In simple terms
A friendly intro before the formal notes — no formulas yet.
Life's Family Tree, Read from DNA
Classification organises all living things into a nested hierarchy, like folders inside folders. Cladistics is the modern method that decides where each organism belongs by asking one question: who shares the most recent common ancestor? DNA and protein sequences answer that question far more reliably than looks alone, and the result is drawn as a branching diagram called a cladogram.
Think of building a family tree with a DNA ancestry kit rather than old photographs. Photographs can mislead — two unrelated people can happen to look alike — but the genetic code records who is descended from whom. In the same way, two animals can look similar because they live similar lives (a shark and a dolphin are both streamlined), yet their DNA reveals that one is a fish and the other a mammal. Cladistics reads the genetic code so the tree reflects true ancestry, not surface resemblance.
- 1
Choose the group of species you want to compare, and gather characteristics — most reliably the base sequences of a gene or the amino acid sequences of a shared protein.
- 2
Count the differences between the sequences. Fewer differences imply a more recent common ancestor; more differences imply a more distant one.
- 3
Use the differences (and the molecular clock, which assumes mutations accumulate at a roughly steady rate) to work out the order in which lineages split.
- 4
Draw the cladogram: every node is a common ancestor, and the branching pattern — not the left-to-right order of the tips — shows the inferred evolutionary history.
Explore the concept
Use the live diagram and synced steps — play it or tap a step card to walk through.
Full topic notes
Formal explanation with the rigour you need for the exam.
The taxonomic hierarchy and binomial names
Every species is given a unique two-part Latin name by the binomial system: the genus (capitalised) followed by the species epithet (lower case), both written in italics — for example Panthera leo (lion) and Homo sapiens (human). Universal Latin names avoid the ambiguity of common names, which differ between languages and even between regions of one country. Species are then nested into a hierarchy of ranks. In the modern scheme the full hierarchy, from broadest to narrowest, is: domain, kingdom, phylum, class, order, family, genus, species. As you move down the hierarchy the groups become smaller and their members share a more recent common ancestor, so they have more characteristics in common.
Ranks (broad → narrow): domain, kingdom, phylum, class, order, family, genus, species.
The binomial name uses the two narrowest ranks: Genus species (italic; genus capitalised, species lower case).
Lower down the hierarchy = smaller groups, more shared features, more recent common ancestry.
The rank names are shared across all of biology, so a classification is a compact statement of how an organism relates to every other.
The three domains: Bacteria, Archaea and Eukarya
The broadest rank divides all cellular life into three domains: Bacteria, Archaea and Eukarya. Bacteria and archaea are both prokaryotes — small cells without a nucleus — so on appearance alone they were long lumped together. Molecular comparison changed that. When ribosomal RNA and other conserved sequences were compared, archaea turned out to differ from bacteria as much as either differs from eukaryotes, and in several molecular respects archaea are actually closer to Eukarya (the domain containing protists, fungi, plants and animals). This is a landmark example of the theme of this whole topic: a relationship that superficial features hid entirely was revealed by molecular evidence, forcing a reclassification of life at its very deepest level.
Bacteria and Archaea are prokaryotes (no nucleus); Eukarya have membrane-bound nuclei and organelles.
Bacteria and Archaea look alike but differ in membrane lipids, cell-wall chemistry and gene-expression machinery.
Molecular (rRNA) evidence shows Archaea are more closely related to Eukarya than to Bacteria.
The three-domain system is a natural classification built on molecular data, not appearance.
Natural versus artificial classification
A natural classification groups organisms according to shared evolutionary ancestry, aiming for groups that each contain a common ancestor and all its descendants. Because such groups reflect real evolutionary history, they have predictive power: if you discover a new feature in one member of a natural group, related members are likely to share it. An artificial classification, by contrast, groups organisms by convenient but superficial traits — 'animals that fly', 'plants with red flowers', 'seaweeds' — regardless of ancestry. Artificial groupings can be useful shorthand, but they force together unrelated lineages (bats, birds and insects all 'fly') and lack predictive power. The modern goal, pursued through cladistics, is to make every named group a natural one.
Natural: based on common ancestry; reflects phylogeny; has predictive power.
Artificial: based on convenient superficial features; may unite unrelated organisms; little predictive power.
'Animals that fly' or 'green plants' can be artificial groupings; a clade is a natural one.
Molecular evidence is steadily converting artificial groupings into natural ones.
Clades and cladistics
Cladistics is the method of classifying organisms by common ancestry so that every group is a clade — an ancestor together with all of its descendants (a monophyletic group). The evidence for grouping comes from shared characteristics, and the most powerful characteristics are molecular: the base sequences of DNA and the amino acid sequences of proteins. Crucially, cladistics counts only homologous features — features inherited from a common ancestor — and deliberately excludes analogous features, which resemble one another only because of convergent evolution. Distinguishing the two is the intellectual heart of the method.
