 5. b. does not produce yellow flowers…………………………………. apple tree

• 3.b. not a real plant ……………………………………………… astroturf

(https://kpu.pressbooks.pub/plant-identification/chapter/introduction-to-dichotomous-keys/)

Publicly available examples of dichotomous keys can be found here:

Benefits of Dichotomous Keys

Dichotomous keys offer numerous advantages across scientific and educational domains, simplifying the process of identification and classification. Here’s why they are beneficial:

  • Accurate Identification: Dichotomous keys provide a systematic approach to identifying organisms or objects by focusing on key characteristics, reducing the risk of misidentification.
  • Ease of Use: Designed to be user-friendly, dichotomous keys can be utilized by individuals of varying expertise levels, from students to experts.
  • Consistency: These keys adhere to a standardized format, employing specific terminology and criteria, ensuring consistent identification outcomes across different users.
  • Educational Value: Dichotomous keys aid in teaching students how to observe, compare, and comprehend the characteristics of different organisms, facilitating a deeper understanding of the natural world.
  • Research Utility: In scientific research, dichotomous keys are instrumental in cataloging and documenting biodiversity, contributing to the comprehension of species relationships and ecological dynamics.
  • Fieldwork Facilitation: Portable and adaptable for field use, dichotomous keys are invaluable for on-site surveys, ecological studies, and biodiversity assessments.
  • Simplicity and Accessibility: By breaking down complex identification tasks into binary choices, dichotomous keys make the process accessible to both experts and novices, streamlining identification efforts.
  • Guidance for Non-Experts: Particularly beneficial for non-expert users, dichotomous keys direct individuals to examine known, significant organisms, aiding in accurate identification even for those unfamiliar with the subject matter.
  • Enhanced Understanding: Through structured questioning and informative feedback, dichotomous keys promote enhanced understanding and engagement, fostering deeper knowledge of the characteristics and traits of various organisms.
  • Efficiency in Identification: By organizing characteristics methodically, dichotomous keys facilitate efficient and systematic identification processes, saving time and resources in both educational and research settings.

Obstacles of Dichotomous Keys

Using keys for identification purposes can present several challenges, including:

  • Limited distinguishing forms: Keys may only identify certain forms of the same species, such as males or specific instars of larvae, limiting their applicability.
  • Incomplete coverage of the characteristics: Some species or groups, particularly those that are difficult to define or poorly studied, may be absent from the key or mentioned only briefly.
  • Differing observation conditions: A few keys provide information about necessary observation conditions like lighting or magnification, potentially causing issues in accurately assessing certain characteristics.
  • Language as a Barrier: Many keys are available in only one language, and translations may be inaccurate or incomplete, leading to confusion.
  • Obsolescence: Older keys may lack information about recently described species and may contain outdated species names.

Additionally, using a dichotomous key can sometimes result in none of the choices being correct, as the clues are based on general information that may not apply to every individual specimen.

Summary (Key Takeaways)

  • Dichotomous key is the most employed classification method.
  • Dichotomous keys excel in identifying organisms belonging to closely- related groups, though complications may arise when similar species inhabit the same geographic area, necessitating precise observation for accurate choices.
  • Factors such as qualitative (physical attributes) and quantitative ( number of features) considerations are taken into account during key creation.
  • Dichotomous keys can be presented graphically (branching flowcharts) or in written format (paired statements), primarily utilized for identifying plant and animal species based on observable characteristics.
  • The dichotomous key is used mainly for the identification process particularly aiding in biodiversity studies.

References

  1. Byju’s. https://byjus.com/question-answer/what-is-dichotomous-key-used-in-the-classification-and-identification-of-organisms/
  2. Dichotomous key. (2019). Biology dictionary. Retrieved from https://biologydictionary.net/dichotomous-key/
  3. Dichotomous key. (2023). Creately. Retrieved from https://creately.com/guides/what-is-a-dichotomous-key/
  4. Dichotomous Key. Oregon State University. Retrieved from https://treespnw.forestry.oregonstate.edu/dichotomous_key.html
  5. Dichotomous Keys. Herbarium. University of Wisconsin Green Bay. Retrieed from https://www.uwgb.edu/herbarium/dichotomous-keys/
  6. How to use a Dichotomous Key. Retrieved from https://www.hanovercounty.gov/DocumentCenter/View/3803/How-to-Use-a-Dichotomous-Key-PDF
  7. Hubbard, R. N. L. B. (1992). Dichotomous keys for the identification of the major Old World crops. Review of Palaeobotany and Palynology, 73(1-4), 105-115.
  8. Introduction to dichotomous keys. KPU Book press. https://kpu.pressbooks.pub/plant-identification/chapter/introduction-to-dichotomous-keys/
  9. Ramuthi, D. (2023). What is Dichotomous key and how to make one?. Retrieved from https://venngage.com/blog/dichotomous-key/#10
  10. Shipunov, A. Dichotomous Keys. Libretexts Biology. Retrieved from https://bio.libretexts.org/Bookshelves/Botany/Introduction_to_Botany_(Shipunov)/10%3A_Methods_of_Taxonomy_and_Diagnostics/10.03%3A_Dichotomous_keys
  11. Timme, S. L. (1991). How to construct and use a dichotomous key. Tested studies for laboratory teaching, 12, 218. Retrieved from https://irrec.ifas.ufl.edu/media/irrecifasufledu/teach-aquaculture-/Timme-1991-How-to-Construct-a-Dichotomous-Key.pdf
  12. What is Dichotomous key: Definition, examples. Weje. Retrieved from https://weje.io/blog/dichotomous-key

About Author

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Agrani Paudel

Agrani Paudel is doing her B.Sc. in Microbiology from St. Xavier’s College, Kathmandu, Nepal. She is also working as a President of Alumni Club, Department of Microbiology, SXC Alumni Forum, St. Xavier’s College. She did her mini thesis on the topic, "Isolation and Identification of Multi-Drug Resistant E. coli and Salmonella in Feral Pigeons’ Droppings". She also published her review on the topics, "Pre-leukemic Cell Detection and Leukemic Transformation of a Normal Marrow Cell: A Mini-Review".

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