How are organisms classified, and how does molecular evidence reveal their evolutionary relationships?
4.2.2 Classification and evolutionary relationships: the binomial system and the taxonomic hierarchy; the five kingdoms and the three-domain classification; the meaning of phylogeny; and how molecular evidence (DNA base sequences, amino acid sequences) and other evidence are used to clarify evolutionary relationships.
A focused answer to the OCR H420 4.2.2 dot point on classification. Covers the binomial system and taxonomic hierarchy, the five kingdoms and the three-domain system, the meaning of phylogeny, and how molecular and other evidence is used to establish evolutionary relationships.
Reviewed by: AI editorial process; not yet individually human-reviewed
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What this dot point is asking
OCR wants you to explain the binomial system and the taxonomic hierarchy, describe the five kingdoms and the three-domain classification, define phylogeny, and explain how molecular and other evidence is used to establish evolutionary relationships.
The answer
Classification and the taxonomic hierarchy
Classification is the organisation of living things into groups (taxa). It is hierarchical: smaller groups are nested within larger ones, with no overlap. The hierarchy, from largest to smallest, is domain, kingdom, phylum, class, order, family, genus, species.
Each species has a unique binomial name of its genus (capitalised) and species (lower case), written in italics (for example Homo sapiens). A species is often defined as a group of organisms that can interbreed to produce fertile offspring.
The five kingdoms and the three domains
The older five-kingdom system grouped life into Prokaryotae (bacteria), Protoctista, Fungi, Plantae and Animalia, mainly on observable features.
Molecular evidence led to the three-domain system, which splits the prokaryotes:
- Bacteria (true bacteria);
- Archaea (prokaryotes that differ in membrane lipids, cell wall chemistry, ribosomal RNA and enzymes, and live in many extreme environments);
- Eukarya (Eukaryota) (all eukaryotes: protoctists, fungi, plants and animals).
The split was justified because Archaea differ fundamentally from Bacteria at the molecular level (and resemble Eukarya in some features), a difference invisible to the eye.
Phylogeny and evidence
Phylogeny classifies organisms by their evolutionary relationships; modern classification aims to be phylogenetic, so groups share a common ancestor.
Evidence used to establish relationships:
- Molecular evidence (the most powerful). Comparing DNA base sequences, the sequences in ribosomal RNA, and the amino acid sequences of proteins. More closely related species share more similar sequences, because fewer mutations have accumulated since they diverged from a common ancestor. This is objective and quantitative.
- Other evidence: comparing observable features (morphology and anatomy), embryology, and the fossil record.
A key advantage of molecular data is that it avoids being misled by convergent evolution, where unrelated organisms evolve similar features (for example the streamlined shapes of sharks and dolphins) without being closely related.
Examples in context
Example 1. Whales among the mammals. Whales look fish-like, but DNA and anatomical evidence place them firmly among the mammals (close to hippos), a classic case where molecular data overrides superficial appearance.
Example 2. The reclassification of fungi. Fungi were once grouped with plants, but molecular and cellular evidence (chitin walls, heterotrophic nutrition, no chlorophyll) showed they form their own kingdom, illustrating how evidence reshapes classification.
Try this
Q1. Write the taxonomic hierarchy from kingdom down to species. [2 marks]
- Cue. Kingdom, phylum, class, order, family, genus, species (domain sits above kingdom).
Q2. Explain why two species with very similar cytochrome c amino acid sequences are likely to be closely related. [2 marks]
- Cue. They diverged from a common ancestor relatively recently, so few mutations have accumulated and the sequences remain similar.
Q3. Name the three domains in the three-domain classification system. [1 mark]
- Cue. Bacteria, Archaea and Eukarya (Eukaryota).
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR H420/02 20184 marksExplain why comparing DNA base sequences and amino acid sequences gives a more reliable classification than comparing observable features alone.Show worked answer →
Link molecular similarity to shared ancestry and the limits of physical features.
The DNA base sequence (and the amino acid sequence of proteins it codes for) directly reflects the genes an organism has. More closely related species share more similar sequences because they diverged from a common ancestor more recently and have accumulated fewer mutations since.
