QCE Biology - Unit 4 - Continuity of life on Earth
Evolutionary Evidence, Phylogenies and Evolutionary Time | QCE Biology
Learn comparative genomics, conserved sequences, cladograms, phylograms, molecular data, evolutionary radiation and mass extinctions.
Updated 2026-05-18 - 6 min read
QCAA official coverage - Biology 2025 v1.3
Exact syllabus points covered
- Explain how comparative genomics provides evidence for the theory of evolution and how conserved sequences can be used to date divergence.
- Infer species relatedness from cladograms, phylograms and molecular sequence data.
- Determine episodes of evolutionary radiation and mass extinctions from an evolutionary timescale of life on Earth, approximately 3.5 billion years.
- Appreciate that ICTs such as genetic databases and The Basic Local Alignment Search Tool (BLAST) have allowed large-scale mapping and analysis of DNA and protein sequences.
- Appreciate that scientific theories are explanations of the natural world that have been repeatedly tested and corroborated in accordance with the scientific method.
Evolutionary evidence, phylogenies and evolutionary time is part of the way QCE Biology turns living systems into evidence students can describe, analyse and evaluate. The safest way to study it is to connect each term to a data pattern, a biological mechanism and a limitation.
Original Sylligence diagram for biology phylogeny.
Core explanation
Comparative genomics
Comparative genomics compares DNA or protein sequences between organisms. Similar sequences can indicate shared ancestry, especially when the sequences are conserved across many lineages.
Cladograms and phylograms
A cladogram shows branching order and inferred relationships. A phylogram also represents amount of evolutionary change or genetic distance through branch length.
Molecular sequence data
Sequence comparisons work by counting similarities and differences. Fewer differences usually suggest a more recent common ancestor, although mutation rates and data quality must be considered.
Evolutionary time
Evolutionary radiations are periods where many lineages diversify. Mass extinctions remove many lineages and can open ecological opportunities for surviving groups.
Evidence types and how to evaluate them
| Evidence | What it compares | Strong inference | Main limitation | | --- | --- | --- | --- | | Fossils | Preserved remains, traces or dates | Sequence of forms through time | Fossil record is incomplete and biased toward hard parts | | Comparative anatomy | Homologous and analogous structures | Common ancestry or similar selection pressures | Similar appearance can be convergent | | Embryology | Developmental patterns | Shared developmental ancestry | Requires careful comparison of stages | | DNA or amino acid sequences | Base or amino acid differences | Relatedness and possible time since divergence | Mutation rates vary between genes and lineages | | DNA hybridisation | Strength of binding between single strands from different species | More binding suggests more similar sequences | Less precise than direct sequencing | | Bioinformatics databases | Sequence alignment using tools such as BLAST | Similarity to known sequences | Database quality and gene choice affect conclusions |
Conserved sequences are useful because they change slowly and can reveal deep evolutionary relationships. Fast-changing sequences are useful for comparing recently diverged populations or species. Molecular clock estimates need calibration, often from fossils or known divergence events.
When reading phylogenies, branch points show common ancestors. In a cladogram, branch length does not necessarily show time or amount of change. In a phylogram, longer branches indicate more genetic change. In a chronogram, branch length represents time.
Using multiple lines of evidence
Evolutionary conclusions are strongest when independent evidence points to the same relationship. Fossils can show when forms appeared, comparative anatomy can show structural similarity, and molecular data can test whether the DNA or amino acid sequences support the same branching pattern.
Homologous structures suggest common ancestry even if the structures now perform different functions. Analogous structures suggest similar selection pressures rather than close relatedness. Vestigial structures are reduced features inherited from ancestors, and they can support evolutionary explanations when compared with related organisms.
DNA hybridisation is an older molecular technique that estimates similarity by measuring how strongly single DNA strands from different species bind together. Direct sequencing is more precise because it compares the base order itself. BLAST-style database searches compare a query sequence with known sequences and can help infer identity or relatedness, but the conclusion depends on database quality and which sequence was used.
Molecular clocks work best when calibrated with independent evidence and when the sequence has an appropriate mutation rate. A gene that changes too slowly may not distinguish recently separated species; a gene that changes too quickly may be unreliable for deep evolutionary time.
Mass extinctions and evolutionary radiations change the tree of life at large scales. Extinctions remove lineages, while surviving lineages may diversify into newly available niches.
Geological time gives context for macroevolutionary patterns. For example, the Mesozoic era is associated with the diversification of dinosaurs and many reptile lineages, while the Cenozoic era is associated with major mammal diversification after the end-Cretaceous mass extinction. Students do not need to memorise every date, but they should recognise that evolutionary evidence is interpreted across very different time scales.
The pentadactyl limb is a useful homology example. Humans, whales, bats and many other tetrapods have the same basic pattern of limb bones, even though the limbs are used for grasping, swimming or flying. The shared pattern supports common ancestry, while the different functions show divergent evolution.
How to use this in data questions
Start by identifying what has been measured. In Biology, a graph or table is rarely just asking for a trend; it is asking whether you can connect the trend to a process. Quote enough data to show the pattern, then use the concept language from the syllabus. If the evidence is limited, name the limitation precisely: sample size, sampling method, uncontrolled variables, measurement precision, population choice or the time scale of the data.
A useful study habit is to turn each heading into a data prompt. Ask what you would expect to happen if the relevant variable increased, decreased or was removed. For ecology topics, think about abundance, distribution, biodiversity, biomass and carrying capacity. For genetics topics, think about genotype, phenotype, gene expression, allele frequency and inheritance pattern. For evolution topics, think about variation, selection pressure, gene flow, isolation and relatedness.
When a question asks you to evaluate, do not just list problems with the experiment. Link the limitation to the confidence of the conclusion. For example, a small sample size matters because a few unusual individuals can distort the pattern. An uncontrolled abiotic factor matters because it gives another possible explanation for the same biological trend. This is the difference between naming a limitation and using it scientifically.
Worked example
Common exam traps
Other traps to watch for:
- using a general word when a syllabus term is available
- ignoring units, sample size or time scale
- treating a model as a perfect copy of the real ecosystem or cell
- writing a memorised paragraph that does not use the given data