What is Evolution? Revision Notes | GCSE AQA Higher Biology

1. What is Evolution?

Why do giraffes have long necks? Why are polar bears white? These aren't random features - they're the result of evolution, one of the most powerful ideas in science.

Think of evolution like this: imagine a population of beetles. Some are green, some are brown. Over many generations, you notice more brown beetles and fewer green ones. The population has evolved - its characteristics have changed over time.

Trap: Many students think individual organisms evolve during their lifetime. Actually, populations evolve over many generations - individuals don't change their inherited characteristics.

2. The Engine of Evolution: Natural Selection

Evolution doesn't happen randomly. There's a mechanism driving it: natural selection. This is where Darwin's genius shines through.

Natural selection works like a filter. Picture those beetles again: if birds find it easier to spot green beetles on brown tree bark, more green beetles get eaten. Brown beetles survive longer, reproduce more, and pass on their 'brown' genes. Over time, the population becomes mostly brown.

For natural selection to work, you need three key ingredients: variation (different characteristics), inheritance (traits passed to offspring), and selection pressure (something that affects survival or reproduction).

3. The Big Picture: Life's Journey

The theory of evolution by natural selection states that all species of living things have evolved from simple life forms that first developed more than three billion years ago. That's a mind-boggling timescale.

Imagine compressing Earth's history into a single year. Simple life forms appear in March. Complex cells don't show up until August. Animals with shells appear in mid-November. Dinosaurs rule for just a few days in December. Humans appear in the last few minutes of New Year's Eve.

This vast timescale is crucial. Small changes accumulate over millions of generations. A tiny advantage - running slightly faster, seeing slightly better, digesting food slightly more efficiently - compounds over time into dramatic differences.

4. From Adaptation to New Species

Evolution doesn't just change existing species - it can create entirely new ones. This happens when populations become so different they can no longer interbreed to produce fertile offspring.

Think of Darwin's finches on the Galápagos Islands. Different islands had different food sources. Finches with beaks suited to their island's food survived better. Over time, each island's finch population developed distinct beak shapes. Eventually, they became separate species.

Worked example: How Natural Selection Creates New Species

Explain how a single ancestral species can evolve into multiple new species through natural selection.

1. A single population becomes separated (by geography, behaviour, etc.) — Populations must be isolated to evolve differently
2. Each separated population faces different environmental challenges — Different environments select for different traits
3. Each population evolves different adaptations over many generations — Natural selection shapes each population differently
4. Populations become so different they cannot interbreed successfully — This is the key test - separate species cannot produce fertile offspring together

Answer: Geographic separation leads to different selection pressures, causing populations to evolve different adaptations until they become reproductively isolated as separate species.

5. Exam Technique: Mastering Evolution Questions

Evolution questions often trip students up because they confuse individual change with population change, or they forget the timescales involved. Here's how to nail these questions every time.

Exam depth extension: In what is evolution?, connect what is evolution? to the engine of evolution: natural selection using clear cause-and-effect statements. When you explain biological processes, include what changes, why it changes, and the consequence for cells, organisms, or populations. If calculations appear, show each step, include units, and end with a sentence that interprets the result in biological context.

Command word focus: 'Explain' questions want you to give reasons. Don't just describe what happens - explain WHY it happens using natural selection principles.

Notice how the sample answer follows the natural selection sequence: advantage → survival → reproduction → inheritance → population change. This structure works for any evolution question.