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Classification and Diversity - Using Dichotomous Keys

Grade 7IB

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

🔑Concepts

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Classification is the process of grouping organisms based on shared physical characteristics and genetic similarities to make them easier to study.

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The Linnaean Hierarchy organizes life into eight levels: Domain →\rightarrow Kingdom →\rightarrow Phylum →\rightarrow Class →\rightarrow Order →\rightarrow Family →\rightarrow Genus →\rightarrow Species.

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A Dichotomous Key is an identification tool consisting of a series of two-part (couplet) choices. Each choice leads to another step or the final identification of the organism.

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Binomial Nomenclature is the formal system of naming species using two parts: the Genus (capitalized) and the species (lowercase), both written in italics or underlined, such as Homo sapiensHomo\ sapiens or Panthera tigrisPanthera\ tigris.

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Diversity refers to the variety of life in an ecosystem. High biodiversity is often measured by the number of different species (species richness) and the abundance of each species.

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Variation within a species can be continuous (e.g., height, which can be any value within a range) or discontinuous (e.g., blood type, which falls into distinct categories like AA, BB, ABAB, or OO).

📐Formulae

Magnification=Image sizeActual size\text{Magnification} = \frac{\text{Image size}}{\text{Actual size}}

1 mm=103 μm=106 nm1\ \text{mm} = 10^3\ \mu m = 10^6\ \text{nm}

Species Richness=∑i=1nsi\text{Species Richness} = \sum_{i=1}^{n} s_i

💡Examples

Problem 1:

Identify an unknown organism using the following key: 1a. Organism has a backbone (Go to 2) 1b. Organism does not have a backbone (Invertebrate) 2a. Organism has fur (Mammal) 2b. Organism has feathers (Bird)

The organism in question has a spine and is covered in feathers. Identify its group.

Solution:

The organism is a Bird.

Explanation:

Starting at step 1, the organism has a backbone (spine), so we follow choice 1a to step 2. At step 2, the organism has feathers, matching choice 2b, which identifies it as a Bird.

Problem 2:

Calculate the actual size of a biological specimen if the image size on a diagram is 45 mm45\ \text{mm} and the magnification is ×150\times 150. Provide the answer in micrometers (μm\mu m).

Solution:

Actual size = 300 μm300\ \mu m.

Explanation:

Using the formula Actual size=Image sizeMagnification\text{Actual size} = \frac{\text{Image size}}{\text{Magnification}}, we get 45 mm150=0.3 mm\frac{45\ \text{mm}}{150} = 0.3\ \text{mm}. To convert to μm\mu m: 0.3×103=300 μm0.3 \times 10^3 = 300\ \mu m.

Problem 3:

Correctly format the scientific name for the Domestic Cat, given its genus is felisfelis and its species is catuscatus.

Solution:

Felis catusFelis\ catus

Explanation:

In binomial nomenclature, the Genus name must be capitalized (FelisFelis) and the species name must be lowercase (catuscatus). Both parts should be italicized.

Using Dichotomous Keys - Revision Notes & Key Formulas | IB Grade 7 Science