5.4.3Topic 5 · On the Wild Side

Evidence of Evolution and the Scientific Community — Edexcel A Level Biology A (SNAB) 9BI0 Revision Notes & Topical Questions

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5.4.3 Evidence of Evolution and the Scientific Community — Edexcel A Level Biology A (SNAB) 9BI0 notes: Describe the fossil, anatomical, molecular and behavioural evidence for evolution, and explain how ideas about evolution have been tested and accepted by the scientific communi…

Homologous structures
Analogous structures
molecular clock
Biogeography
Exam-style question 1
Exam-style question 2

Part 1 of 7Fossil evidence

Fossils are the preserved remains or traces of organisms, found in sedimentary rock layers.

Fossil evidence

Older fossils are generally in deeper layers, and they can be dated by radiometric methods.

Fossil sequences show gradual change, for example in the evolution of the horse from small multi-toed ancestors to larger single-hoofed animals,

and in transitional forms such as Tiktaalik and Archaeopteryx (feathered dinosaur).
- (fish-to-tetrapod)

The fossil record is incomplete because few organisms fossilise, soft tissues rarely survive and many deposits have not been found,

so conclusions are limited.

Part 2 of 7Comparative anatomy

Homologous structures have the same basic structure but different functions, such as the pentadactyl limb of humans, bats, whales and horses.

Comparative anatomy

Analogous structures

They suggest a common ancestor, with adaptive radiation modifying the structure for different uses. Analogous structures have a similar function but different origin,

such as the wings of insects and birds,
and indicate convergent evolution in similar environments.

Vestigial structures, such as the human appendix or the pelvic bones of whales, are reduced and have lost their original function.

Part 3 of 7Molecular evidence

All organisms use DNA, the same genetic code and similar proteins, which supports common ancestry.

Comparing the base sequences of genes,

or amino acid sequences of proteins such as cytochrome c or haemoglobin, shows that the more closely related two species are, the fewer differences there are.

Molecular clock

Because mutations accumulate at an approximately steady rate, the number of differences can be used as a molecular clock to estimate when two lineages diverged.

DNA hybridisation and sequencing of whole genomes give a detailed picture of relationships and allow construction of phylogenetic trees.

Molecular evidence generally agrees with the conclusions from fossils and anatomy, which strengthens it.

Part 4 of 7Other evidence

Biogeography, the geographical distribution of species, shows that isolated islands have unique species closely related to those of the nearest mainland.

Other evidence

Observed evolution provides direct evidence: antibiotic resistance in bacteria, pesticide resistance in insects and changes in beak size of Galapagos finches in drought years, measured by Peter and Rosemary Grant.

Artificial selection in crops and domestic animals shows how selection can change a population quickly.

Evidence	Strength	Limitation
Fossils	Direct, shows sequence in time	Incomplete
Anatomy	Shows relationships	Convergence can mislead
Molecular	Quantifiable, many species	Mutation rates can vary

Part 5 of 7Acceptance by the scientific community

Darwin delayed publication for twenty years while he gathered evidence.

Wallace independently reached the same idea, and the two presented their work together in 1858.

The theory was accepted over time because the evidence was extensive,
but it was initially challenged on religious grounds and because the mechanism of inheritance was unknown.

Mendel's work on inheritance and later discovery of DNA provided the missing mechanism.

Earlier, Lamarck suggested that acquired characteristics could be inherited, but experiments did not support this and no mechanism was found.

Scientists accept a theory when its predictions are repeatedly supported by independent evidence, and they remain open to modification.

Peer review, publication of data and replication by other scientists maintain the reliability of conclusions. Public acceptance can lag, and discussion of evolution also includes social and ethical questions.

Part 6 of 7Observing evolution and dating methods

The best-known direct observation of natural selection in action is the long-term study of medium ground finches on Daphne Major in the Galapagos.

In a drought, small soft seeds became scarce and only large, hard seeds were available.

Birds with larger, deeper beaks survived better, and the next generation had a larger mean beak depth,

which showed that beak size is heritable and subject to selection.

When rains returned the trend reversed.

Similarly, populations of insects exposed to insecticide become resistant within a small number of generations.

Dating is important in assessing fossil evidence.

Observing evolution and dating methods

Radiometric dating uses the known rate of decay of radioactive isotopes: carbon-14 (half-life about 5730 years) is used for remains up to about 50 000 years old,

while isotopes such as potassium-40 and uranium-238 date much older rocks.

The age of a fossil can be found by measuring the proportion of the parent isotope remaining and applying its half-life.

Index fossils, found only in a narrow range of rock layers, also allow relative dating of strata in different places.

The pattern of agreement across independent lines of evidence is a central idea: fossils, anatomy, DNA, biogeography and observed change all support descent with modification. When different methods give consistent answers, confidence rises, and this is how the scientific community judges a theory.

Part 7 of 7Exam-style questions

Explain how comparing the amino acid sequences of a protein in different species provides evidence for evolution. [3]3 marks
Distinguish between homologous and analogous structures. [2]2 marks
Suggest why the fossil record gives an incomplete picture of evolution. [3]3 marks

Key terms

Term	Meaning
Fossil	Preserved remains or traces of an organism from the past.
Homologous structure	Structure with the same origin but a different function.
Analogous structure	Structure with the same function but a different origin.
Molecular clock	Using the rate of accumulating mutations to estimate divergence time.
Vestigial structure	A reduced structure that has lost its main function.
Biogeography	The study of the geographical distribution of species.
Convergent evolution	Unrelated organisms evolving similar features in similar environments.
Phylogenetic tree	A diagram of evolutionary relationships.

Avoid these mistakes

Confusing homologous and analogous structures; use the origin versus function distinction.

Saying evolution is 'only a theory'. In science a theory is a well-tested explanation supported by many lines of evidence.

Claiming Lamarck's idea of inheritance of acquired characteristics is supported by evidence.

Ignoring limitations of evidence, such as gaps in the fossil record.

Last updated: 18 October 2026

Part 1 of 7Fossil evidence

Part 2 of 7Comparative anatomy

Part 3 of 7Molecular evidence

Part 4 of 7Other evidence

Part 5 of 7Acceptance by the scientific community

Part 6 of 7Observing evolution and dating methods

Part 7 of 7Exam-style questions

Explain how comparing the amino acid sequences of a protein in different species provides evidence for evolution. [3]3 marks

Distinguish between homologous and analogous structures. [2]2 marks

Suggest why the fossil record gives an incomplete picture of evolution. [3]3 marks

Key terms

Avoid these mistakes

Marked practiceQuizzes for this lesson