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Evolution is supported by an overwhelming body of evidence from multiple independent fields of biology. At A-Level, you need to understand and evaluate the different lines of evidence in detail, including the fossil record, comparative anatomy, molecular biology, biogeography, and direct observation.
The fossil record provides direct physical evidence of organisms that lived in the past. It demonstrates:
Transitional fossils (or "intermediate forms") show features of two different groups, providing evidence that one group evolved from another.
| Transitional Fossil | Intermediate Between | Key Features |
|---|---|---|
| Archaeopteryx | Reptiles and birds | Feathers (bird-like), but also teeth, bony tail, and clawed wings (reptile-like) |
| Tiktaalik | Fish and tetrapods (land vertebrates) | Fish-like scales and fins, but also a flat head, neck, and limb-like fin bones with wrist joints |
| Ambulocetus | Land mammals and whales | Had four legs and could walk on land, but also had features adapted for swimming; intermediate between Pakicetus (land-dwelling whale ancestor) and modern whales |
| Homo erectus | Earlier hominins and modern humans | Brain size intermediate between H. habilis and H. sapiens; used more advanced tools |
Despite these limitations, the fossil record provides powerful evidence for evolution. Each new fossil discovery has been consistent with evolutionary theory.
Exam Tip: When discussing fossil evidence, always mention specific examples of transitional fossils and explain which two groups they bridge. Simply stating "fossils show evolution" is not sufficient for full marks.
Homologous structures are anatomical features in different species that share a common structural origin (inherited from a common ancestor) but may serve different functions.
The pentadactyl limb is the classic example. All tetrapod vertebrates (amphibians, reptiles, birds, mammals) have limbs built on the same basic plan:
This shared structure, despite vastly different functions (swimming, flying, running, grasping), is best explained by inheritance from a common tetrapod ancestor, with subsequent modification by natural selection.
Vestigial structures are anatomical features that have lost their original function through evolution but are still present in reduced form.
| Vestigial Structure | Organism | Ancestral Function |
|---|---|---|
| Human appendix | Humans | Thought to have aided digestion of plant material in herbivorous ancestors |
| Pelvic bones in whales | Whales and dolphins | Their land-dwelling ancestors had functional hind limbs; the reduced pelvic bones are vestigial |
| Wings of flightless birds | Ostriches, emus, kiwis | Ancestral birds could fly; the wings have been reduced but not eliminated |
| Eyes of cave-dwelling fish | Astyanax mexicanus (blind cavefish) | Ancestral fish had functional eyes; in the dark cave environment, eyes are no longer needed |
| Human coccyx (tailbone) | Humans | Remnant of a tail present in primate ancestors |
Vestigial structures are evidence for evolution because they only make sense if organisms descended from ancestors in which these structures were functional.
Analogous structures have similar functions but different evolutionary origins — they evolved independently in unrelated lineages through convergent evolution.
| Structure | Organisms | Evidence of Convergent Evolution |
|---|---|---|
| Wings | Birds, bats, insects | All used for flight, but structurally very different (feathered limbs, skin membrane, chitinous) |
| Eyes | Vertebrates, cephalopods (octopus) | Both are camera-type eyes but evolved independently; the octopus eye has no blind spot |
| Streamlined body | Dolphins (mammals), sharks (fish) | Both adapted to aquatic locomotion but from very different ancestors |
Analogous structures demonstrate that natural selection can produce similar solutions to similar environmental challenges in completely unrelated organisms.
Molecular biology provides some of the most compelling and quantitative evidence for evolution.
