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Classification - Dichotomous Keys

Grade 6Cambridge (IGCSE)

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

🔑Concepts

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Classification is the process of sorting living organisms into groups based on shared observable features and differences.

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A Dichotomous Key is a scientific tool used to identify unknown organisms. The term 'dichotomous' means 'divided into two parts.'

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The key consists of a series of numbered steps. Each step offers two contrasting choices (e.g., 'Has feathers' vs. 'Does not have feathers').

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By following the choices that match the organism's physical characteristics, the user is led to the organism's name or the next step in the key.

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Scientific names are often used in keys following the binomial system, consisting of the Genus and species (e.g., PantheraPanthera leoleo for a lion).

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Identification is primarily based on morphology (physical structure), such as leaf shape, number of legs, or presence of a backbone (VertebrataVertebrata).

📐Formulae

Magnification=Image size (I)Actual size (A)\text{Magnification} = \frac{\text{Image size (I)}}{\text{Actual size (A)}}

Leaf Area Index (LAI)=Leaf AreaGround Area\text{Leaf Area Index (LAI)} = \frac{\text{Leaf Area}}{\text{Ground Area}}

💡Examples

Problem 1:

Use the following simple dichotomous key to identify an organism that has no wings and has 8 legs:

  1. a) Has wings... Go to 2 b) No wings... Go to 3
  2. a) Wings are colorful... LepidopteraLepidoptera b) Wings are transparent... DipteraDiptera
  3. a) Has 6 legs... InsectaInsecta b) Has 8 legs... ArachnidaArachnida

Solution:

The organism is an ArachnidaArachnida.

Explanation:

Starting at Step 1, the organism has no wings, so we follow choice 1b, which directs us to Step 3. At Step 3, the organism has 8 legs, matching choice 3b, which identifies it as ArachnidaArachnida.

Problem 2:

An unknown leaf has a length (ll) of 10 cm10\text{ cm} and a width (ww) of 2 cm2\text{ cm}. A key asks if the ratio R=lwR = \frac{l}{w} is greater than 44. Should you follow the path for 'Narrow Leaf' or 'Broad Leaf'?

Solution:

Follow the path for 'Narrow Leaf'.

Explanation:

Calculate the ratio: R=102=5R = \frac{10}{2} = 5. Since 5>45 > 4, the leaf fits the criteria for the ratio being greater than 4, which is typically used to classify 'Narrow' or 'Linear' leaf shapes in botanical keys.