Cladistics at Higher Level: This statement is Higher Level only. Classification groups organisms; cladistics groups them by who they evolved from, so the groups show real evolutionary relationships.
Practise this as you read
- Read a cladogram: find where two branches join.
- Give each advantage with a real example.
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One ancestor and all its descendants: A clade shows evolutionary relationships: every member evolved from the same common ancestor.
The points to remember
- A clade is a common ancestor and all of its descendants, living and extinct.
- A cladogram is a branching diagram that shows clades.
- Each node (branch point) is a common ancestor.
- Species that branch from a more recent node are more closely related.
- Clades are found from shared characteristics and, above all, from DNA comparisons.
Remember it as: One ancestor plus all its descendants: that is a clade.
Real example: crocodiles look like lizards, but they share a more recent common ancestor with birds. Both have a four-chambered heart and a gizzard, and their DNA agrees. So crocodiles and birds form a clade.
A cladogram is not a ladder: Read how recently two groups share an ancestor, not which is 'more advanced'. Every living species has been evolving for the same length of time.
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Older systems grouped organisms by how they look. Clades group them by who they evolved from, and this has several advantages.
Advantages of classifying in clades
- It shows evolutionary relationships: which species share the most recent common ancestor.
- It rests on evidence that can be tested, above all DNA, so groups can be checked and updated.
- It helps predict characteristics: close relatives often share features, such as useful chemicals.
- It reveals hidden relationships that looks alone would miss.
- It shows which species are evolutionarily distinct, so conservation can protect unique branches.
Remember it as: Relationships, testable evidence, predictions, hidden links, conservation priorities.
Grouping by looks
- Similar-looking species go together.
- Hard to test.
- Fooled by similar ways of life.
Grouping by clades
- Grouped by common ancestor.
- Tested with DNA.
- Shows how closely species are related.
Real example: DNA shows that the closest living relatives of whales are hippos. Nobody could see this from their bodies alone: a hidden relationship.
Conservation: the tuatara: The tuatara is the only survivor of a branch of reptiles that split from lizards and snakes about 250 million years ago. Losing it would lose a whole branch of the tree, so it is a high priority for protection.
Real example of prediction: the cancer drug taxol was first found in the bark of the rare Pacific yew. Its close relative, the common European yew, has a similar chemical in its needles, so the drug can now be made without cutting down rare trees.
How this comes up: Paper 2, Section B (a) [4], or a Section A question on a cladogram: read it, then give an advantage.
A natural history museum is rearranging its reptile and bird galleries so that they follow clades instead of the traditional classes.
Outline two advantages of using a method of classification that shows evolutionary relationships in a clade.
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