OCR · A-Level · Biology

    Course version: H420 · Version from Sept 2025

    Classification and evolution: (a) the biological classification of species

    Biological classification groups organisms into a hierarchy of taxa based on shared characteristics and evolutionary relationships.

    Checked against the OCR H420 specification (opens in a new tab) on .

    • Point 1 of 10
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    Biological classification groups organisms into a hierarchy of taxa based on shared characteristics and evolutionary relationships. The main ranks from broadest to narrowest are domain, kingdom, phylum, class, order, family, genus and species. A species is the basic unit: members of a species share similar features and can interbreed to produce fertile offspring. Classification uses observable features, biochemical evidence such as DNA and protein sequences, and evolutionary relationships to place organisms in the correct taxon. The binomial naming system gives each species a genus name and a species name, for example Homo sapiens. Remember that the more ranks two organisms share, the more closely related they are.

    What earns the marks

    • The taxonomic hierarchy runs from domain through kingdom, phylum, class, order, family, genus to species.
    • A species is a group of organisms with similar features that can interbreed to produce fertile offspring.
    • Classification uses shared characteristics, biochemical evidence such as DNA and protein sequences, and evolutionary relationships.
    • Binomial nomenclature gives each species a two-part name: genus followed by species.
    • Organisms sharing more taxonomic ranks are more closely related than those sharing fewer.
    • Classification is hierarchical, with each level containing groups from the level below.

    Common mistakes and how to put them right

    • Reversing the order of taxonomic ranks.

      Put it right: Domain is the broadest rank and species the narrowest, so the sequence runs domain, kingdom, phylum, class, order, family, genus, species.

    • Thinking classification relies only on appearance.

      Put it right: Modern classification also uses biochemical evidence such as DNA and protein sequences and reflects evolutionary relationships.

    • Confusing genus and species in a binomial name.

      Put it right: The genus comes first and the species second, as in Homo sapiens.

    Exam tips

    • Memorise the rank order with a mnemonic and practise writing it out.
    • When asked about relatedness, count the shared ranks rather than judging by appearance.
    • Check the position of genus and species in any binomial name given in an option.

    Try the lesson

    The start of the Classification and evolution: (a) the biological classification of species lesson, made from checked teaching notes for this specification point.

    Have a go first

    In the binomial naming system, what does the first part of a scientific name represent?

    Listen

    Biological classification groups organisms into a hierarchy of taxa based on shared characteristics and evolutionary relationships. The main ranks from broadest to narrowest are domain, kingdom, phylum, class, order, family, genus and species. A species is the basic unit: members of a species share similar features and can interbreed to produce fertile offspring. Classification uses observable features, biochemical evidence such as DNA and protein sequences, and evolutionary relationships to place organisms in the correct taxon. The binomial naming system gives each species a genus name and a species name, for example Homo sapiens. Remember that the more ranks two organisms share, the more closely related they are. A strong answer earns marks for these points. The taxonomic hierarchy runs from domain through kingdom, phylum, class, order, family, genus to species. A species is a group of organisms with similar features that can interbreed to produce fertile offspring. Classification uses shared characteristics, biochemical evidence such as DNA and protein sequences, and evolutionary relationships. Binomial nomenclature gives each species a two-part name: genus followed by species. A common slip: Reversing the order of taxonomic ranks. Instead, domain is the broadest rank and species the narrowest, so the sequence runs domain, kingdom, phylum, class, order, family, genus, species. Exam tip: Memorise the rank order with a mnemonic and practise writing it out.

    See it

    A vertical flowchart showing eight boxes connected in a single chain from top to bottom. The top box is labelled Domain, which leads down to Kingdom, then Phylum, then Class, then Order, then Family, then Genus, and finally Species at the bottom. Each rank contains the groups of the level below it, and the chain runs from the broadest rank at the top to the narrowest at the bottom.
    The taxonomic hierarchy from the broadest rank, domain, down to the narrowest, species.

    Put it in order

    Put these steps in the order they happen.

    1. 1. Order
    2. 2. Species
    3. 3. Family
    4. 4. Domain
    5. 5. Kingdom
    6. 6. Genus
    7. 7. Phylum
    8. 8. Class

    Key words

    Say what each word means before you turn the card.

    Taxonomic hierarchy

    Try answering aloud or in your head before turning the card.

    Species concept

    Try answering aloud or in your head before turning the card.

    Binomial nomenclature

    Try answering aloud or in your head before turning the card.

    Evidence for classification

    Try answering aloud or in your head before turning the card.

    See every definition
    Taxonomic hierarchy
    The system that groups organisms into ranks from domain through kingdom, phylum, class, order, family, genus to species, each level containing groups from the level below.
    Species concept
    A group of organisms with similar features that can interbreed to produce fertile offspring.
    Binomial nomenclature
    A two-part naming system giving each species a genus name followed by a species name, for example Homo sapiens.
    Evidence for classification
    Shared characteristics, biochemical evidence such as DNA and protein sequences, and evolutionary relationships are used to place organisms in the correct taxon.

    Label the diagram

    Some boxes have lost their names. Pick the right name for each number.

    The diagram again, with 3 boxes numbered instead of named. A vertical flowchart showing eight boxes connected in a single chain from top to bottom. The top box is labelled Domain, which leads down to Kingdom, then Phylum, then Class, then Order, then Family, then Genus, and finally Species at the bottom. Each rank contains the groups of the level below it, and the chain runs from the broadest rank at the top to the narrowest at the bottom.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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