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Characteristics and Classification of Living Organisms - Dichotomous keys

Grade 12IGCSEBiology

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

🔑Concepts

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A dichotomous key is a diagnostic tool used to identify organisms based on a sequence of choices between two alternative observable traits. The term 'dichotomous' is derived from the Greek words meaning 'divided into two parts'.

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Classification is based on morphological (shape and structure) and anatomical (internal structure) features. Modern classification also utilizes DNADNA sequencing and amino acid sequences in proteins to determine evolutionary relationships.

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The Binomial System, developed by Linnaeus, names organisms using two parts: the GenusGenus (capitalized) and the speciesspecies (lowercase). For example, the African elephant is Loxodonta africanaLoxodonta \ africana.

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Hierarchical Classification: Organisms are grouped into a hierarchy: Kingdom→Phylum→Class→Order→Family→Genus→SpeciesKingdom \rightarrow Phylum \rightarrow Class \rightarrow Order \rightarrow Family \rightarrow Genus \rightarrow Species.

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When using a dichotomous key, always start at step 1. Each step consists of a 'couplet' (two contrasting statements). Following the correct statement leads you to either the organism's name or the next step number.

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Effective keys use objective, constant features (e.g., number of legs, presence of antennae) rather than subjective or variable features (e.g., size or exact color shade).

📐Formulae

Nsteps=n−1N_{steps} = n - 1

Scientific Name=GenusCapitalized+specieslowercase\text{Scientific Name} = Genus_{Capitalized} + species_{lowercase}

%Similarity=(Identical Base PairsTotal Base Pairs)×100\% \text{Similarity} = \left( \frac{\text{Identical Base Pairs}}{\text{Total Base Pairs}} \right) \times 100

💡Examples

Problem 1:

Identify a vertebrate specimen with the following characteristics: It has scaly skin, does not have feathers, and possesses four limbs. Use the simplified key below:

  1. a) Has feathers ... Class AvesClass \ Aves b) Does not have feathers ... Go to 2
  2. a) Has scales ... Go to 3 b) Has hair ... Class MammaliaClass \ Mammalia
  3. a) Has four limbs ... Class ReptiliaClass \ Reptilia b) Has fins and no limbs ... Class PiscesClass \ Pisces

Solution:

Class ReptiliaClass \ Reptilia

Explanation:

Starting at step 1, the specimen does not have feathers (1b), so we move to step 2. At step 2, the specimen has scales (2a), so we move to step 3. At step 3, the specimen has four limbs (3a), identifying it as Class ReptiliaClass \ Reptilia.

Problem 2:

Given five species of bacteria, calculate the minimum number of couplets (NstepsN_{steps}) required to construct a functional dichotomous key.

Solution:

Nsteps=4N_{steps} = 4

Explanation:

Using the formula Nsteps=n−1N_{steps} = n - 1, where nn is the number of organisms to be identified: 5−1=45 - 1 = 4. A key for 5 species requires exactly 4 pairs of contrasting statements to isolate each species individually.

Dichotomous keys - Revision Notes & Key Diagrams | IGCSE Grade 12 Biology