Year 9 Science · Lesson 8

Natural selection

Why some adaptations win

There are two moths on this tree. Can you find both?

A light peppered moth and a dark peppered moth resting on a birch trunk
Light and dark peppered moths on birch. Martinowksy, CC BY-SA 3.0

Starter · true or false?

"Giraffes stretched their necks to reach high leaves, so their babies were born with longer necks."

False

This was Jean-Baptiste Lamarck's idea, in 1809. Stretching your neck doesn't change your genes, so it can't be passed on to your offspring.

Keep this in mind. By the end of the lesson you'll be able to give the right explanation.

The scientists

Two people had the same idea

Charles Darwin sailed round the world on HMS Beagle from 1831 to 1836, including the Galápagos Islands.

Alfred Russel Wallace collected animals in the Amazon and in the islands of South East Asia.

Each worked out natural selection on his own. Their ideas were presented together in 1858. In 1859 Darwin published On the Origin of Species.

Photograph of Charles Darwin, seated
Darwin. Maull and Fox, public domain
Photograph of Alfred Russel Wallace, seated
Wallace. London Stereoscopic Co., public domain

Learn this sequence

Natural selection in five steps

1
Variation
Individuals of a species are different from each other.
2
Selection
The environment (predators, food, climate) means some survive better than others.
3
Reproduction
The survivors breed and have more offspring.
4
Inheritance
The offspring inherit the helpful genes.
5
Time
Over many generations, the helpful trait becomes common.

Nothing changes because it needs to. The variation is already there; the environment picks which survives.

Case study · Lab manual p19

The peppered moth

  • Before 1850: almost all peppered moths in England were light, speckled like the lichen on tree bark.
  • 1848: a dark moth was recorded in Manchester. Factory soot was blackening the trees.
  • 1895: about 98% of Manchester's peppered moths were dark.
  • After 1956: clean air laws cut the soot. Light moths became common again.
A light, speckled peppered moth
A dark peppered moth

Both: Chiswick Chap, CC BY-SA 3.0 and CC BY-SA 3.0

Lab manual p19 Q1

Explain the moths with the five steps

StepPeppered moths during the Industrial Revolution
VariationSome moths were light, a few were dark.
SelectionOn sooty trees, birds spotted and ate more light moths.
ReproductionMore dark moths survived to breed.
InheritanceTheir offspring inherited the genes for dark wings.
TimeOver many generations, dark moths became the most common.

Back to the starter

Now explain the giraffe properly

Use all five words: variation, selection, reproduction, inheritance, time.

Giraffes varied in neck length. When food was short, those with longer necks could reach more leaves and were more likely to survive. They had more offspring, which inherited the genes for longer necks. Over many generations, long necks became common.

Lab manual p16

Activity: Birds and Beaks

  1. Write the foods in the top row of the results table.
  2. With one beak, pick up as much of the first food as you can in 30 seconds. Drop it in your cup (the "stomach").
  3. Count and record how many pieces you collected.
  4. Repeat with each food, then with each beak.

Beaks

  • Tweezers
  • Clothes peg
  • Spoon
  • Chopsticks

Lab manual p16 Q2

Beaks of Aotearoa

A kiwi

Kiwi

Long beak with nostrils at the tip. Probes the soil to smell out worms.

A tūī

Tūī

Curved beak and a brush-tipped tongue to reach nectar in flowers.

A kererū

Kererū

Wide gape to swallow big fruits whole, like taraire and karaka.

Two kākā on a branch

Kākā

Strong, hooked beak to rip open bark and dig out grubs.

Kiwi: Maungatautari Ecological Island Trust, public domain. Tūī and kererū: Charles J. Sharp, CC BY-SA 4.0. Kākā: Kate Macbeth, CC BY-SA 4.0.

Exit question · Lab manual p16 Q3

An island where the only food is hard seeds

Which beak shape would help a bird survive and reproduce? Explain using the idea of natural selection.

Use all five steps: variation, selection, reproduction, inheritance, time.

Start with: "The birds on the island had different beaks…"

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