The big idea: We cannot watch most of evolution happen — it took millions of years.
Instead, scientists build the case from lines of evidence left behind: shared body plans, fossils, where species live, and — the strongest of all — similarities in DNA.
On its own each clue is weak. Together they point the same way: today's species descended, with change, from earlier ones.
The main lines of evidence
- Homologous structures — the same body plan reused for different jobs
- Selective breeding — humans changing species fast, showing change is possible
- Fossils — older rock layers hold simpler, often extinct forms
- Biogeography — where species live matches how they spread and split
- DNA / base sequences — the closer the match, the closer the relatives
Homologous structures — the strongest anatomical clue. A human arm, a bat wing and a whale flipper do completely different jobs, yet all share the SAME bones (humerus, radius+ulna, carpals, digits), inherited from a common ancestor.
Interactive diagram
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Key words first: Evolution = a change in the heritable characteristics of a population over many generations.
Evidence here means an observation that supports the idea of common ancestry — not proof of a single event.
Each line of evidence is checked before it is used, so first pin down the terms.
The exam's favourite trap is telling apart two structures that look similar: homologous structures (similar because of a shared ancestor) and analogous structures (similar only because they do the same job).
- Homologous structures
- Structures with the same basic plan but different functions, inherited from a common ancestor (e.g. the matching limb bones of a human, a bat and a whale).
- Analogous structures
- Structures with a similar function but a different basic plan, with no recent common ancestor for that structure (e.g. a bird wing and an insect wing).
- Divergent evolution
- One ancestral form gives rise to several different forms — it produces homologous structures.
- Convergent evolution
- Unrelated species facing similar conditions evolve similar features independently — it produces analogous structures.
- Selective (artificial) breeding
- Humans choosing which organisms reproduce, changing a population quickly — evidence that heritable change really happens.
So when you see two species sharing a feature, ask why they are similar:
- Similar inside plan, different use → homologous → common ancestor (divergent evolution).
- Similar use, different inside plan → analogous → convergent evolution (no recent shared ancestor).
Homologous structures
- Same underlying plan, different jobs
- Inherited from a shared ancestor
- Example: the same arm bones in a human arm, a whale flipper and a bat wing
- Evidence of divergent evolution (one ancestor → many forms)
Analogous structures
- Same job, built on a different plan
- No recent shared ancestor for that structure
- Example: a bird wing and an insect wing (both fly, totally different inside)
- Evidence of convergent evolution (different ancestors → similar form)
DNA is the strongest evidence: The most powerful clue is molecular: comparing DNA base sequences (or the protein sequences they code for).
Species that share more of their sequence are more closely related and split from a common ancestor more recently. This is why a cladogram built from DNA usually agrees with the one built from body plans — two independent clues telling the same story.
Reading an evolutionary tree (cladogram): Evidence is often drawn as a cladogram — a branching tree where each branch tip is a species and each branch point (node) is a shared common ancestor the species above it are descended from.
To find a species' closest relative, trace back down both branches until they meet: the pair that meets at the most recent (lowest) node are the most closely related, because they shared a common ancestor most recently.
The trick: the closest relative is NOT the one drawn nearest on the page — it is the one you reach by going back the fewest branch points.
- Cladogram (evolutionary tree)
- A branching diagram showing how species are related by descent from common ancestors.
- Node (branch point)
- A point where the tree splits — it represents the most recent common ancestor of every species above that point.
- Most recent common ancestor
- The latest shared ancestor of two species — found at the node where their two branches join; the two species joined at the most recent node are the most closely related.
IB-style question — reading a cladogram
A cladogram of four cat species shows: a leopard and a jaguar join at one node; that branch then joins a lion at a deeper (older) node; and a domestic cat branches off at the deepest node of all. Which species is the leopard's closest relative, and how can you tell? [2]
How to read it
- Find the leopard's branch and trace back to its first node. Going back from the leopard, the very first branch point you reach is the one it shares with the jaguar — this is their most recent common ancestor.
- Compare nodes. The lion and the domestic cat only join the leopard's line at deeper (older) nodes, so their shared ancestors with the leopard are further back in time.
- Answer the command term. The leopard's closest relative is the jaguar, because the two share the most recent common ancestor (they meet at the lowest / most recent node).
Final answer
The jaguar — the leopard and jaguar branches join at the most recent (lowest) node, so they share a common ancestor more recently than the leopard does with the lion or the domestic cat.
The leopard and jaguar meet at the most recent node (highlighted), so the jaguar is the leopard's closest relative; the domestic cat branches off earliest.
Interactive diagram
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Watch out — not all 'evidence' counts: Inheriting a trait an organism gained during its life (e.g. a giraffe stretching its neck, then passing on a longer neck) is Lamarckism — it is wrong and is not evidence for evolution.
Valid evidence relies on heritable features only: shared body plans, fossils, biogeography, selective breeding and DNA.
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How this is tested: On Paper 1A (multiple choice) you are most often asked to identify the type of evidence for a relationship, or to distinguish homologous from analogous structures (bat vs insect wing, dolphin vs ichthyosaur, human vs octopus eye).
On Paper 2 the headline question is a 4-mark Explain: how do analogous structures arise through convergent evolution? Build your answer step by step around natural selection.
IB-style question — convergent evolution
Sharks (fish) and dolphins (mammals) both have a streamlined body and a dorsal fin, yet their most recent common ancestor had neither. Explain how these analogous structures arose through convergent evolution. [4]
How to score all four marks
- Set up the variation. Within each population there was heritable variation in body shape, caused by mutation — some individuals were slightly more streamlined than others.
- State the shared pressure. Both groups live in the same kind of environment (open water), so they face the same selection pressure — a streamlined shape lets you swim faster to catch prey and escape predators.
- Apply natural selection separately. In each population the more streamlined individuals survive and reproduce more, passing the helpful alleles on; over many generations the allele frequency for a streamlined body rises in each group independently.
- Answer the command term — explain why they are analogous. Because the two lineages evolved the feature separately (not from a shared streamlined ancestor), the structures are analogous, and similar form from independent evolution under the same pressure is convergent evolution.
Final answer
Heritable variation in body shape + the same selection pressure (open water) means natural selection favours streamlined individuals in EACH lineage independently; the trait was not inherited from a shared streamlined ancestor, so the structures are analogous and the pattern is convergent evolution.
✓ What scores the marks: Make sure your answer names all four: heritable variation, the same selection pressure, natural selection acting separately in each lineage, and the conclusion that the similarity evolved independently (so it is analogous / convergent).