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Spec Mapping — OCR H420 Module 4.2.2 — Classification and evolution, content statements covering phylogenetic trees, the principles of cladistic analysis, and the molecular evidence (DNA, rRNA, protein sequences) used to reconstruct evolutionary relationships (refer to the official OCR H420 specification document for exact wording). This lesson is the bridge between the classification framework (Lesson 7) and the evidence for evolution (Lesson 10).
Phylogeny is the study of the evolutionary relationships between species. Where classical classification was content to group organisms by similarity, modern phylogenetics asks a deeper question: which species share a common ancestor, and how recently? Phylogenetic trees are the answer, and the discipline of cladistics provides the rules for building them. OCR A-Level Biology A Module 4.2.2 requires you to interpret phylogenetic trees and to understand the molecular evidence used to construct them.
Cladistics as a formal method was developed by the German entomologist Willi Hennig (1913–1976) in the 1950s and 1960s and translated into English in 1966. Carl Woese's 1977 work on rRNA was the first great triumph of molecular cladistics. Richard Owen (the British anatomist who coined the term "Dinosauria" in 1842) deserves a mention for the older concept of homology that cladistics formalised — paraphrased here, his idea of "the same organ in different animals under every variety of form and function" is essentially the principle on which homologous-character cladistic analysis still rests.
Key Definitions:
- Phylogeny — the evolutionary history and relationships of species.
- Phylogenetic tree — a branching diagram showing evolutionary relationships.
- Cladistics — a method of classifying organisms based on shared derived characteristics.
- Clade — a group of organisms that share a common ancestor (a monophyletic group).
- Common ancestor — an ancestral species from which two or more descendants evolved.
A phylogenetic tree is a hypothesis about how species are related.
flowchart TD
A[Common Ancestor] --> B[Ancestor 1]
A --> C[Ancestor 2]
B --> D[Species 1]
B --> E[Species 2]
C --> F[Species 3]
C --> G[Ancestor 3]
G --> H[Species 4]
G --> I[Species 5]
Species that share a more recent common ancestor (node) are more closely related. To see how close two species are, trace back to the first shared node; the older the node, the more distant the relationship.
Modern phylogenetic trees are built using cladistics, a method that groups species by shared derived characteristics (synapomorphies). The reasoning: if two species share a feature that arose after they split from most other species, they probably inherited it from a common ancestor that none of the others had.
A character can be:
Having a backbone, for instance, is a derived feature for all vertebrates (no non-vertebrate has one), making all vertebrates a clade.
Consider five animals: amoeba, trout, frog, lizard, mouse. Which are most closely related?
| Character | Amoeba | Trout | Frog | Lizard | Mouse |
|---|---|---|---|---|---|
| Multicellular | No | Yes | Yes | Yes | Yes |
| Backbone | No | Yes | Yes | Yes | Yes |
| Four limbs | No | No | Yes | Yes | Yes |
| Amniotic egg | No | No | No | Yes | Yes |
| Fur and milk | No | No | No | No | Yes |
Each new character defines a smaller nested clade:
flowchart TD
A[All] --> B[Amoeba]
A --> C[Multicellular]
C --> D[Non-vertebrate]
C --> E[Vertebrates]
E --> F[Trout]
E --> G[Tetrapods]
G --> H[Frog]
G --> I[Amniotes]
I --> J[Lizard]
I --> K[Mammals/Mouse]
The tree shows that mouse and lizard are more closely related to each other than either is to frog, because they share the amniotic egg — a synapomorphy that arose in their common ancestor.
Cladistic analyses can use morphology, but today molecular data dominate. Molecular evidence includes:
The DNA sequence of a gene (or the whole genome) can be compared between species. Similar sequences suggest recent common ancestry; dissimilar sequences suggest more distant ancestry. The number of differences can even be used as a rough molecular clock to estimate how long ago two species diverged.
Commonly used genes for phylogenetics:
RNA phylogenetics was pioneered by Carl Woese's work on rRNA, which revealed the Archaea as a separate domain (Lesson 8). Because rRNA is present in every cell (no exceptions) and changes slowly, it is the "gold standard" for the deepest evolutionary questions.
Before DNA sequencing became cheap, protein sequences were the main molecular data. Cytochrome c, for instance, differs by:
These differences faithfully mirror the phylogeny derived from morphology and DNA — independent confirmation that evolution happened.
Immunological distances (how strongly antibodies to one species' proteins react with another species' proteins) can also measure relatedness.
Single-stranded DNA from two species is mixed; the more similar the sequences, the more stably they pair.
Exam Tip: When OCR asks "explain how DNA sequencing has improved classification", include all of the following: objective and quantitative, reveals relationships that morphology misses, resolves controversial groupings, and enables molecular clocks to date divergences.
The molecular clock hypothesis proposes that mutations accumulate in a gene at an approximately constant rate over time. If so, the number of differences between two species reflects how long ago they diverged.
Example: Cytochrome c differs by 12 amino acids between humans and horses. If substitutions accumulate at, say, one per 20 million years, the split occurred around 240 million years ago — close to the actual figure from fossils.
Different genes tick at different rates, so the clock must be calibrated against the fossil record. The assumption of constant rate is not exact but works well enough to be useful.
In every case, molecular phylogenetics has reorganised our understanding of the tree of life.
OCR usually focuses on the branching order — i.e. "which species is most closely related to which?" — rather than branch lengths.
Cladistics insists on monophyletic groups (clades) that contain an ancestor and all its descendants:
Modern taxonomy aims to eliminate paraphyletic groups where possible.
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