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KS3 Science · Year 9Chapter 3 · Variation, natural selection and evolution
Year 9 · Chapter 3

Variation, natural selection and evolution

By the end of this chapter you will be able to

  • Explain genetic, environmental and combined variation with examples
  • Tell continuous from discontinuous variation, and draw a suitable graph
  • Describe the stages of natural selection in order
  • Use fossils and the work of Darwin and Wallace as evidence for evolution
  • Explain how antibiotic resistance develops by natural selection, with worked numbers

You should already know

  • Living things and their features (Year 7 Science)
  • Bacteria and antibiotics (Year 8 Science, Chapter 4: health, disease and immunity)

Where this leads

  • Chromosomes, genes and alleles (Chapter 2), which give the genetic variation in this chapter

National Curriculum for England, Key Stage 3 science, Year 9. Content: 9B2 variation is genetic, environmental or both; continuous and discontinuous variation, Natural selection: variation, competition, survival, reproduction and inheritance of advantageous alleles, Fossils and the evidence for evolution; extinction; the work of Darwin and Wallace (basic), Antibiotic resistance in bacteria explained by natural selection (deepens Year 8, Chapter 4: resistance).

Exercise P  Do you remember?

1
FWhat is 12 multiplied by 5? Write the number.
2
FWhat is 20 percent of 50? Write the number.
3
FWhat is a sudden change in the genetic material of an organism called? Write the word.
4
FWhen one animal kills and eats another, what is it called? Write the word.
5
FHow many years are there in one century? Write the number.
3.1

Genetic, environmental and continuous variation

Year 9
9B2: variation is genetic, environmental or both; continuous and discontinuous variation, with measurement and graphs
Video lesson: Genetic, environmental and continuous variationComing soon
Story. In Year 9 science, a class measures the height of every pupil and writes down each blood group. Some pupils are much taller than others, and each pupil is in only one blood group. The teacher asks: “Which of these differences comes from genes, and which from the environment?”

Variation is the difference between individuals of the same species. It can be genetic, caused by different alleles; environmental, caused by food, light, climate or experience; or both. Plants grown in full sun grow different leaves from plants grown in shade.

Continuous variation has any value in a range, such as height, and is shown on a bar chart of bands. Discontinuous variation has separate groups with no values in between, such as blood group A, B, AB or O.

Heights of 20 pupils2140-1496150-1599160-1693170-179height band (cm); number of pupils
Bar chart of the heights of 20 pupils, grouped in 10 cm bands

Key ideas

  • Genetic variation comes from different alleles.
  • Environmental variation comes from surroundings such as light and food.
  • Continuous variation has values across a range; discontinuous has separate groups.
  • Height is continuous; blood group is discontinuous.
Worked example 3.1.1
Blood group A, B, AB and O are a set of separate groups. Is this continuous or discontinuous variation? Write the word.

Method. Check whether values fall between the groups.

  1. Check. No pupil can be between group A and group B.
    no values in between
  2. Name it. Separate groups with no values in between are discontinuous.
    discontinuous
  3. Write the answer.
    discontinuous

Check. Height, by contrast, has many values in between. ✓

Worked example 3.1.2
Using the height bar chart, which band has the most pupils? Write the band.

Method. Find the tallest bar.

  1. Compare. The bars are 2, 6, 9 and 3 pupils high.
    2, 6, 9, 3
  2. Choose. The tallest bar is for 9 pupils, in the 160-169 band.
    160-169
  3. Write the answer.
    160-169

Check. 9 is the largest of the four counts. ✓

Worked example 3.1.3
Plants grown in full sun have a mean leaf length of 62 mm, and plants in shade have 48 mm. What type of variation is this? Write the word.

Method. Ask what changed between the two groups.

  1. Look. The plants have the same genes but grew in different light.
    different light
  2. Name it. The difference is caused by the environment.
    environmental
  3. Write the answer.
    environmental

Check. Same genes, different light gives environmental variation. ✓

Watch out

Watch out: height is not only genetic. Food and exercise change it, so height is both genetic and environmental.

Exam tip

Exam tip. Name the variation type with a reason, for example “environmental, because the leaves grew in different light”.

Try it (with your teacher, or a grown-up at home). With your teacher, measure the length of ten leaves from one plant, and ten from another plant of the same kind grown in a different place. Work out each mean. Say whether any difference is likely to be genetic, environmental or both.

