The big idea: For centuries, living things were sorted into groups by how they looked — their morphology (shape, anatomy, flower type).
Cladistics now sorts them by shared ancestry, read mostly from DNA base sequences. When these two methods disagree, the DNA-based picture is trusted, and the organisms are reclassified.
The goal of every reshuffle is the same: make each named group a clade — an ancestor plus ALL of its descendants — so the classification finally mirrors real evolutionary history.
- Traditional classification
- Sorting organisms into groups by visible similarities — their morphology (body shape, anatomy, flower structure).
- Cladistics
- Classifying organisms by shared ancestry, worked out mainly from DNA (and protein) sequence data.
- Clade
- A group made of one common ancestor and EVERY species descended from it — nothing left out, nothing added in.
- Monophyletic
- Describes a valid clade — an ancestor and all of its descendants. Cladistics aims to make every named group monophyletic.
- Reclassification
- Changing which group an organism is placed in once new (usually molecular) evidence shows its true relationships differ from the old, looks-based grouping.
Why looks can mislead: Two species can look alike without being close relatives — they may have evolved similar features separately (convergent evolution). And close relatives can look very different if they adapted to different ways of life.
So a grouping built on appearance alone can accidentally mix unrelated lineages or split up real ones — which is exactly what DNA evidence later exposes.
Cladistics only allows groups that are monophyletic — an ancestor and all its descendants. So whenever DNA shows an old group fails that test, the group must be redrawn.
Read each case below as a chain of cause and effect: old grouping → what the DNA revealed → why the old group was not a clade → the reclassification.
Case 1 — the figwort family (Scrophulariaceae) was split
- Many plants were lumped into one big family because their flowers looked similar.
- DNA analysis showed these plants did not all share one common ancestor — the 'family' was actually several separate lineages.
- So the old family was not a clade (it was polyphyletic — built on a look-alike trait).
- It was split and reorganised into several smaller families, each one a true clade that reflects real ancestry.
Case 2 — birds were placed inside the reptile clade
- Traditionally birds (Aves) were a separate class from reptiles, because they look so different (feathers, flight, beaks).
- Skeletal cladistics and DNA place birds deep inside the dinosaur/reptile lineage — their closest living relatives are crocodiles.
- A 'Reptilia' that excludes birds is therefore not a clade (it leaves out one descendant branch — it is paraphyletic).
- To make Reptilia monophyletic, birds are counted as reptiles — effectively, living dinosaurs.
Why birds were reclassified: DNA and skeletal cladistics place birds right next to crocodiles, deep INSIDE the reptile/dinosaur clade. A traditional 'Reptilia' that left birds out is therefore NOT a clade — to make every group monophyletic, birds must be counted as reptiles (specifically, living dinosaurs).
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Case 3 — species moved between genera
- A species was placed in a particular genus because it looked like the others in it.
- DNA then showed its true closest relatives sit in a different genus.
- Keeping it where it was would make the genus not a clade, so the species is moved to the genus of its real relatives.
- Its scientific (binomial) name changes to match — a visible sign that the classification has been corrected.
| Type of group | What it contains | Is it a valid clade? |
|---|---|---|
| Monophyletic | An ancestor AND every one of its descendants | ✓ Yes — this is what cladistics requires |
| Paraphyletic | An ancestor and SOME (not all) descendants — one branch left out | ✗ No — e.g. 'reptiles' that exclude birds |
| Polyphyletic | Members from different ancestors, grouped on a look-alike trait | ✗ No — e.g. the old figwort family |
The rule behind all three cases: Each reclassification fixes the same fault: the old group was not monophyletic.
- The figwort family was polyphyletic (unrelated lineages grouped by look) → split.
- 'Reptiles without birds' was paraphyletic (a descendant branch left out) → birds added in.
- A misplaced species made its genus non-monophyletic → moved to its true clade.
Cladistics demands that every named group be an ancestor + ALL its descendants, and reclassification is simply enforcing that rule once the DNA is in.
| Group | Old (morphology-based) view | What the DNA / cladistics showed | Reclassification |
|---|---|---|---|
| Figwort family (Scrophulariaceae) | One large plant family lumped together by flower shape | The members did NOT share a single common ancestor — the family was not one clade | SPLIT and reorganised into several smaller families, each a true clade |
| Birds vs reptiles | Birds = their own class (Aves), separate from reptiles | Birds are nested deep inside the reptile/dinosaur lineage — closest to crocodiles | Birds placed WITHIN the reptile clade (a monophyletic Reptilia must include birds) |
| A misplaced species | Grouped in one genus because it LOOKED like its genus-mates | Its DNA matched a different lineage — its real closest relatives were elsewhere | MOVED to the genus of its true relatives (its scientific name changes) |
What this tells us about classification itself: Because new molecular data can overturn long-standing groups, classification is provisional and evidence-led — it is the current best hypothesis of how organisms are related, not a fixed truth.
This is a strength, not a weakness: cladistics gives objective, testable groupings that anyone can check against the data, so the scheme improves as better evidence arrives.
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How this is tested: On Paper 1A (multiple choice) you may be asked why a group was reclassified, or to pick the statement that correctly describes a clade / monophyletic group.
On Paper 2 the typical question is a short Explain / Outline: explain why evidence from DNA can lead to organisms being reclassified, usually anchored to a named example (birds, the figwort family, or a species moved between genera). Score the marks by linking conflict with the old grouping → not a clade → reclassify to make it monophyletic → classification is provisional.
IB-style question — why DNA evidence leads to reclassification
Birds were traditionally classified in their own class, separate from reptiles. Using cladistic evidence, explain why birds have been reclassified within the reptile clade. [4]
How to score all four marks
- State the cladistic evidence. DNA (and skeletal) data place birds deep inside the dinosaur/reptile lineage — their closest living relatives are crocodiles, not a separate bird-only branch.
- Show the old grouping fails. A group called 'reptiles' that excludes birds leaves out one descendant branch, so it is not a clade (it is paraphyletic — not monophyletic).
- Apply the cladistics rule. Cladistics only allows monophyletic groups — an ancestor and ALL its descendants — so birds must be included for Reptilia to be a valid clade.
- Answer the command term. Because the molecular evidence conflicts with the traditional looks-based class, birds are reclassified within the reptile clade so the group reflects true ancestry. (Award 1 mark per distinct point, up to 4.)
Final answer
DNA and skeletal cladistics place birds inside the dinosaur/reptile lineage (closest to crocodiles). A 'reptile' group that excludes birds is paraphyletic, not a clade. Cladistics requires monophyletic groups (an ancestor + all its descendants), so birds are reclassified within the reptile clade to reflect their true ancestry.
✓ Why this scores full marks: It uses the cladistic evidence (DNA / closest relative = crocodile), shows the old group is not a clade, names the rule (monophyletic = ancestor + all descendants), and ties it back to reflecting true ancestry.
A common way to lose marks is to say only 'birds evolved from dinosaurs' without explaining that the old group wasn't a clade, which is the actual reason for the reclassification.