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Variation exists in a population, and a selection pressure affects survival. Free GCSE AQA Biology notes with key terms and 12 questions answered.
How Evolution Works Through Natural Selection is part of Variation and evolution in GCSE AQA Higher Biology.
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Variation exists in a population, and a selection pressure affects survival. Free GCSE AQA Biology notes with key terms and 12 questions answered.
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Use this lesson to stay inside Variation and evolution while connecting the detail back to the wider Inheritance, variation and evolution topic.
Read the core explanations from How Evolution Works Through Natural Selection before testing yourself from memory.
Why do giraffes have long necks? Why are polar bears white? These aren't random features - they're the result of a powerful process that shapes all life on Earth.
Picture a population of beetles living in a forest. Some are brown, some are green. When birds hunt for food, which beetles are easier to spot against the brown tree bark? The green ones stand out like beacons, whilst the brown ones blend in perfectly.
This isn't just about beetles - it's about the fundamental mechanism that drives evolution: natural selection. Understanding this process explains how species change over time and why organisms are so perfectly suited to their environments.
Natural selection works like a filter, but instead of filtering water, it filters traits. For this process to work, four conditions must be met:
1. Variation exists: Individuals in a population must have different characteristics. Without variation, there's nothing to select from.
2. Traits are inherited: Characteristics must be passed from parents to offspring through genes. Learnt behaviours don't count.
3. Selection pressure occurs: Environmental factors must affect survival or reproduction. This could be predators, disease, climate, or competition for resources.
4. Differential reproduction: Individuals with advantageous traits must produce more offspring than those without. This is where the 'selection' happens.
Trap: Many students think organisms 'try' to evolve or that they evolve during their lifetime. Actually, evolution happens to populations over many generations, not to individuals.
Variation is the raw material of evolution. Without it, all individuals would be identical and natural selection couldn't work. Think of variation as nature's lottery tickets - the more different tickets you have, the better your chances of winning.
Variation comes from two main sources:
Genetic variation: Caused by mutations, sexual reproduction, and independent assortment during meiosis. This creates the differences in DNA that can be inherited.
Environmental variation: Caused by factors like diet, exercise, or climate. A plant might grow taller in rich soil, but this trait won't be passed to its offspring.
Only genetic variation matters for evolution because only genetic traits can be inherited. Environmental changes to an individual don't affect their DNA.
Selection pressures are like nature's quality control system. They determine which traits are 'good' and which are 'bad' in a particular environment. The same trait can be advantageous in one environment but deadly in another.
Common selection pressures include:
Predation: Faster gazelles escape lions; slower ones don't. Speed becomes advantageous.
Disease: Individuals with better immune systems survive epidemics and reproduce.
Climate: Thicker fur helps animals survive cold winters; thin fur is better in hot climates.
Competition: Better competitors get more food, mates, or territory.
The key insight is that selection pressures change over time and location. What's advantageous today might be harmful tomorrow if the environment changes.
Worked example: Analysing Natural Selection in Action
In a population of moths, 60% are light-coloured and 40% are dark-coloured. After industrial pollution darkens the tree bark, the population becomes 20% light and 80% dark. Explain how natural selection caused this change.
Answer: Natural selection favoured dark moths when pollution darkened the trees, causing the population to evolve from mostly light to mostly dark over several generations.
Natural selection isn't just theory - we can observe it happening in real time. These examples show evolution in action:
Antibiotic resistance in bacteria: When antibiotics are used, most bacteria die but a few resistant ones survive. These multiply rapidly, creating a population that's mostly resistant. This is why doctors warn against overusing antibiotics.
Darwin's finches: During droughts, finches with larger beaks can crack tough seeds whilst those with smaller beaks starve. The population evolves larger average beak size until the drought ends.
Peppered moths: Before industrialisation, light moths were camouflaged against pale tree bark. Pollution darkened the trees, so dark moths gained the advantage. When pollution decreased, light moths became common again.
Notice the pattern: variation exists, environment changes, some variants survive better, population composition shifts. This is natural selection in four steps.
Evolution is often misunderstood. Let's clear up the biggest misconceptions that trip up students in exams:
The biggest trap: Students often write that organisms 'need' to evolve or that they evolve 'in order to' survive. This suggests purpose and planning. Instead, write that organisms 'with advantageous traits are more likely to survive and reproduce'.
Natural selection questions follow predictable patterns. Master these techniques to maximise your marks:
The 6-mark structure for 'Explain how natural selection leads to evolution':
1. Variation exists in the population for the trait in question
2. Selection pressure affects survival (name the specific pressure)
3. Individuals with advantageous traits are more likely to survive
4. These individuals reproduce more and pass on their genes
5. Advantageous alleles become more common in the population
6. Over many generations, the population evolves
Published by Revise Right. Use the official specification reference to check the requirements for your course. Content updated: 2026-03-20.
Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
Before industrialisation, light peppered moths were camouflaged against pale tree bark. Pollution darkened the trees, so birds caught more light moths and dark moths survived to reproduce, making dark moths more common. When pollution decreased, light moths became common again.
Variation exists in a population, and a selection pressure affects survival. Individuals with advantageous traits are more likely to survive, reproduce and pass on their genes, so advantageous alleles become more common in the population, and over many generations the population evolves.
Genetic variation is caused by mutations, sexual reproduction and independent assortment during meiosis. These create differences in DNA that can be inherited, unlike environmental variation caused by diet, exercise or climate.
When antibiotics are used, most bacteria die but a few resistant ones survive. These resistant bacteria multiply rapidly, creating a population that is mostly resistant, which is why doctors warn against overusing antibiotics.
A selection pressure is an environmental factor that affects an organism's ability to survive and reproduce, such as predation, disease, climate or competition. The same trait can be advantageous in one environment but harmful in another, and selection pressures change over time and location.
Only genetic variation matters for evolution because only genetic traits can be inherited. Genetic variation comes from mutations, sexual reproduction and independent assortment during meiosis, while environmental variation, such as a plant growing taller in rich soil, does not affect DNA and is not passed to offspring.
The four conditions for natural selection are: variation exists in the population, traits are inherited through genes, a selection pressure affects survival or reproduction, and individuals with advantageous traits produce more offspring (differential reproduction).
Examples of selection pressures are predation, where faster gazelles escape lions; disease, where individuals with better immune systems survive; climate, where thicker fur helps in cold winters; and competition for food, mates or territory.
During droughts, finches with larger beaks can crack tough seeds while those with smaller beaks starve. The population evolves a larger average beak size until the drought ends.
Saying organisms 'need' to evolve or evolve 'in order to' survive suggests purpose and planning, which is wrong. Instead, organisms with advantageous traits are more likely to survive and reproduce.
A single trait can be advantageous in one environment but deadly in another, and selection pressures change over time and location. Light moths were camouflaged on pale bark, but after pollution darkened the trees, dark moths had the advantage.
A 6-mark answer should cover: variation exists in the population; a named selection pressure affects survival; individuals with advantageous traits are more likely to survive; they reproduce and pass on their genes; advantageous alleles become more common; and over many generations the population evolves.