Exercise 3A  Genetic, environmental and continuous variation

1
FWhich type of variation is caused only by genes? Write the word.
2
FWhich type of variation is caused only by the surroundings? Write the word.
3
FWhich type of variation is caused by both genes and the surroundings? Write the words.
4
FIs blood group continuous or discontinuous variation? Write the word.
5
FIs height continuous or discontinuous variation? Write the word.
6
FWhich height band has the most pupils in the bar chart? Write the band.
7
PSUsing the height table, how many pupils are in the 150-159 band and the 170-179 band together? Write the number.
8
PSIn the same class, how many pupils have blood group O? Write the number.
9
PSHow many more pupils have group A than group B? Write the number.
10
PSPlants in shade have a mean leaf length of 48 mm, and plants in full sun have 62 mm. How much longer are the leaves in full sun? Write the number of millimetres.
11
RExplain why plants of the same kind can have leaves of different sizes in different places. Write one sentence.
12
RChallenge: give one human example of genetic variation and one of environmental variation. Accept any pair, such as eye colour for genetic and a scar from a cut for environmental.
3.2

Natural selection

Year 9
9B2: natural selection: variation, competition, survival, reproduction and inheritance of advantageous alleles
Video lesson: Natural selectionComing soon
Story. A field has brown grass, and a population of beetles has green and brown individuals. Birds eat the beetles that are easiest to see. The class follows one generation of beetles to see which colour survives more often.

Natural selection happens in five stages. There is variation in a population. Individuals compete for food, space and mates. Those with an advantage survive and reproduce. They pass on the advantageous alleles to their offspring, through inheritance. Over many generations, the population changes.

VariationCompetitionSurvivalReproductionInheritancethe advantageous alleles are passed on, so the population changes
Five stages of natural selection, in order

Key ideas

  • Natural selection: variation, competition, survival, reproduction, inheritance.
  • Only individuals that survive can reproduce and pass on their alleles.
  • Advantageous alleles become more common over generations.
  • Natural selection needs variation to work.
Worked example 3.2.1
The beetles start as 60 green and 20 brown. How many beetles are there in total? Write the number.

Method. Add the two groups.

  1. Add. Add the green and brown beetles.
    60 + 20
  2. Calculate. So there are 80 beetles in the population.
    60 + 20 = 80
  3. Write the answer.
    80

Check. 80 is the starting total. ✓

Worked example 3.2.2
Birds eat 5 brown beetles and 30 green beetles. How many brown beetles survive? Write the number.

Method. Subtract the number eaten from the start.

  1. Subtract. Start with 20 brown beetles, and 5 are eaten.
    20 - 5
  2. Calculate. So 15 brown beetles survive.
    20 - 5 = 15
  3. Write the answer.
    15

Check. 15 + 5 = 20. ✓

Worked example 3.2.3
Which colour of beetle has more survivors in this field? Write the word.

Method. Compare the survivors.

  1. Compare. Green: 60 - 30 = 30 survivors. Brown: 15 survivors.
    30 and 15
  2. Decide. Green has more survivors, so the answer is green.
    green
  3. Write the answer.
    green

Check. The birds take fewer brown beetles. ✓

Watch out

Watch out: natural selection does not make individuals change during their lives. Different individuals survive, and the population changes over generations.

Exam tip

Exam tip. Write the five stages in order when asked to explain natural selection, and link each stage to a named feature.

Try it (with your teacher, or a grown-up at home). With your teacher, scatter 20 red and 20 white counters on a tray, as beetles on a field. Use a spoon as a bird to pick up 10 counters one at a time, and count the survivors. Repeat once, then say which stage of natural selection you modelled.