All living organisms use DNA as their genetic material and share the same genetic code. This universality strongly suggests common ancestry. More specifically:
| Species Comparison | % DNA Similarity | Relationship |
|---|---|---|
| Human vs chimpanzee | ~98.7% | Very closely related (diverged ~6–7 million years ago) |
| Human vs gorilla | ~95.7% | Closely related |
| Human vs mouse | ~85% | More distantly related (diverged ~96 million years ago) |
| Human vs fruit fly | ~60% | Very distantly related |
| Human vs banana | ~60% (for shared genes) | Extremely distantly related |
Comparing amino acid sequences of homologous proteins across species reveals evolutionary relationships. Cytochrome c — an electron carrier in the mitochondrial electron transport chain — is found in virtually all aerobic organisms and is highly conserved.
| Species | Amino Acid Differences (vs Human Cytochrome c) |
|---|---|
| Chimpanzee | 0 |
| Rhesus monkey | 1 |
| Horse | 12 |
| Chicken | 13 |
| Tuna | 21 |
| Yeast | 44 |
The pattern of differences matches the expected pattern of evolutionary relationships: more closely related species have fewer differences.
Immunological comparisons can reveal how similar proteins are between species:
By calibrating the rate of molecular change against known divergence dates from the fossil record, molecular clocks can estimate when species diverged. The principle is that neutral mutations accumulate at an approximately constant rate.
Exam Tip: Molecular evidence questions are very common. Be specific: state that homologous DNA or protein sequences are compared, that the number of differences is counted, and that fewer differences indicate more recent common ancestry. Always link molecular evidence back to the concept of common descent.
Biogeography — the study of the geographical distribution of species — provides evidence for evolution.
Island species resemble nearby mainland species — suggesting colonisation from the mainland followed by evolutionary divergence. Galapagos finches resemble South American finches because they evolved from South American ancestors.
Isolated landmasses have unique species — Australia's marsupials evolved in isolation after the continent separated from other landmasses. Madagascar's lemurs diversified in isolation.
Continental drift explains distributions — similar fossils are found on continents that were once connected. Glossopteris (a fossil plant) is found in South America, Africa, India, Australia and Antarctica — all once joined in the supercontinent Gondwana.
Ocean islands lack certain groups — remote volcanic islands (which were never connected to a continent) typically lack native amphibians and terrestrial mammals, because these groups cannot cross ocean barriers. The organisms present are those that could fly or be carried by wind/water.
Evolution can be directly observed in populations with short generation times:
The same process occurs with insecticide resistance. For example, DDT resistance in mosquitoes evolved rapidly after widespread DDT use began.
As described in the previous lesson, the change in frequency of light and dark forms of Biston betularia during and after industrialisation is directly observed directional selection.
Peter and Rosemary Grant studied Galapagos finches for over 40 years on the island of Daphne Major. They documented:
Embryos of different vertebrate species look remarkably similar in early stages of development, sharing features such as:
These shared embryological features suggest that vertebrates share a common ancestor with a similar developmental programme. The differences in adult form arise from modifications to this shared developmental plan.
The strongest aspect of the evidence for evolution is that multiple independent lines of evidence all point to the same conclusions:
| Evidence Type | What It Shows |
|---|---|
| Fossils | Life has changed over time; transitional forms exist |
| Comparative anatomy | Homologous structures indicate common ancestry |
| Molecular biology | DNA/protein similarities reflect evolutionary relationships |
| Biogeography | Species distributions match predictions of evolutionary theory |
| Direct observation | Evolution occurs in real time in organisms with short generation times |
| Comparative embryology | Shared developmental stages reflect common ancestry |
Exam Tip: Extended-response questions may ask you to discuss the evidence for evolution. Cover at least three different types of evidence, give specific examples for each, and conclude by noting that the convergence of independent lines of evidence makes the case for evolution overwhelming.
| Key Concept | Detail |
|---|---|
| Fossil record | Shows change over time, transitional forms, sequential appearance |
| Transitional fossils | Bridge gaps between groups (Archaeopteryx, Tiktaalik) |
| Homologous structures | Same origin, different function — common ancestry |
| Vestigial structures | Reduced remnants of once-functional features |
| DNA/protein comparisons | More similar sequences = more closely related |
| Biogeography | Species distributions match evolutionary predictions |
| Observed evolution | Antibiotic resistance, peppered moths, Darwin's finches |
| Convergent evidence | Multiple independent lines all support evolution |
Exam Tip: Remember that no single piece of evidence "proves" evolution — it is the convergence of evidence from fossils, anatomy, molecular biology, biogeography and direct observation that makes the theory of evolution the most well-supported explanation for the diversity of life.
