How Evolution Works Through Natural Selection Revision Notes | GCSE AQA Higher Biology

1. The Survival Game

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.

2. The Four Pillars of Natural Selection

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.

3. How Variation Drives Evolution

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.

4. Selection Pressures Shape Populations

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.

1. Two colour variants exist: light-coloured (60%) and dark-coloured (40%) moths — Variation in colour provides the raw material for selection
2. Industrial pollution darkened tree bark, changing the environment — This created a new selection pressure - camouflage effectiveness changed
3. Dark moths now blend in with dark bark; light moths stand out — The 'best' trait switched from light to dark colouring
4. Birds catch more light moths (easier to see), so more dark moths survive to reproduce — Survival advantage leads to reproductive advantage
5. Dark moths produce more offspring, increasing their frequency from 40% to 80% — The population evolves - trait frequencies change over generations

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.

5. Real-World Examples of Natural Selection

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.

6. Common Misconceptions About Evolution

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'.

7. Exam Technique for Natural Selection Questions

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