Evidence for Evolution

Spec Mapping — OCR H420 Module 4.2.2 — Classification and evolution, content statements covering the evidence for evolution from palaeontology (fossils), comparative anatomy (homologous and analogous structures), and comparative biochemistry (DNA, rRNA, protein sequences) (refer to the official OCR H420 specification document for exact wording). This lesson assembles the multi-disciplinary case for evolution and is the natural source of AO3 evaluation questions in OCR H420 papers.

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 Module 4.2.2 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.

The case for evolution was built incrementally by many scientists. Charles Darwin's 1859 On the Origin of Species set out the argument and assembled the evidence available at the time — fossils, comparative anatomy (drawing on Richard Owen's concept of homology), biogeography (drawing on the Beagle observations), and artificial selection. Alfred Russel Wallace's 1858 joint paper with Darwin contributed the independent co-discovery of natural selection and the biogeographic insight that animal distributions reflect deep history. Thomas Henry Huxley ("Darwin's bulldog") was an early and forceful defender. Theodosius Dobzhansky's twentieth-century synthesis articulated the principle that all biology becomes coherent only when viewed through evolution — a paraphrase, not a quotation. The molecular evidence that now dominates the case for evolution emerged in the late twentieth century with cytochrome c sequencing, Carl Woese's rRNA work, and (since 2000) whole-genome comparison.

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.

Vestigial features pose a serious problem for alternative theories: why would a designer leave useless structures behind?

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3. Comparative Biochemistry

Biochemistry provides perhaps the most powerful evidence for evolution. Because all life descends from a common ancestor, all organisms share:

Universal Biochemistry

• The same four DNA bases (A, T, G, C).
• The same 20 amino acids in proteins.
• The same genetic code (with minor variations in mitochondria).
• The same core metabolic pathways (glycolysis, Krebs cycle, oxidative phosphorylation).
• The same ribosomes (though 70S vs 80S).

This universality is explained simply by common ancestry.

Cytochrome c

Cytochrome c is a small protein involved in the electron transport chain, present in every aerobic organism. Because its function is vital, its sequence changes slowly — making it an excellent molecular clock.

Comparisons reveal that species thought to be closely related by morphology also have similar cytochrome c sequences. Humans differ from:

• Chimpanzees by 0 amino acids.
• Rhesus monkeys by 1 amino acid.
• Horses by 12 amino acids.
• Rabbits by 9 amino acids.
• Dogs by 13 amino acids.
• Chickens by 13 amino acids.
• Tuna by 20 amino acids.
• Moths by 31 amino acids.
• Yeast by 43 amino acids.

The pattern exactly matches the phylogeny derived from morphology and the fossil record. Independent lines of evidence agree — strong confirmation of evolution.

Ribosomal RNA

The rRNA within ribosomes is even more conserved than cytochrome c because ribosomes are essential to all life. Carl Woese used rRNA sequences to discover the three domains (Lesson 8). rRNA is the molecule of choice for the deepest phylogenetic questions.

DNA Sequencing

Modern DNA sequencing allows comparison of whole genomes. Key findings:

• Humans and chimpanzees share about 98.8% of their DNA (by most measures), consistent with a split 6–7 million years ago.
• Humans and mice share about 85% of protein-coding sequences.
• Every organism carries "scars" of evolution — pseudogenes, retroviral insertions, and other non-functional DNA shared with relatives.

Endogenous Retroviruses

Retroviruses occasionally insert their DNA into germ cells, where it is passed to offspring. Humans and chimpanzees share exactly the same endogenous retroviral insertions in the same positions — over 200 of them. This would be essentially impossible by chance; it is only explicable if humans and chimps inherited these insertions from a common ancestor.

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4. Biogeography

Charles Darwin himself noted that species distributions around the world made sense only under evolution:

• Island endemics are usually related to nearby mainland species.
• Marsupials dominate Australia because it separated before most placental mammals evolved elsewhere.
• Galápagos finches each evolved on a different island, showing how isolation drives speciation.

Vicariance

When continents split (plate tectonics), populations are separated and evolve independently. For example, Old World monkeys are in Africa and Asia while New World monkeys are in South America — they diverged as the continents did.

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5. Direct Observation

Evolution has been observed in action, on human timescales:

• Antibiotic resistance in bacteria (MRSA, MDR-TB).
• Pesticide resistance in insects (malaria mosquitoes to DDT).
• Peppered moth industrial melanism (see Lesson 11).
• Darwin's finches — beak shapes have measurably changed across droughts on the Galápagos.
• Lizards on Caribbean islands — limb length changes in populations moved between islands.
• Lenski long-term E. coli experiment — more than 75,000 generations of evolution observed, including the emergence of a new metabolic ability (citrate use).

Direct observation answers the complaint that "we can't see evolution happening".

Artificial Selection

Humans have driven dramatic evolutionary changes through selective breeding:

• Dogs — all 400+ breeds descend from grey wolves in 15,000 years.
• Brassicas — broccoli, cabbage, kale, cauliflower, Brussels sprouts and kohlrabi all come from one species (Brassica oleracea).
• Maize — descended from teosinte grass through thousands of years of selection.

Darwin himself used artificial selection as a powerful analogy for natural selection in The Origin of Species.

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Integrating the Evidence

Each line of evidence is individually powerful; together they are conclusive. Fossils, anatomy, biochemistry, biogeography and direct observation tell the same story of gradual change from common ancestors over billions of years. More importantly, the evidence is consistent — a phylogeny built from fossils agrees with one built from DNA which agrees with one built from cytochrome c.

No other theory explains all the facts — which is why modern biology cannot function without evolution.

Exam Tip: OCR questions on evolution evidence reward breadth (mention several lines of evidence) and specific examples (Archaeopteryx, cytochrome c numbers, pentadactyl limb). Avoid vague claims like "fossils prove evolution" — explain how.

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Common Exam Mistakes