Exercise 3B  Natural selection

1
FWhat is the first stage of natural selection? Write the word.
2
FWhich word means that individuals struggle to get food, space or mates? Write the word.
3
FWhat is passed on to the next generation in natural selection? Write the words.
4
FDoes natural selection happen in a cycle that repeats each generation? Write yes or no.
5
FName the two scientists who proposed natural selection in 1858. Write the names.
6
PSThe beetles start as 60 green and 20 brown. How many beetles are there in total? Write the number.
7
PSBirds eat 30 green beetles. How many green beetles survive? Write the number.
8
PSBirds eat 5 brown beetles. How many brown beetles survive? Write the number.
9
PSHow many beetles survive in total? Write the number.
10
PSWhich colour has the larger share of the survivors? Write the word.
11
RExplain why brown beetles are more likely to survive in a different field, with brown soil. Write one sentence.
12
RChallenge: explain why a population with no variation cannot be changed by natural selection. Write one sentence.
3.3

Fossils, Darwin and Wallace, and evidence for evolution

Year 9
9B2: fossils and the evidence for evolution; the work of Darwin and Wallace (basic)
Video lesson: Fossils, Darwin and Wallace, and evidence for evolutionComing soon
Story. A museum guide shows three layers of rock from a cliff. Each layer holds fossils of different animals. The guide says: “The fossils in the lower layers are older, and the animals changed over time.”

Fossils are the remains or traces of living things, preserved in rock. In an undisturbed rock sequence, the lowest layer is the oldest. Fossils in each layer show what lived at that time, and how the animals changed.

In 1858, Charles Darwin and Alfred Russel Wallace each proposed natural selection as the way species change. Darwin published his book, On the Origin of Species, in 1859. Their work is the basis of the theory of evolution.

Rock layerFossils found
Oldest layersimple shelled sea animals
Middle layersmall fish-like animals
Youngest layeranimals with legs and lungs
Fossils found in three rock layers, from oldest to youngest

Key ideas

  • Fossils are remains or traces of living things preserved in rock.
  • In an undisturbed sequence, the lowest layer is the oldest.
  • Darwin and Wallace proposed natural selection in 1858; Darwin published in 1859.
  • Evolution is supported by fossil evidence and other evidence.
Worked example 3.3.1
In the table, which layer is the oldest? Write the words.

Method. Use the rule for layers.

  1. Use the rule. In an undisturbed sequence, the lowest layer is the oldest.
    lowest layer
  2. Name it. The lowest layer is the oldest, with simple shelled sea animals.
    oldest layer
  3. Write the answer.
    the oldest layer

Check. The lowest layer is the oldest. ✓

Worked example 3.3.2
In what year did Darwin publish On the Origin of Species? Write the number.

Method. Recall the year.

  1. Recall. Darwin published the book in 1859.
    1859
  2. Check. Darwin and Wallace proposed the idea in 1858.
    1858
  3. Write the answer.
    1859

Check. 1859 is one year after 1858. ✓

Worked example 3.3.3
The fossil of an animal with legs and lungs is in the youngest layer. Is it older or younger than the shelled sea animals? Write the word.

Method. Compare the layers.

  1. Compare. The youngest layer is at the top, above the oldest layer.
    at the top
  2. Name it. So the animals with legs are younger.
    younger
  3. Write the answer.
    younger

Check. Younger animals are found in the higher layers. ✓

Watch out

Watch out: the fossil record is incomplete. Missing fossils do not mean the animal did not exist.

Exam tip

Exam tip. Name the layer by its position, for example “the lowest layer”, and give the fossil found in it.

Try it (with your teacher, or a grown-up at home). With your teacher, look at a picture or a real sample of sedimentary rock with layers. Say which layer is the oldest, and why. Do not dig outside or break rock.

Exercise 3C  Fossils, Darwin and Wallace, and evidence for evolution

1
FWhat are fossils? Write the words.
2
FIn an undisturbed rock sequence, which layer is the oldest? Write the word.
3
FName the scientist who published On the Origin of Species in 1859. Write the name.
4
FWhich scientist also proposed natural selection in 1858, alongside Darwin? Write the name.
5
FWhat is a species? Write the words.
6
FIn an undisturbed sequence, are fossils in higher layers usually younger? Write yes or no.
7
PSHow many layers of rock are shown in the fossil table? Write the number.
8
PSWhich group of animals is found in the youngest layer? Write the words.
9
PSIn what year did Darwin and Wallace first propose natural selection? Write the number.
10
PSName one piece of evidence for evolution that is not from fossils. Accept any one, such as comparing the body structures of different animals, or the DNA of different species.
11
RExplain why the fossil record is incomplete. Write one sentence.
12
RChallenge: explain why scientists accept evolution even though some fossils are missing. Accept any sentence saying that many independent kinds of evidence support the same idea.
3.4

Extinction and changing environments

Year 9
9B2: extinction; the importance of maintaining biodiversity and the use of gene banks to preserve hereditary material
Video lesson: Extinction and changing environmentsComing soon
Story. A farmer finds that a sudden frost kills many of the plants in one field. Only a few plants survive, because they had alleles that helped them tolerate cold. The farmer wonders whether the whole plant species could die out.

