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B14.5 Classification

GCSE Combined Science · AQA · Chapter B14: Genetics and Evolution

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: the Linnaean system of groups, the five kingdoms and the binomial system for naming species. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • List the groups of the Linnaean system in order, from kingdom to species
  • Explain the basis on which Linnaeus classified organisms
  • Name the five kingdoms
  • Write and interpret binomial (two-part) scientific names
  • Use classification to judge how closely organisms are related

Key words

  • Classification: Sorting living things into groups based on their shared features.
  • Kingdom: The largest group in the Linnaean system, e.g. the animal kingdom.
  • Genus: A group of closely related species. It is the first part of a scientific name.
  • Species: The smallest group: organisms that can interbreed to produce fertile offspring.
  • Binomial system: Naming each species with two words: its genus and its species, e.g. Homo sapiens.
  • Hybrid: The offspring of two different species, e.g. a mule (horse × donkey). Hybrids are usually infertile.

The Linnaean system

Carl Linnaeus, a Swedish scientist working in the 1700s, classified living things according to their structure and characteristics – what they looked like and how their bodies were built. He sorted organisms into a series of groups, each one inside the next:

Memory aid: Order of groups: Kingdom, Phylum, Class, Order, Family, Genus, Species. Try: “Kind People Can Often Find Good Snacks”.

As you go down from kingdom to species, each group contains fewer organisms that are more similar to each other. For example, the vertebrates are split into five classes: fish, amphibians, reptiles, birds and mammals.

The five kingdoms

Linnaeus started with just a few kingdoms. As microscopes improved, scientists saw the cells of tiny organisms and the system grew to five kingdoms:

What is a species?

A species is a group of organisms that can interbreed to produce fertile offspring. A horse and a donkey can mate, but their offspring (a mule) is infertile, so horses and donkeys are different species, even though they are in the same genus, Equus.

Naming living things: the binomial system

Linnaeus gave every species a two-part Latin name:

  • The first word is the genus. It starts with a capital letter.
  • The second word is the species. It starts with a small letter.
  • The name is written in italics when typed, or underlined when handwritten.
  • After the first time, the genus can be shortened to its first letter: Escherichia coli becomes E. coli.
  • Brown bear; Scientific name Ursus arctos; Genus Ursus; Species arctos
  • Polar bear; Scientific name Ursus maritimus; Genus Ursus; Species maritimus
  • Grey wolf; Scientific name Canis lupus; Genus Canis; Species lupus
  • Human; Scientific name Homo sapiens; Genus Homo; Species sapiens

Worked example: who is most closely related?

Brown bears and polar bears share the genus Ursus. The wolf is in a different family (Canidae) but the same order (Carnivora).
Organisms that share more groups – especially a genus – are more closely related.

Answer: the two bears are more closely related to each other than either is to the wolf

Scientific names matter because common names vary between countries and languages, and one common name can mean different species. A scientific name is the same everywhere in the world.

Common mistakes

  • Writing the species name with a capital letter (Homo Sapiens). Only the genus has a capital.
  • Mixing up the order of the groups, especially order and family. Use a memory aid.
  • Thinking organisms in the same order are more closely related than organisms in the same genus. Smaller groups mean closer relationships.
  • Putting viruses in a kingdom. They are not made of cells and are not in any of the five kingdoms.

Revision questions

Try these, then check your answers below each one.

Question 1. Which list shows the groups of the Linnaean system in order, from the largest to the smallest? (Choose from: Kingdom, phylum, class, order, family, genus, species, Kingdom, class, phylum, order, genus, family, species, Kingdom, phylum, order, class, family, genus, species, Species, genus, family, order, class, phylum, kingdom.)

Answer: Kingdom, phylum, class, order, family, genus, species

Working: Kingdom → phylum → class → order → family → genus → species.

Question 2. Which is the correct way to write the scientific name of the grey wolf? (Choose from: Canis lupus, canis lupus, Canis Lupus, Lupus canis.)

Answer: Canis lupus

Working: Canis lupus: genus (Canis) with a capital letter, species (lupus) with a small letter, in italics.

Question 3. The diagram shows the groups used to classify living things, each one inside the next. Which group is missing at X?

Answer: family

Working: Family. The groups are kingdom, phylum, class, order, family, genus, species.

Related lessons

Continue in the full course

This lesson is section B14.5 (AGCSC1405) of our free GCSE Combined Science course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Combined Science course.

 

 

 

 

 

 

 

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