1.2 Concept and Uses of Classification Systems

2026 Syllabus Objectives

  1. State that organisms can be classified into groups by the features that they share
  2. Describe a species as a group of organisms that can reproduce to produce fertile offspring
  3. Describe the binomial system of naming species as an internationally agreed system in which the scientific name of an organism is made up of two parts showing the genus and species
  4. Construct and use dichotomous keys based on identifiable features
  5. Explain that classification systems aim to reflect evolutionary relationships
  6. Explain that the sequences of bases in DNA are used as a means of classification
  7. Explain that groups of organisms which share a more recent ancestor (are more closely related) have base sequences in DNA that are more similar than those that share only a distant ancestor

Classification by Shared Features 🔬

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Organisms can be classified into groups based on the features that they share. This approach allows biologists to organize the vast diversity of life into manageable categories. By identifying and comparing observable characteristics, organisms with similar features are grouped together, while those with different features are placed in separate groups.

Dichotomous keys are practical tools that utilize this principle, using identifiable features to systematically classify organisms into appropriate groups.

Species Definition 🧬

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What is a Species?

The species is the smallest and most specific group into which biologists classify living organisms.

🔑 Key Definition: A species is a group of organisms that can reproduce with each other to produce fertile offspring.

Important terminology:

  • Fertile: able to reproduce
  • Infertile: not able to reproduce

Characteristics of a Species

For organisms to belong to the same species, they must satisfy two critical criteria:

  1. They can interbreed (reproduce with each other)
  2. Their offspring are fertile (the offspring can also reproduce)

Example: Horses and Donkeys 🐴

This classic example illustrates the species concept clearly:

Horses (Equus caballus):

  • Members of this species can reproduce with each other
  • Offspring are horses that belong to the same species
  • These offspring are fertile and can produce more horses

Donkeys (Equus asinus):

  • Donkeys reproduce with each other to produce fertile donkeys
  • They belong to a different species than horses

What happens when horses and donkeys interbreed?

  • A male donkey can reproduce with a female horse
  • The offspring is called a mule
  • Mules are usually strong and healthy organisms
  • However, mules are infertile (cannot reproduce)

Conclusion: Because the offspring (mule) is infertile, this confirms that horses and donkeys belong to different species, despite their ability to interbreed.

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