When the environment changes, a species may be less well adapted to survive and reproduce. If no individuals are left, the species is extinct. A species with more variation is more likely to have some individuals that cope with the change.

Biodiversity is the variety of living things. Keeping biodiversity helps species survive changes. A gene bank stores seeds or hereditary material, so that alleles are kept for the future.

PopulationIndividualsEffect of a sudden change
Large and variedmanysome survive, so the species continues
Small and uniformfewlikely to die out, so the species is extinct
Two populations and the effect of a sudden change

Key ideas

  • Extinction: no individuals of a species are left alive.
  • Variation gives some individuals a chance to survive a change.
  • Biodiversity is the variety of living things.
  • Gene banks store hereditary material to protect alleles.
Worked example 3.4.1
A population of 200 plants has 40 plants die in a drought. How many survive? Write the number.

Method. Subtract the plants that died.

  1. Subtract. Start with 200 plants, and 40 die.
    200 - 40
  2. Calculate. So 160 plants survive.
    200 - 40 = 160
  3. Write the answer.
    160

Check. 160 + 40 = 200. ✓

Worked example 3.4.2
Name the word for a species that has no individuals left. Write the word.

Method. Recall the word.

  1. Recall. A species with no living members has died out.
    no living members
  2. Name it. This is called extinction, so the species is extinct.
    extinct
  3. Write the answer.
    extinct

Check. Extinction is permanent. ✓

Worked example 3.4.3
In a population of 400 plants, 25% die in a frost. How many die? Write the number.

Method. Find 25% of 400.

  1. Find a quarter. 25% is one quarter, so divide 400 by 4.
    400 / 4
  2. Calculate. So 100 plants die.
    400 / 4 = 100
  3. Write the answer.
    100

Check. 100 is one quarter of 400. ✓

Watch out

Watch out: extinction is permanent. A species is not extinct just because it is rare; it is extinct only when none are left.

Exam tip

Exam tip. Link variation to survival: a varied population has more chances of coping with a change.

Try it (with your teacher, or a grown-up at home). With your teacher, grow two small groups of cress seeds, one from a mixed pack and one all from one plant if possible. Compare how many grow after a cold day. Say which group has more variation and which is more likely to survive a change.

Exercise 3D  Extinction and changing environments

1
FWhat word means that no individuals of a species are left alive? Write the word.
2
FWhich word means the variety of living things? Write the word.
3
FWhat type of store keeps seeds or hereditary material to protect alleles for the future? Write the words.
4
FDoes a sudden environmental change always lead to natural selection? Write yes or no.
5
PSA population of 200 plants has 40 plants die in a drought. How many plants survive? Write the number.
6
PSIn a population of 400 plants, 25% die in a frost. How many die? Write the number.
7
PSIn a population of 80 plants, 20 are tall and 60 are short. Which group is more likely to survive a cold snap if the tall ones have alleles for cold tolerance? Write the word.
8
PSA species has only 3 individuals left. Another has 3000. Which species is more at risk? Write the number of individuals in the species at more risk.
9
RExplain why a varied population is less likely to become extinct than a population that is all alike. Write one sentence.
10
PSGive one reason why biodiversity matters for food security. Accept any sentence that says many crops and wild species depend on each other and on varied genes.
11
RChallenge: explain why a gene bank is useful even if the stored alleles are not needed today. Accept any sentence that says the alleles may be needed in future if the environment changes.
3.5

Antibiotic resistance and natural selection

Year 9
9B2: antibiotic resistance in bacteria explained by natural selection (deepens Year 8, Chapter 4)
Video lesson: Antibiotic resistance and natural selectionComing soon
Story. A patient is given an antibiotic for an infection. Most bacteria are killed, but a few survive, because they have an allele that makes them resistant. The survivors reproduce, and the next infection is harder to treat. The class works through the numbers in a simple model.

Antibiotic resistance is when a bacterium is no longer killed by an antibiotic. Resistance comes from a chance mutation, a random change in the DNA. Most bacteria are killed by the antibiotic. The few resistant bacteria survive, and they reproduce, passing the resistance allele on.

