Ch 1 · 18.1 Classification · AS & A Level Biology 9700
Topic 1 of 3 in Classification, biodiversity and conservation — AS & A Level Biology 9700 — 58 lessons.
Introduction
Classification sorts the millions of living species into groups that reflect how closely related they are. This lesson starts with the surprisingly tricky question of what a species is, then works down from the three domains to the kingdoms of eukaryotes, and ends with viruses, which fit none of them. (Syllabus section 18.1.)
Key ideas — What is a species?
There is no single definition that works for every organism, so biologists use three species concepts:
🧑🏫 Teacher's explanation
Imagine you are organizing a massive library. How do you decide if two books belong to the same 'series'? You could look at their covers (Morphological), check if they are part of the same story arc where characters interact (Biological), or see if they are all found in the 'Science Fiction' section (Ecological).
📖 Definitions
🔍 Worked example — Easy
A biologist observes two populations of beetles. Population A and Population B can mate and produce offspring that are capable of reproducing. However, they live on different mountains and never meet in the wild. Which species concept is most useful here, and why?
🔍 Worked example — Standard
Compare the utility of the morphological species concept when studying living animals versus studying fossils.
🔍 Worked example — Exam-style
A researcher finds a group of bacteria that all share the same metabolic pathways and occupy the same specific nutrient niche in a hydrothermal vent. Which species concept is being applied?
⚠️ Common errors
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🎯 Exam question
(a) Identify the species concept that would be used to distinguish these organisms based on their habitat and food sources. [1] (b) Explain why the biological species concept might be difficult to apply to these organisms if they were found to be asexual. [2] [3 marks]
Key ideas — Three domains
All organisms are placed in one of three domains: Archaea, Bacteria and Eukarya. Archaea and Bacteria are both prokaryotes (no nucleus, no membrane-bound organelles); Eukarya contains every organism with eukaryotic cells. Studies of ribosomal RNA showed that Archaea are as different from Bacteria as they are from eukaryotes, which is why they have a domain of their own.
🧑🏫 Teacher's explanation
Imagine you are organizing a massive library. Instead of just sorting by 'books' and 'magazines', you first sort them into three massive sections: 'Digital', 'Print', and 'Manuscript'. This is what scientists do with life. We use 'Domains' as the largest possible category to group every living thing on Earth.
📖 Definitions
🔍 Worked example — Easy
An organism is identified as having a cell wall composed of peptidoglycan and membrane lipids with ester bonds. To which domain does it belong?
🔍 Worked example — Standard
A scientist discovers a new single-celled organism. It lacks a nucleus and membrane-bound organelles. Its ribosomal RNA sequence is significantly different from known Bacteria, and its cell membrane contains branched hydrocarbon chains joined to glycerol by ether bonds. Classify this organism into its domain and state why it is not in the Bacteria domain.
🔍 Worked example — Exam-style
A virus is discovered. It contains a single strand of RNA as its genetic material. How would a biologist classify this virus based on the syllabus criteria?
⚠️ Common errors
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🎯 Exam question
(a) Identify the domain to which this organism belongs. [1] (b) Explain how the scientist could use ribosomal RNA to further distinguish this organism from a bacterium. [2] [3 marks]
Key ideas — The taxonomic hierarchy of the Eukarya
Within the Eukarya, organisms are classified in a hierarchy of ranks, each containing the ranks below it: kingdom, phylum, class, order, family, genus, species (above them all is the domain). Organisms in the same genus share more features, and a more recent common ancestor, than organisms that share only a phylum.
🧑🏫 Teacher's explanation
Think of classification like a digital file system on your computer. You have a 'Drive' (Domain), which contains 'Folders' (Kingdoms), which contain 'Sub-folders' (Phyla), and so on, until you reach a specific 'File' (Species). As you move down the hierarchy, the groups become smaller and the members become more similar to one another.
📖 Definitions
🔍 Worked example — Easy
A scientist discovers a new organism. It has a cell wall made of peptidoglycan and 70S ribosomes. Which domain does it belong to, and is it a eukaryote?
🔍 Worked example — Standard
An organism is classified as follows: Kingdom: Fungi, Phylum: Basidiomycota, Class: Agaricomycetes, Order: Agaricales, Family: Agaricaceae, Genus: Agaricus, Species: bisporus. State the binomial name for this organism.
🔍 Worked example — Exam-style
A virus is identified as having a single strand of RNA. How would a biologist classify this virus based on the criteria provided in the syllabus?
⚠️ Common errors
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🎯 Exam question
(a) Identify the most likely Kingdom for this organism. [1] (b) State two differences between the cell wall composition of Bacteria and the cell wall composition of the organism identified in (a). [2] [3 marks]
Key ideas — The four eukaryote kingdoms
🧑🏫 Teacher's explanation
Classification is like a giant filing system for life. Imagine you are organizing a massive library. First, you group books by broad categories (Domains), then by genre (Kingdoms), then by subject (Phylum), and so on, until you reach a specific title (Species). This allows scientists to communicate clearly about organisms without confusion.
📖 Definitions
🔍 Worked example — Easy
A scientist observes a new organism. It is multicellular, has cell walls made of chitin, and feeds by secreting enzymes onto decaying wood. Identify its kingdom and state its mode of nutrition.
🔍 Worked example — Standard
Compare the cell walls of Bacteria and Archaea.
🔍 Worked example — Exam-style
A virus is identified as having a single strand of RNA. Classify this virus based on the syllabus criteria.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
(a) To which kingdom does this organism belong? [1] (b) Explain how the organism's mode of nutrition differs from that of a fungus. [2] [3 marks]
Key ideas — Classifying viruses
Viruses are not cells, so they are not placed in any domain. They are classified by their nucleic acid:
🧑🏫 Teacher's explanation
Think of classification like organizing a massive library. Most books (living organisms) are organized by their genre, author, and title. However, viruses are like digital files or loose sheets of paper—they aren't 'books' because they don't have the binding (cell structure) that all other life forms have. Because they lack cells, they don't fit into the three main domains of life (Archaea, Bacteria, or Eukarya).
📖 Definitions
🔍 Worked example — Easy
A newly discovered virus contains a single molecule of genetic material that is composed of ribonucleotides. Classify this virus based on the two criteria required by the syllabus.
🔍 Worked example — Standard
Compare the classification criteria used for a bacterium with the classification criteria used for a virus.
🔍 Worked example — Exam-style
A scientist observes a virus under an electron microscope. The virus has a double helix of deoxyribonucleotides. State the classification of this virus.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
(a) State the type of nucleic acid present in the virus. [1] (b) Classify the virus based on its nucleic acid type and strandedness. [2] [3 marks]
Worked example
Question. Lions and tigers can be bred in zoos, producing hybrids called ligers. Male ligers are infertile. (a) Explain why lions and tigers are classified as separate species, using the biological species concept. [2] (b) Give one limitation of the biological species concept. [1]
🔍 Worked example — Easy
A scientist observes two populations of birds. They look identical in plumage and size (morphology) and live in the same forest (ecology). However, they have different mating songs, so they never mate with each other. Using the biological species concept, determine if they are the same species.
🔍 Worked example — Standard
Compare the domains Archaea and Bacteria regarding their cell wall composition and membrane lipids.
🔍 Worked example — Exam-style
An unknown virus is identified. It contains a single strand of RNA. Classify this virus based on the criteria provided in the syllabus.
⚠️ Common errors
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Practice
Answers