Observable features can be misleading: unrelated organisms may evolve similar features (convergent evolution) in similar environments, and similar-looking organisms may not be closely related. Molecular data is quantitative and objective, so it resolves relationships that morphology cannot.
Markers reward more similar sequences meaning more recent common ancestor, and that molecular evidence avoids being misled by convergent or superficial features.
OCR H420/02 20203 marksThe three-domain system reclassified life from the older five-kingdom system. Describe the three domains and explain why the prokaryotes were split into two of them.Show worked answer →
Name the domains, then justify the split with molecular evidence.
The three domains are Bacteria, Archaea and Eukarya (Eukaryota). Bacteria and Archaea are both prokaryotic; Eukarya contains all eukaryotes.
The prokaryotes were split because molecular evidence (especially ribosomal RNA sequences, and differences in cell membrane lipids, cell wall chemistry and enzymes such as RNA polymerase) showed that Archaea differ fundamentally from Bacteria and are in some ways more similar to Eukarya. This molecular difference was not visible in their similar appearance, which is why the older system grouped them together.
Markers reward the three named domains and the use of molecular (rRNA) evidence to justify separating Archaea from Bacteria.
Related dot points
- 4.2.2 Evolution: the process of evolution by natural selection acting on variation; the role of mutation in generating variation; the types of natural selection (directional, stabilising and disruptive); the evidence for evolution from fossils, comparative anatomy and molecular biology; and examples such as antibiotic resistance and industrial melanism.
A focused answer to the OCR H420 4.2.2 dot point on evolution. Covers natural selection acting on variation, mutation as the source of variation, directional, stabilising and disruptive selection, the evidence for evolution, and examples such as antibiotic resistance and peppered moths.
- 4.2.1 Biodiversity: the levels of biodiversity (habitat, species and genetic); how to sample plants and animals (random sampling, quadrats, transects and mark-release-recapture); the calculation and interpretation of Simpson's index of diversity; and the ecological, economic and aesthetic reasons for maintaining biodiversity.
A focused answer to the OCR H420 4.2.1 dot point on biodiversity. Covers habitat, species and genetic diversity, sampling methods including quadrats, transects and mark-release-recapture, the calculation and interpretation of Simpson's index of diversity, and the reasons for maintaining biodiversity.
- 4.1.1 Communicable diseases: the range of pathogens (bacteria, viruses, fungi and protoctists) and the communicable diseases they cause in animals and plants; the means of transmission; the primary non-specific defences of plants and animals; and the role of phagocytes in the non-specific immune response.
A focused answer to the OCR H420 4.1.1 dot point on communicable diseases. Covers the four pathogen groups and example diseases, means of transmission, the primary non-specific defences of plants and animals, and the role of phagocytes in non-specific immunity.
- 2.1.3 Nucleotides and nucleic acids: the semi-conservative replication of DNA and the roles of DNA helicase, DNA polymerase and the complementary base pairing rule; the nature of the genetic code as a triplet code that is degenerate and non-overlapping; the roles of mRNA and tRNA in protein synthesis.
A focused answer to the OCR H420 2.1.3 dot point on DNA replication and the genetic code. Covers semi-conservative replication, the roles of DNA helicase and DNA polymerase, the Meselson-Stahl evidence, and the triplet, degenerate, non-overlapping code with transcription and translation.
- 6.1.2 Populations and evolution: the meaning of a gene pool and allele frequency; the use of the Hardy-Weinberg principle to calculate allele and genotype frequencies; the factors that change allele frequencies (natural selection, genetic drift, the founder effect and migration); and the process of speciation (allopatric and sympatric).
A focused answer to the OCR H420 6.1.2 dot point on populations and evolution. Covers gene pools and allele frequency, the Hardy-Weinberg principle and its calculations, the factors that change allele frequencies including genetic drift and the founder effect, and allopatric and sympatric speciation.
Sources & how we know this
- OCR A Level Biology A (H420) Specification — OCR (2023)