The old system sorted species by how they looked. DNA sequencing changed that, because it shows who is actually related to whom. This HL page covers clades, cladograms and the messy reality that life does not fit neatly into the eight tidy ranks we invented for it.
📚 What you need to know
A clade is a common ancestor plus all of its descendants.
Clades are monophyletic groups, and can contain living and extinct species.
Cladistics groups organisms by shared descent; taxonomy names and ranks them.
A cladogram shows the order of divergence; each node is a common ancestor.
Fewer nodes between two groups means a closer relationship.
Classification is difficult because of convergent morphology, fixed ranks, hybrids and introgression.
Cladistics uses unranked groups, which avoids forcing nature into human categories.
What a clade actually is
Pick any ancestor on the tree of life. The clade is that ancestor and every single species that descended from it, with nothing left out. Leave one branch out and you no longer have a clade — you have an arbitrary human grouping.
Reptiles are the classic example. Crocodiles are more closely related to birds than they are to lizards, so any group that includes crocodiles but excludes birds is not a clade.
Reading a cladogram
A cladogram is a diagram of relationships, not a family photo. What matters is the branching pattern, not how close two names happen to sit on the page.
Chimpanzees and bonobos are each other’s closest relatives because only one node separates them. Humans and orangutans are separated by three, so they are the most distantly related pair shown.
🧩 How to answer a cladogram question
Find the tips you are being asked about.
Trace back from each tip until the two paths meet. That meeting point is their most recent common ancestor.
Count the nodes you passed. Fewer nodes means more closely related.
Check for a clade by picking a node and asking whether every branch above it is in the group.
Watch for a time scale. Without one, you cannot say how long ago anything happened.
Taxonomy and cladistics are not the same job
Feature
Taxonomy
Cladistics
Purpose
Naming organisms and sorting them into ranked groups
Working out evolutionary relationships
Basis
Traditionally shared visible features
Shared descent, usually from DNA evidence
Groups
Ranked: kingdom, phylum, class and so on
Unranked clades of any size
Unit
A taxon
A clade
If taxonomy has been done properly, every taxon should also be a clade. Where it has not, DNA evidence forces a rewrite: some species get moved, some groups get split, and some get merged.
A taxon is what we call a group. A clade is what evolution actually produced. The whole point of modern classification is to make those two things line up.
Why classifying is harder than it looks
Misleading morphology
Dolphins and sharks are both streamlined, fast swimmers with fins. Sorted by appearance they look like close relatives. They are not — dolphins are mammals and sharks are fish. Those body shapes evolved separately because both animals face the same problem of moving through water. Genome sequencing avoids this trap because it compares ancestry, not appearance.
Ranks are a human invention
Nothing in nature says a group must be a “family” rather than an “order”. The ranks were set up because they are tidy, not because life comes in eight neat levels. That creates real problems: if one group has to be moved, every other group at that rank may have to be shuffled to make room.
Species boundaries blur
The point where two populations become two species is often a judgement call. Plants in quite distant groups can sometimes cross and produce fertile hybrids, which are technically new species but impossible to file.
Introgression makes it worse. If a hybrid breeds back with only one of the parent species, generation after generation, you end up with a population carrying a few genes from the other species. Modern humans carry a small proportion of Neanderthal DNA for exactly this reason. Those individuals do not fit cleanly into either species, but calling them a new species makes no sense either.
The big idea to write in an evaluation: variation in nature is gradual, but the ranked system demands sharp boundaries. Cladistics avoids the problem by dropping ranks altogether and following the evidence instead.
Worked examples
WORKED EXAMPLE
Using the cladogram above
(a) State which two species share the most recent common ancestor. (b) Explain whether humans and gorillas alone form a clade. [4]
Part (a): trace back from each tipchimpanzees and bonobos meet at node 1, only one node from each tip(a) chimpanzees and bonobosPart (b): find their shared ancestorhumans and gorillas meet at node 3but chimpanzees and bonobos also descend from node 3(b) no — the group leaves out descendants, so it is not monophyletic“all of the descendants” is the phrase that earns the explanation mark
WORKED EXAMPLE
Explaining a reclassification
Two plants were placed in the same family because they have almost identical flowers. DNA sequencing later showed they belong to different families. Explain how this could happen. [3]
Step 1: the old basisthey were grouped on morphology, that is, on visible featuresStep 2: why that misledsimilar flowers can evolve separately in response to the same pollinatorsStep 3: what DNA showsDNA compares shared descent, so it reveals the true relationshipthe similarity was convergent, not inherited, so the plants were reclassifiedthe words to use are “similar appearance does not mean recent common ancestor”
💡 Exam tip
Define a clade in one sentence: a common ancestor and all of its descendants.
On cladograms, count nodes rather than measuring distance between labels.
If there is no time scale, say so — questions often test whether you spot that.
Use dolphins and sharks as your named example of misleading morphology.
Use Neanderthal DNA in modern humans as your named example of introgression.
When evaluating the ranked system, the key criticism is that it forces sharp lines onto gradual variation.
⚠ Common mix-up
Reading a cladogram left to right as “more advanced”. All the tips are equally modern.
Thinking neighbouring labels are always closest relatives. Only the branching pattern tells you that.
Confusing clade with taxon. A taxon is a named group; a clade is defined by ancestry.
Assuming every clade is small. A clade can be two species or every vertebrate alive.
Forgetting extinct species. Clades include extinct descendants and often an extinct ancestor.
Saying DNA classification is always right. It is stronger evidence, but relationships are still revised as data improves.
Up next: The Niche a Species Could Fill, and the One It Does — from how species are related to how they actually make a living.
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