The Edexcel 9BI0 specification places this material at the close of Topic 4: Biodiversity and Natural Resources, where the evidence for evolution is assessed alongside the mechanism (Lesson 6 — Natural Selection) and the outcome (Lesson 7 — Speciation). Synoptic ties run backwards to Lesson 3 (Phylogenetics and Cladistics), which provides the comparative-tree framework into which fossil, morphological and molecular evidence is fed; to Topic 1 — protein structure, which underpins the cytochrome-c amino-acid-difference comparison; and to Topic 8 — DNA sequencing and genomics, which has transformed molecular phylogenetics. The specification rewards candidates who treat evolution as a scientific theory in the technical sense — a unifying explanation supported by multiple independent lines of evidence — and who can name and apply at least three of the five canonical lines (fossil, comparative anatomy including vestigial structures, comparative embryology, molecular evidence, biogeography) with worked examples (refer to the official Pearson Edexcel 9BI0 specification document for exact wording).
Question (8 marks):
A palaeontologist working in the Canadian Arctic searches sedimentary rocks dated to ∼375 million years ago for a predicted intermediate between fish and tetrapods. The team recovers a specimen with fish-like scales and gill arches, a flat skull with dorsal eyes, a mobile neck (absent in fish), and lobed fins containing internal bones homologous to the tetrapod humerus, radius, ulna and proximal wrist. The specimen is Tiktaalik roseae. Independent comparative-anatomy work shows all modern tetrapods share a pentadactyl limb plan; molecular phylogenetics places lungfish as the closest living relatives of tetrapods.
(a) Explain why the prior prediction of Tiktaalik's rock-age and morphology is stronger evidence for evolution than the discovery alone. (3)
(b) Identify the morphological features that classify Tiktaalik as a transitional fossil, naming the two groups it bridges. (3)
(c) Evaluate how the pentadactyl-limb evidence and the molecular-phylogenetic placement of lungfish converge on the same conclusion as the fossil. (2)
Solution with mark scheme:
(a) M1 (AO1.2) — evolutionary theory predicts that fish-to-tetrapod intermediates should be found in Late Devonian rocks (∼380–365 mya), the time interval bracketed by fully-aquatic lobe-finned fish in older strata and limbed tetrapods in younger strata.
M1 (AO2.1) — the Ellesmere Island fieldwork was targeted on rocks of exactly this age on the basis of the prior prediction; finding the predicted morphology in the predicted age-interval is a risky prediction that could have failed.
A1 (AO3.1a) — the success of a risky prediction is logically stronger than retrospective interpretation of an accidental find; it satisfies the Popperian criterion of falsifiability — had no intermediate been found across many sites of the right age, evolutionary theory would have been weakened.
(b) M1 (AO1.2) — Tiktaalik retains fish-like features: scales, gill arches and lobed fins (rather than limbs).
M1 (AO1.2) — Tiktaalik shows tetrapod-like features: a flat dorsoventrally-compressed skull, a mobile neck (independent skull movement is impossible in fish), dorsally-placed eyes, and internal fin bones homologous to the tetrapod humerus, radius, ulna and proximal wrist.
A1 (AO2.1) — the simultaneous presence of fish and tetrapod characters in one organism classifies it as a transitional form bridging lobe-finned fish (sarcopterygians) and early tetrapods.
(c) M1 (AO2.1) — the pentadactyl limb shared by all modern tetrapods is a homologous structure inherited from a common tetrapod ancestor; Tiktaalik's fin-bone arrangement is the predicted precursor to that pentadactyl plan.
A1 (AO3.2a) — the fossil (palaeontology), the homologous limb (comparative anatomy) and the lungfish placement (molecular phylogenetics) are three independent methodologies that converge on the same evolutionary tree — convergent evidence is the operational hallmark of a robust scientific theory.
Total: 8 marks.
Question (6 marks): Outline the five major lines of evidence for evolution, distinguish homology from analogy with named examples, and evaluate why the convergence of independent lines is treated as stronger evidence than any one line alone.
Mark scheme decomposition by AO:
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