Homologous versus analogous features
Homologous features are similar because they were inherited from a shared ancestor, even when their present functions differ. The classic example is the pentadactyl limb: the same five-boned skeletal plan underlies the human arm, the whale flipper and the bat wing, despite their very different jobs. Analogous features look or work alike but evolved independently in separate lineages that faced similar pressures — the wings of an insect and a bird, or the streamlined bodies of sharks (fish) and dolphins (mammals). Analogies signal similar lifestyles, not shared ancestry, so relying on them produces artificial groupings. Cladistics therefore builds trees from homologies — above all from shared molecular sequences, which are abundant, objective, and very hard for convergent evolution to fake in exactly the same way.
Homologous: same underlying structure from a common ancestor; may differ in function (e.g. pentadactyl limb). USE these.
Analogous: similar function/appearance evolved independently (convergent evolution, e.g. insect vs bird wings). EXCLUDE these.
Cladistics groups by homologies, especially shared DNA and protein sequences.
Convergent evolution is exactly why appearance alone misleads and molecular evidence is preferred.
Molecular evidence and the molecular clock
Cladistics leans most heavily on molecular data. The base sequence of a gene, or the amino acid sequence of a protein such as haemoglobin or cytochrome c, is compared between species, and the number of differences is counted. The reasoning is simple: two species that diverged recently have had little time to accumulate independent mutations, so their sequences are very similar; species that diverged long ago have accumulated many differences. This is the molecular clock — the assumption that mutations build up at a roughly constant average rate, so the number of sequence differences is proportional to the time since two lineages shared a common ancestor. Because the clock rests on counting objective, plentiful differences rather than judging appearance, it is not fooled by convergent evolution, and it can date branch points when calibrated against fossils.
This is why molecular evidence so often reclassifies organisms. The figwort family (Scrophulariaceae) was traditionally a huge group united by flower structure. When several chloroplast genes were sequenced, the DNA showed the old family was not a clade at all — its members did not all descend from one common ancestor, having evolved similar flowers by convergent evolution. The family was broken up and its members redistributed, some moved into the plantain family and new families created. It is a clean illustration that a classification is a testable hypothesis: physical features proposed it, molecular evidence falsified it, and the natural classification replaced the artificial one.
Compare DNA base sequences or protein amino acid sequences between species.
More differences ⇒ more distantly related (longer since the common ancestor); fewer differences ⇒ more closely related.
Molecular clock: mutations accumulate at a roughly steady average rate, so differences estimate time since divergence.
Molecular data are objective, abundant and not misled by analogous features — so they can reclassify organisms grouped wrongly by appearance.
Constructing and interpreting cladograms
A cladogram is a branching diagram that presents the results of cladistics. Read it carefully: each node (branch point) represents a common ancestor from which two lineages diverged, and tracing any two tips back until their lines first meet identifies their most recent common ancestor (MRCA). Two organisms are most closely related when their lines join at the most recent node — not when they happen to be drawn side by side or near the top. The branching pattern is the only information a basic cladogram carries: you may rotate the branches about any node without changing the meaning, and the left-to-right order of the tips is arbitrary. An organism drawn at the far end is not more 'advanced'; every living tip has been evolving for exactly the same amount of time.
Node = common ancestor (usually extinct); branch point = a lineage splitting in two.
MRCA of two taxa: trace both back to the first node where their lines meet.
Most closely related = lines meet at the most RECENT node, regardless of tip position.
Only the branching pattern matters; branches can rotate about a node, and tip order is arbitrary.
No living tip is 'more evolved' than another — all have evolved for the same time.
Common mistakes examiners penalise
Calling a node a living species — a node is an inferred COMMON ANCESTOR, usually extinct, not one of the tip organisms. 'Humans evolved from chimps' is wrong; they share an ancestor at the node.
Reading relatedness from tip position — two tips are most closely related when their lines meet at the most RECENT node, not when they are drawn side by side or 'at the top'. No tip is more 'advanced'.
Confusing homologous and analogous — homologous = shared ancestry (pentadactyl limb); analogous = convergent evolution (insect vs bird wing). Only homologous features (and molecular data) are used to build cladograms.
Using analogous features as evidence of relationship — similar shape or function can come from convergent evolution and misleads classification; this is exactly why molecular evidence is preferred.
Forgetting the domain level — the modern hierarchy starts at DOMAIN (Bacteria, Archaea, Eukarya) above kingdom; leaving it out loses easy marks.
Getting the molecular-clock direction backwards — MORE sequence differences mean MORE distantly related (longer since divergence), not more closely related.
Treating a classification as fixed fact — classifications are testable hypotheses; state that molecular evidence can revise them (e.g. the figworts, the three domains).
Sloppy binomial names — genus capitalised, species lower case, whole name italicised: Homo sapiens, not 'Homo Sapiens' or 'homo sapiens'.
