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Primary Science · Year 6Chapter 6 · Offspring, adaptation and evolution
Year 6 Science · Offspring, adaptation and evolution · Lesson 6.5

Year 6 Science: Evolution by natural selection

Part of the Year 6 Science offspring, adaptation and evolution chapter. Chapter overview · whole chapter on one page

6.5

Evolution by natural selection

Year 6
EV3: adaptation may lead to evolution; natural selection changes a population over many generations
Story. In a field, a group of beetles lives on the grass. Some are green and some are brown. Birds eat more of the brown beetles, which are easier to see. “Will the green beetles take over?” asks Tom. “Let’s count them each generation, and find out,” says Mrs Chen.

Evolution is the change in living things over many generations. One way that it happens is natural selection:

  1. living things of a species vary;
  2. some variations help the living thing survive in its environment;
  3. those living things are more likely to survive, reproduce and pass on their features;
  4. over many generations, the population changes.

Adaptation may lead to evolution, because helpful features become more common over time.

variationsurvival of helpful featuresmore offspring with themspecies changes over generationsnatural selection, shown in four steps
Four boxes showing natural selection: variation, survival of helpful features, more offspring with them, and the species changes over generations.

The table shows the percentage of green beetles in each generation of a field population. The rest are brown, and each generation has 100 beetles.

GenerationGreen beetles (%)Brown beetles (%)
Generation 16040
Generation 27030
Generation 38020
Generation 49010
020406080100green beetles (%)gen 1gen 2gen 3gen 460%70%80%90%
Line graph of the percentage of green beetles in four generations: 60%, 70%, 80% and 90%.

Key ideas

  • Evolution is change in living things over many generations.
  • Natural selection: helpful variations help survival, so they become more common.
  • Adaptation may lead to evolution, as helpful features spread through a population.
  • The data shows a change from one generation to the next, not a change in one living thing.
Worked example 6.5.1
Look at the table in section 6.5. What percentage of beetles were green in generation 1? Write the number.

Method. Read the first row.

  1. Read the row. Generation 1 has 60 green beetles out of 100.
    60
  2. Write the number. So the percentage of green beetles is 60.
    60%
  3. Write the answer.
    60%

Check. The brown beetles were 40 out of 100. ✓

Worked example 6.5.2
How many percentage points did the share of green beetles rise from generation 1 to generation 4? Write the number.

Method. Subtract the generation 1 value from the generation 4 value.

  1. Read the values. Generation 4 has 90 per cent green, and generation 1 has 60 per cent.
    90 and 60
  2. Subtract. The rise is 90 minus 60.
    90 - 60 = 30
  3. Write the answer.
    30

Check. 60 + 30 = 90. ✓

Worked example 6.5.3
In generation 3, how many brown beetles were there? Write the number.

Method. Use the total of 100 beetles.

  1. Read the value. Generation 3 has 80 green beetles out of 100.
    80
  2. Subtract from the total. The brown beetles are 100 minus 80.
    100 - 80 = 20
  3. Write the answer.
    20

Check. 80 green plus 20 brown is 100. ✓

Watch out

Watch out: natural selection acts on a whole population over many generations. An individual beetle does not change colour to suit the field.

Exam tip

Exam tip. When you describe evolution, mention variation, survival, reproduction, and many generations.

Try it (with your teacher, or a grown-up at home). Use 20 coloured beads or dried beans: 10 of one colour and 10 of another, scattered on a sheet of paper. Ask a partner to pick up ten beads in five seconds, as a bird would. Keep the ones picked up as "eaten". Repeat for three generations, then count the colours. Which colour survived most?

Exercise 6E  Evolution by natural selection

1
FWhat is the percentage of green beetles in generation 2? Write the number.
2
FHow many brown beetles were in generation 1? Write the number.
3
FIn which generation were the most green beetles? Write the generation number.
4
FBy how many percentage points did the share of green beetles rise from generation 1 to generation 2? Write the number.
5
FIs the share of green beetles rising in each generation? Write yes or no.
6
FWhat is the name of the process in which helpful variations become more common over generations? Write the words.
7
FWhat word describes the change in living things over many generations? Write the word.
8
PSDoes one beetle change its colour during its life to suit the field? Write yes or no.
9
PSIn generation 3, how many green beetles were there? Write the number.
10
PSPriya says natural selection means that the green beetles change colour each year. Is Priya right? Write yes or no.
11
RExplain why a helpful variation is more likely to become common over many generations. Write one sentence.
12
RExplain how the data in section 6.5 could show that natural selection is taking place. Write one sentence.
13
RChallenge: explain why the data shows a population change, and not a single beetle changing. Write one sentence.

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