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Evolution by natural selection is the unifying theory of biology. Today it is supported by overwhelming evidence from multiple independent fields, each of which on its own would be persuasive and which together are conclusive. OCR A-Level Biology A specification 4.2.2 (g) requires you to know the main lines of evidence for evolution — palaeontology (fossils), comparative anatomy (homologous and analogous structures) and comparative biochemistry (proteins, DNA and rRNA). This lesson gathers the evidence and shows how different disciplines tell the same story.
Key Definitions:
- Evolution — change in the inherited characteristics of a population over successive generations.
- Palaeontology — the study of fossils.
- Homologous structure — a structure with a common evolutionary origin, though it may have different functions today.
- Analogous structure — a structure with similar function but different evolutionary origin.
- Convergent evolution — independent evolution of similar features in unrelated lineages.
flowchart TD
A[Evidence for Evolution] --> B[Palaeontology]
A --> C[Comparative Anatomy]
A --> D[Comparative Biochemistry]
A --> E[Biogeography]
A --> F[Direct Observation]
B --> B1[Fossil record]
B --> B2[Transitional forms]
C --> C1[Homologous structures]
C --> C2[Vestigial structures]
D --> D1[DNA sequences]
D --> D2[Cytochrome c]
D --> D3[rRNA]
E --> E1[Distribution of species]
F --> F1[Artificial selection]
F --> F2[Evolution in action]
Fossils are the preserved remains or traces of past life. They form when organisms die and are buried in sediment; under the right conditions (rapid burial, anaerobic environment, mineralisation) hard parts like bones and shells can be preserved for hundreds of millions of years. Soft tissues sometimes fossilise in exceptional circumstances (e.g. the Burgess Shale, amber).
Transitional forms show characteristics of both ancestor and descendant groups. Classic examples:
Creationist critics once claimed transitional forms did not exist; every decade has produced more.
Despite these gaps, the fossil record we have is powerful evidence of evolutionary change through time.
Fossils are dated by:
Independent methods agree closely, giving confidence in the chronology.
Homologous structures have the same underlying anatomical plan but different functions — a sign that they evolved from a common ancestral structure. The classic example is the pentadactyl limb of vertebrates:
flowchart LR
A[Ancestral Pentadactyl Limb] --> B[Human Arm: tool use]
A --> C[Bat Wing: flight]
A --> D[Whale Flipper: swimming]
A --> E[Horse Leg: running]
A --> F[Mole Forelimb: digging]
All tetrapod vertebrates — mammals, birds, reptiles, amphibians — share a forelimb with:
Despite wildly different functions (grasping, flying, swimming, running, digging), the underlying plan is the same, because they all descended from a common ancestor with this plan. If each species had been "designed" independently, there would be no reason to expect the same basic architecture.
Analogous structures look similar and perform similar functions but have different evolutionary origins. They arise through convergent evolution — independent lineages meeting similar challenges with similar solutions.
Examples:
Homology points to shared ancestry; analogy to shared environments. Cladistics relies on distinguishing them correctly.
Vestigial structures are reduced or non-functional remnants of features that were useful in an ancestor. They are exactly what you would expect under evolution — features lingering after their selection pressure was removed. Examples:
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