Model answer — marked the way our engine marks it
A3.2 explanation questions are marked analytically: each distinct valid biological point is worth one mark, up to the total available. Method/knowledge marks (M) credit a correct idea; application/reasoning marks (A) credit correct use of that idea or a valid conclusion; and error-carried-forward (ECF) means an earlier slip does not automatically cost the later marks if your reasoning is sound. Study how each mark below is tied to a specific, named idea rather than to loose phrasing.
Where this leads
The habit built here — group by shared ancestry, read nodes as common ancestors, trust homologies and molecular data over appearance — is the backbone of evolutionary biology. The same sequence-comparison logic underlies phylogenetics across the whole tree of life, from tracing virus strains during an outbreak to placing a newly discovered microbe among the three domains. Once you can construct and, above all, interpret a cladogram, you can turn any set of molecular or morphological data into a testable statement about how organisms are related.
Worked examples
See the formulas applied — reveal one step at a time, like the exam.
The table shows the number of amino acid differences in a shared protein between humans and four other primates.
| Species | Amino acid differences from human |
|---|---|
| Chimpanzee | 0 |
| --- | --- |
| Gorilla | 1 |
| Gibbon | 3 |
| Rhesus monkey | 8 |
(a) Which species is most closely related to humans? Justify your answer. [2] (b) Construct a cladogram showing the relationships between all five species. [2]
- 1
(a) [2 marks] Chimpanzee. [A1] It has zero amino acid differences from human, the fewest of any species in the table, so by the molecular clock it shares the most recent common ancestor with humans (least time to accumulate mutations). [A1 — reasoning]
Interpreting a cladogram. The cladogram below shows five animals. Read the nodes as common ancestors.
┌────── Salmon
┌──┤
│ └────── Frog
┌──┤
┌──┤ └───────── Lizard
│ └──────────── Mouse
───┤
└─────────────── Shark
(a) Which animal is the outgroup — the most distantly related to the other four? [1] (b) A student says, 'The mouse is more closely related to the lizard than to the frog.' Use the nodes to evaluate this claim. [2] (c) Sharks and salmon are both streamlined fish-shaped swimmers, yet the cladogram separates them widely. Explain, using the terms analogous and homologous, why their body shape does not place them together. [2]
- 1
(a) [1 mark] The shark. Its line branches off at the deepest (oldest) node, so it shares only the most distant common ancestor with the other four. [A1]
Explain how molecular evidence is used to construct cladograms and why it can be more reliable than physical features. [4]
- 1
Model answer. The base sequences of a gene (or the amino acid sequences of a shared protein) are compared between the species being classified, and the number of differences between them is counted. Fewer differences indicate that two species diverged more recently and so share a more recent common ancestor, while more differences indicate a more distant relationship; because mutations accumulate at a roughly constant rate (the molecular clock), the number of differences estimates the time since divergence. The species are then grouped and the branching order of the cladogram is set so that the most similar sequences share the most recent nodes. Molecular evidence can be more reliable than physical features because it is objective and quantitative, and because it is not misled by convergent evolution: analogous features can make unrelated organisms look similar, whereas shared DNA and protein sequences reflect true common ancestry.
How it all connects
The big idea sits in the middle — tap a linked idea to explore the link.
Tap a linked idea to see how it connects back to the main topic — that connection is what examiners reward.
Glossary
Try to recall each definition before you reveal it.
Quick check
Answer in your head first — then tap to check. No pressure.
Revision flashcards
Flip the card. Test yourself before the exam.
Binomial system
Each species has a unique two-part Latin name: the genus (capitalised) followed by the species epithet (lower case), written in italics — for example Homo sapiens. Universal names avoid the confusion of common names that differ between languages and regions.
Key takeaways
Review these before you close the topic — retrieval beats re-reading.
- ✓
Ranks (broad → narrow): domain, kingdom, phylum, class, order, family, genus, species.
- ✓
The binomial name uses the two narrowest ranks: Genus species (italic; genus capitalised, species lower case).
- ✓
Lower down the hierarchy = smaller groups, more shared features, more recent common ancestry.
- ✓
The rank names are shared across all of biology, so a classification is a compact statement of how an organism relates to every other.
Practice — then mark it
The whole point: a real Cambridge question, marked mark-by-mark.
Get a Paper 2 question marked: explain how molecular evidence builds cladograms, and interpret a cladogram with full reasoning
Get a Paper 2 question marked: explain how molecular evidence builds cladograms, and interpret a cladogram with full reasoning
Extra simulations & links
PhET, GeoGebra and other curated tools — open in a new tab.
Frequently asked
Checkpoint
One marked question is worth ten re-reads — close the loop before you move on.
Reading it isn’t knowing it — prove it.
Before you move on: do Get a Paper 2 question marked: explain how molecular evidence builds cladograms, and interpret a cladogram with full reasoning on paper, snap a photo, and get examiner-style feedback on exactly where you win and lose marks.