KS3 · Biology

Natural selection

Did giraffes get their long necks by stretching for leaves and passing the stretch on? It sounds sensible. It's wrong — and here's what really happened.

How giraffes got long necks — six steps

1Variation23456

Stage 1 of 6: Variation. paused

Variation · 1/6scrub the six steps →

Giraffe ancestors had necks of different lengths — purely by chance. Nobody chose their neck.

Scrub from step 1 to step 6 and watch each step set up the next.

Biology · Natural selection

Can natural selection pass it on?

Put each feature where it belongs. Could natural selection make it more common over many generations?

Still to sort

Varies by chance and is inherited (0)

Natural selection can make these more common.

Where the line is: Ask: did this come from the parents, or was it built up during this individual's own life?

Gained during a lifetime (0)

Natural selection cannot pass these on.

Where the line is: A change to an individual's body during its life is not passed on to its offspring, however useful it is.

5 of 5 still to sort.

The make-or-break question: where did this feature come from?

Watch out: However useful a feature gained during a lifetime is, it is not passed on to offspring.

Peppered moths, part 1

Peppered moths vary in colour: some are pale, some are dark. Commit before you reveal.

It is the 1800s. Pollution from burning coal has killed the pale lichen on the trees, and soot has darkened the tree trunks. Over many generations, what happens to the moths?

Peppered moths, part 2 — the twist

The dark moths are now common. Then people clean up the air.

Air pollution is reduced (the Clean Air Act of 1956 was part of this) and tree bark becomes lighter again. What happens to the moths now?

When the environment changes

?

Reason it through

How can a change in the environment lead to a species becoming extinct?

Link 1 of 4

First link · your turn

The conditions a species lives in change — sometimes because of human actions. What can happen to how well its individuals are adapted?

2
Locked — reveal the link above first
3
Locked — reveal the link above first
4
Locked — reveal the link above first

Check your thinking

What is really going on?

A population changes over many generations until a helpful feature is common to almost all of them.

Which of these is closest to what you think is going on?
How sure are you?

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

What you need to know

  • Variation within a species arises by random chance. Organisms do not choose or decide to change themselves.
  • Natural selection runs in six steps: variation, advantage, competition, survival, reproduction, inheritance.
  • Features gained during a lifetime, such as bigger muscles from exercise, are not passed on to offspring.
  • When the environment changes, which individuals are best adapted can change too — and human actions can cause those changes.

The big picture

Natural selection is the process by which individuals that are better adapted to their environment are more likely to survive, reproduce and pass on their heritable features, so those features become more common over many generations. It starts with variation that arises by chance, and it changes direction when the environment changes.

Key points

1An adaptation is a feature that helps an organism survive and reproduce where it lives, such as a hummingbird's long, thin beak for reaching nectar in narrow flowers. Adaptations are a result of natural selection.
2Because of variation, some individuals are better adapted to compete for resources such as food, mates or shelter.
3The 'fittest' individuals are the best adapted to their environment — not necessarily the biggest or strongest.
4Natural selection describes what is more likely to happen, not what happens to every individual. Repeated over many generations, it makes an advantageous heritable trait common.
5Peppered moths: soot from burning coal in the 1800s made dark moths better camouflaged, so they became more common; after air pollution was reduced, including through the 1956 Clean Air Act, pale moths became more common again.
6A change in the environment can leave individuals, or a whole species, less well adapted to compete and reproduce, which may lead to extinction.

Worked example

Problem

A hummingbird's long, thin beak lets it reach nectar inside narrow flowers. Use the six steps to explain how natural selection could have made long, thin beaks common.

⚠ Watch out

Writing that an organism changed because it needed to or tried to — for example, that a giraffe stretched its neck and passed the longer neck on. The variation was already there by chance, and features gained during a lifetime are not inherited.

🧠

Memory hook

Six steps, one chain: Very Awkward Camels Sometimes Run Indoors — Variation, Advantage, Competition, Survival, Reproduction, Inheritance. Break any link and the chain stops.

✓

Check yourself

Cover the page. Could you explain why the peppered moth 'winner' flipped twice — once to dark, then back to pale — and what that tells you about the meaning of 'fittest'?

Flashcards

(14)
What is natural selection?
The process by which better-adapted individuals are more likely to survive, reproduce and pass on their heritable traits, so those traits become more common over many generations.
What is an adaptation?
A feature that helps an organism survive and reproduce where it lives — for example, a hummingbird's long, thin beak reaches nectar in narrow flowers.
Where does variation within a population come from?
Random chance. Some individuals simply have different traits from others — nobody chooses them.
Can bigger muscles from exercise be passed on to offspring?
No. Features gained during a lifetime are not passed on, so natural selection cannot make them common.
In natural selection, what does 'fittest' mean?
Best adapted to the environment — not necessarily the biggest or strongest.
What does 'heritable' mean, and why does natural selection need it?
A heritable trait can be passed from parents to offspring. Without that, an advantage would end with the individual and could never spread.
Name three resources individuals compete for.
Food, mates and shelter.
Why does natural selection talk about what is 'more likely'?
It describes what tends to happen across a population, not what happens to every individual every time.
How long does it take natural selection to make a trait common?
Many generations — the steps repeat in each generation until the trait is common across the population or species.
Why did long necks become common in giraffes?
Ancestors with longer necks, by chance, reached higher leaves, competed better for food, were more likely to survive and reproduce, and passed the trait on — over many generations.
Why did dark peppered moths become more common in the 1800s?
Soot from burning coal darkened tree trunks, so dark moths were better camouflaged, less likely to be eaten by predators, and more likely to survive and reproduce.
What happened to peppered moths after air pollution was reduced?
Bark became lighter, pale moths were better camouflaged again, and pale colouring became more common.
How can a change in the environment affect natural selection?
Some well-adapted individuals may become less well adapted, and some poorly adapted ones may become better adapted. Human actions can cause such changes.
How can a change in the environment lead to extinction?
It can leave a whole species less well adapted to compete and reproduce, which may lead to extinction.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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How this lesson was checked. This KS3 Biologylesson was published through Lightbulb Learning's human-designed editorial process — the educational standards, accuracy rules and publication checks it must pass were authored and approved by Philip Halpin. It passed subject-specific assessment, automated educational checks and technical publication verification before going live (publication checks completed 30 September 2026). Published pages are monitored, human spot-checking is ongoing across the lesson library, and anything found wrong is corrected or withdrawn. How our lessons are made and checked. Spotted a mistake? Email hello@lightbulblearning.co and we'll review it.