Evidence for Evolution

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.

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Overview of the Evidence

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]

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1. Palaeontology (The Fossil Record)

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).

What the Fossil Record Shows

1. The simplest life forms appear in the oldest rocks, complex forms only in younger ones. The sequence — bacteria → simple eukaryotes → multicellular organisms → vertebrates → mammals → primates — reflects the order predicted by evolution.
2. Species appear and disappear through time; extinction is real, and most species that ever lived are no longer with us.
3. Transitional forms bridge major groups.
4. Fossils in the same layers are found worldwide in predictable positions — the record is consistent.

Transitional Fossils

Transitional forms show characteristics of both ancestor and descendant groups. Classic examples:

• Archaeopteryx — a 150-million-year-old bird with reptile features (teeth, long bony tail, clawed fingers) and bird features (flight feathers, wishbone).
• Tiktaalik (375 million years old) — a "fishapod" with fish-like fins and land-animal features (wrist-like joints, lungs, neck).
• Ambulocetus and Pakicetus — early whales with legs, showing the transition from land-living ancestors.
• Australopithecus and Homo habilis — transitional forms between apes and modern humans.

Creationist critics once claimed transitional forms did not exist; every decade has produced more.

Limitations of the Fossil Record

• Soft-bodied organisms rarely fossilise, so early life and many major groups are under-represented.
• Fossilisation is rare — it requires specific conditions.
• Preservation bias — marine organisms with hard shells fossilise better than land-living soft-bodied ones.
• Erosion destroys fossils.
• Many fossils are yet undiscovered.

Despite these gaps, the fossil record we have is powerful evidence of evolutionary change through time.

Dating Fossils

Fossils are dated by:

• Stratigraphy — position in rock layers (older at the bottom).
• Radiometric dating of surrounding igneous rocks (e.g. potassium-argon, uranium-lead).
• Radiocarbon dating for younger fossils (< 50,000 years).

Independent methods agree closely, giving confidence in the chronology.

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2. Comparative Anatomy

Homologous Structures

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:

• One upper bone (humerus).
• Two lower bones (radius and ulna).
• Wrist bones (carpals).
• Hand bones (metacarpals).
• Five digits (phalanges) — modified in many species.

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 and Convergent Evolution

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:

• Wings of insects, birds and bats all enable flight but evolved separately; their skeletal structures are completely different.
• Eye of octopus and vertebrate — both camera-type eyes, but built differently (e.g. the octopus retina has no blind spot).
• Body shape of dolphin and shark — streamlined torpedoes, but a dolphin is a mammal and a shark is a fish.

Homology points to shared ancestry; analogy to shared environments. Cladistics relies on distinguishing them correctly.

Vestigial Structures

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:

• Human appendix — a remnant of a larger cecum used for digesting plant material.
• Human coccyx (tailbone) — remnant of ancestral tail.
• Wisdom teeth — reduced molars from a larger ancestral jaw.
• Whale pelvis — tiny, non-functional hip bones in whales, from terrestrial ancestors.
• Goosebumps — contraction of hair-erector muscles that once made fur stand on end.
• Wings of flightless birds — kiwis, ostriches and penguins have useless wings.