Darwin's Theory of Natural Selection Revision Notes | GCSE AQA Higher Biology

1. The Mystery That Changed Biology

Picture this: you're on a remote island, and you notice that the finches have completely different beak shapes. Some have thick, strong beaks perfect for cracking seeds. Others have thin, pointed beaks ideal for catching insects. Yet they're all finches. Why such dramatic differences?

This exact puzzle fascinated Charles Darwin during his voyage on HMS Beagle. What he discovered didn't just explain the finches — it revolutionised our understanding of life itself. The answer lies in a process so elegant yet powerful that it shapes every living thing on Earth.

2. The Raw Material: Variation Within Species

Look around your classroom. Even though you're all humans, no two people are identical. Height, eye colour, hair texture, even fingerprints — the variation is endless. This same principle applies to every species on Earth.

Trap: Many students think all members of a species are in essence identical. Actually, variation is huge — it's what makes evolution possible. Without differences, there would be nothing for natural selection to work on.

Darwin observed this variation everywhere during his five-year voyage. Finches with different beak shapes, tortoises with different shell patterns, mockingbirds with different songs. The key insight? This variation isn't random noise — it's the foundation of evolutionary change.

3. Survival of the Fittest: The Selection Process

Here's where Darwin's genius shines. He realised that not all variations are equally useful. In any environment, some characteristics help organisms survive and reproduce better than others. Those individuals are more likely to pass on their genes.

Think of it as nature's quality control system. The environment acts like a filter — organisms with helpful traits get through, while those with harmful traits are filtered out. But here's the crucial point: it's not about being 'perfect', it's about being better suited to that specific environment.

Worked example: How Natural Selection Works

Explain how natural selection would affect a population of moths where some are light-coloured and some are dark-coloured, living near a factory that covers trees with soot.

1. Population contains both light and dark moths — This gives natural selection something to work with
2. Trees are covered in dark soot from factory pollution — The environment determines which traits are advantageous
3. Dark moths camouflaged on sooty trees, light moths easily spotted by predators — Better camouflage = higher survival rate
4. More dark moths survive to reproduce and pass on dark colour genes — Survival advantage leads to reproductive advantage
5. Over generations, proportion of dark moths increases in population — This is evolution by natural selection in action

Answer: Natural selection would favour dark moths in this sooty environment, leading to an increase in dark moths over successive generations.

Trap: Students often think 'survival of the fittest' means the strongest or fastest. Actually, 'fittest' means best suited to the environment. A tiny, slow organism might be the 'fittest' if it can hide from predators effectively.

4. Darwin's Journey: From Observation to Theory

Darwin didn't just wake up one day with his theory fully formed. It took decades of careful observation, experimentation, and thinking. His voyage on HMS Beagle (1831-1836) provided the initial spark, but the theory wasn't published until 1859 in 'On the Origin of Species'.

What convinced Darwin? The evidence came from multiple sources: fossils showing gradual changes over time, geographical distribution of species, artificial selection by breeders, and detailed observations of variation in nature. Each piece of evidence supported the same conclusion.

The Galápagos finches were particularly important. Darwin noticed that finches on different islands had different beak shapes, perfectly matched to their food sources. Seed-eaters had thick, strong beaks. Insect-eaters had thin, pointed beaks. Nectar-feeders had long, curved beaks. This wasn't coincidence — it was adaptation in action.

Darwin also drew inspiration from Thomas Malthus's essay on population growth. Malthus argued that populations grow faster than food supplies, leading to competition for resources. Darwin realised this competition would favour individuals with advantageous traits — the mechanism for natural selection.

5. The Complete Picture: How Traits Pass Between Generations

Natural selection only works if advantageous characteristics can be inherited. Darwin observed this in domestic breeding — farmers could selectively breed cattle for milk production or dogs for specific traits. If artificial selection worked, why not natural selection?

Here's the crucial connection: organisms with survival advantages are more likely to reproduce successfully. When they do reproduce, they pass their advantageous genes to their offspring. Over many generations, these beneficial traits become more common in the population.

Trap: Students sometimes think organisms can 'choose' to evolve or that they evolve during their lifetime. Evolution happens to populations over many generations, not to individuals during their lives. A giraffe doesn't grow a longer neck by stretching — giraffes with longer necks are more likely to survive and reproduce.

6. Exam Success: Mastering Natural Selection Questions

Exam questions on natural selection follow predictable patterns. Examiners love to test whether you can apply Darwin's principles to new scenarios. The key is following a logical sequence in your answers.

For 'Explain' questions worth 4-6 marks, use this structure: Variation → Selection pressure → Survival advantage → Reproduction → Inheritance → Population change. Each step should link logically to the next using words like 'therefore', 'because', and 'as a result'.

Mark-winning phrases: 'Individuals with [trait] are more likely to survive', 'These individuals reproduce more successfully', 'The advantageous alleles are passed to offspring', 'Over many generations, the frequency of [trait] increases in the population'.