Evolution and Natural Selection Revision Guide
GCSE Biology | Foundation & Higher
Key Facts
- Evolution is the gradual change in inherited characteristics of a population over many generations.
- Natural selection is the mechanism that drives evolution: organisms best adapted to their environment are more likely to survive and reproduce.
- Charles Darwin proposed the theory of evolution by natural selection in 1859.
- Variation within a species is caused by genetic mutations and sexual reproduction.
- Speciation occurs when populations become so different they can no longer interbreed.
What Is Natural Selection?
Natural selection is often summarised as "survival of the fittest," but "fittest" means best adapted, not strongest. The process works like this:
- Individuals within a species show variation in their characteristics (e.g. fur colour, beak shape, speed).
- Some of these variations are caused by genetic mutations and can be inherited.
- Individuals with characteristics better suited to the environment are more likely to survive and reproduce.
- They pass on the advantageous genes to their offspring.
- Over many generations, the advantageous characteristic becomes more common in the population.
This process is very slow. It takes thousands or millions of generations for significant changes to occur.
Charles Darwin
Charles Darwin (1809-1882) developed the theory of evolution by natural selection after a five-year voyage on HMS Beagle (1831-1836). On the Galapagos Islands, he observed that finches on different islands had different beak shapes adapted to the food available. He published his theory in "On the Origin of Species" in 1859.
Darwin's theory was controversial because it contradicted the religious belief that God created all species in their current form. It took decades for his ideas to be widely accepted. Today, evolution by natural selection is supported by overwhelming evidence and is the foundation of modern biology.
Evidence for Evolution
Fossil Record
Fossils show how organisms have changed over millions of years. Older rock layers contain simpler organisms; newer layers contain more complex ones. Transitional fossils (like Archaeopteryx, which has features of both dinosaurs and birds) show the links between groups. However, the fossil record is incomplete because not all organisms fossilise.
Antibiotic Resistance
Bacteria evolve resistance to antibiotics through natural selection. When antibiotics are used, most bacteria are killed, but a few with a random mutation may survive. These resistant bacteria reproduce rapidly, passing on the resistance gene. This is evolution happening in real time and is a major public health concern (e.g. MRSA).
Comparative Anatomy
Many species share similar bone structures (homologous structures). For example, the human arm, whale flipper, bat wing, and dog leg all have the same basic bone arrangement. This suggests they share a common ancestor and have adapted to different environments.
DNA Evidence
All living organisms share DNA. Species that are closely related have more similar DNA sequences. For example, humans share about 98.7% of their DNA with chimpanzees. DNA analysis provides the strongest modern evidence for evolution and common ancestry.
Speciation
Speciation is the formation of new species. It usually happens when a population is separated by a geographical barrier (e.g. a river, mountain range, or island). This is called geographical isolation.
- A population is split into two groups by a physical barrier.
- The two groups experience different environmental conditions.
- Different mutations occur in each group, and natural selection favours different characteristics.
- Over time, the two groups become so genetically different that they can no longer interbreed to produce fertile offspring.
- They are now two separate species.
Other Key Concepts
- Selective breeding (artificial selection): Humans choose organisms with desirable characteristics to breed together. This is used in agriculture (cows bred for high milk yield) and dog breeding. It reduces genetic variation, which can cause health problems.
- Genetic engineering: Genes from one organism are inserted into another using enzymes. This can create organisms with desired traits (e.g. insulin-producing bacteria, pest-resistant crops).
- Extinction: A species becomes extinct when no individuals remain. Causes include: environmental change, new predators, new diseases, habitat destruction, competition, and catastrophic events (e.g. asteroid impact).
- Classification: Organisms are classified into groups based on shared characteristics. The Linnaean system uses: Kingdom, Phylum, Class, Order, Family, Genus, Species. Modern classification also uses DNA analysis.
Worked Example
Question: Explain how a population of bacteria can become resistant to an antibiotic.
1. Within the bacterial population, there is natural variation due to random genetic mutations.
2. Some bacteria have a mutation that makes them resistant to the antibiotic.
3. When the antibiotic is used, non-resistant bacteria are killed.
4. The resistant bacteria survive and reproduce, passing on the resistance gene.
5. Over time, the proportion of resistant bacteria increases until most of the population is resistant.
This is an example of natural selection. The antibiotic is the selection pressure.
Common Mistakes
- Saying organisms "choose" to evolve or "develop" adaptations on purpose. Evolution is random mutation + natural selection, not a conscious process.
- Confusing natural selection with selective breeding. Natural selection happens without human intervention.
- Saying individuals evolve. Individuals do not evolve; populations do, over many generations.
- Forgetting that variation must be genetic (inherited) for natural selection to work. Environmental variation is not passed on.
Exam Tips
- For natural selection questions, always follow the sequence: variation, selection pressure, survival, reproduction, inheritance.
- Use the correct scientific language: mutation, allele, phenotype, selection pressure, adapted.
- Antibiotic resistance is a common exam topic. Practise explaining the full process step by step.
- For 6-mark questions about speciation, include geographical isolation, different selection pressures, different mutations, and the inability to interbreed.
- Remember: Darwin could not explain how variation arose (DNA had not been discovered). This was a limitation of his original theory.
Practice Questions
- Explain the process of natural selection using an example.
- Describe three pieces of evidence that support the theory of evolution.
- Explain how a new species can form through geographical isolation.
- Why is antibiotic resistance a growing problem? What can be done to reduce it?
- Explain the difference between natural selection and selective breeding.
- Why was Darwin's theory controversial when it was first published?
Recommended Revision Guides
Top-rated guides for GCSE Biology:
- CGP GCSE Biology Complete Revision and Practice — Comprehensive coverage of all topics.
- Hodder GCSE AQA Biology Revision Guide — Exam-focused with clear explanations.
- Oxford Revise AQA GCSE Biology — Visual revision with exam practice.