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Classification of Living Things - Dichotomous Keys

Grade 6IB

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Classification is the process of grouping living things based on shared observable characteristics. This field is known as Taxonomy.

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A Dichotomous Key is a biological tool used to identify unknown organisms. The term 'dichotomous' means 'divided into two parts,' referring to the way the key provides two contrasting choices at each step.

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The hierarchy of classification follows the order: Domain, Kingdom, Phylum, Class, Order, Family, Genus, and Species (D−K−P−C−O−F−G−SD-K-P-C-O-F-G-S).

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Binomial Nomenclature is the formal system of naming species. Each name consists of the Genus\textit{Genus} (capitalized) and the species\textit{species} (lowercase). Both are traditionally written in italics.

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A dichotomous key can be presented as a branching flow diagram or as a series of numbered paired statements.

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Characteristics used in a key must be objective and observable (e.g., 'has 66 legs') rather than subjective (e.g., 'is beautiful').

📐Formulae

Scientific Name=Genus+species\text{Scientific Name} = \textit{Genus} + \textit{species}

Total Steps in a Key=n−1 (where n is the number of organisms to be identified)\text{Total Steps in a Key} = n - 1 \text{ (where } n \text{ is the number of organisms to be identified)}

Binary Choice∈{Option A,Option B}\text{Binary Choice} \in \{ \text{Option A}, \text{Option B} \}

💡Examples

Problem 1:

Use the following dichotomous key to identify an organism that has a backbone, does not have gills, and has feathers: 1a. Has a backbone... Go to 22 1b. No backbone... Invertebrate 2a. Has gills... Fish 2b. No gills... Go to 33 3a. Has feathers... Organism X\text{Organism X} 3b. No feathers... Mammal

Solution:

Organism X is a Bird (Aves).

Explanation:

Starting at step 11, the organism has a backbone, so we move to step 22. At step 22, it does not have gills, directing us to step 33. At step 33, the presence of feathers identifies it as a Bird.

Problem 2:

Identify the correctly formatted scientific name for a domestic cat if the genus is Felis\textit{Felis} and the species is catus\textit{catus}.

Solution:

Felis catus\textit{Felis catus}

Explanation:

According to the rules of binomial nomenclature, the Genus must be capitalized (FF), the species must be lowercase (cc), and the entire name must be italicized: Felis catus\textit{Felis catus}.

Problem 3:

If you are designing a dichotomous key for 88 different species of insects, what is the minimum number of steps required to ensure every insect can be uniquely identified?

Solution:

77 steps.

Explanation:

Using the formula n−1n - 1, where n=8n = 8, the calculation is 8−1=78 - 1 = 7. A complete key always requires one fewer step than the number of items being classified to reach a final identification for all.