This is natural selection: the antibiotic is the environment, the resistant bacteria have the advantage, and over generations most of the population becomes resistant.

Antibiotics do not work on viruses, so they should not be used for colds or flu. Antibiotics should be taken only when a doctor says they are needed, and the whole course should be finished.

StageBacteria in the populationResistant bacteriaPercentage resistant
Before antibiotic1000101%
After antibiotic (survivors)1010100%
After reproduction (model)10001000100%
A simple model of antibiotic resistance, with invented numbers

Key ideas

  • Resistance comes from a chance mutation, not from the antibiotic.
  • The antibiotic kills the non-resistant bacteria; the resistant ones survive.
  • Survivors reproduce and pass on the resistance allele: this is natural selection.
  • Antibiotics do not work on viruses.
Worked example 3.5.1
In the model, 1000 bacteria include 10 resistant ones. What percentage are resistant? Write the number.

Method. Divide the resistant number by the total, then multiply by 100.

  1. Divide. 10 / 1000 = 0.01.
    10 / 1000 = 0.01
  2. Convert. Multiply by 100 to get a percentage.
    0.01 x 100 = 1
  3. Write the answer.
    1

Check. 1% of 1000 is 10. ✓

Worked example 3.5.2
The antibiotic kills all non-resistant bacteria. How many bacteria survive? Write the number.

Method. Find the number that are not resistant, then subtract.

  1. Subtract. The non-resistant bacteria are 1000 - 10 = 990.
    1000 - 10 = 990
  2. Survivors. The 990 are killed, so only the resistant 10 survive.
    10 survive
  3. Write the answer.
    10

Check. 10 survive from 1000. ✓

Worked example 3.5.3
Each resistant survivor makes 100 offspring, which are all resistant. How many resistant bacteria are there now? Write the number.

Method. Multiply the survivors by 100.

  1. Multiply. 10 survivors x 100 offspring each.
    10 x 100
  2. Calculate. So there are 1000 resistant bacteria.
    10 x 100 = 1000
  3. Write the answer.
    1000

Check. The population is now all resistant. ✓

Watch out

Watch out: the antibiotic does not cause the mutation. The mutation happens by chance, and the antibiotic selects the resistant bacteria that already exist.

Exam tip

Exam tip. Explain resistance in five steps: variation, antibiotic, survival, reproduction, inheritance. Each step should be a full sentence.

Try it (with your teacher, or a grown-up at home). Do not handle bacteria. Instead, with your teacher, use a counter model: 100 red counters (bacteria) with 2 blue counters (resistant), and remove the red ones at random. Count the blues left, and say why resistant bacteria are more common after treatment.

Exercise 3E  Antibiotic resistance and natural selection

1
FWhat is antibiotic resistance? Write the words.
2
FDo antibiotics work on viruses? Write yes or no.
3
FWhere does resistance first come from in a bacterium? Write the word.
4
FWhat is the name of the process by which resistant bacteria become more common? Write the words.
5
FShould an antibiotic be taken for a cold? Write yes or no.
6
PSA population of 1000 bacteria has 10 resistant ones. How many are not resistant? Write the number.
7
PSA population of 2000 bacteria has 20 resistant ones. What percentage is resistant? Write the number.
8
PSIn the model, 10 resistant survivors each make 100 offspring. How many offspring are made in total? Write the number.
9
PSIn a population of 500 bacteria, 5 are resistant. The antibiotic kills all the non-resistant bacteria. How many are left? Write the number.
10
PSThose 5 resistant bacteria each make 50 offspring. How many bacteria are there now? Write the number.
11
RExplain why finishing the full course of antibiotics matters, in terms of natural selection. Write one sentence.
12
RChallenge: explain why the population is all resistant after the antibiotic in the model, even though only 10 of 1000 were resistant at the start. Accept any sentence that says the resistant bacteria survived and reproduced, so they make up the whole population.

Chapter summary

SkillMethod
VariationGenetic (alleles), environmental (surroundings) or both; continuous (a range) or discontinuous (groups).
Natural selectionVariation, competition, survival, reproduction, inheritance of advantageous alleles.
FossilsRemains or traces in rock; in an undisturbed sequence, the lowest layer is the oldest.
Darwin and WallaceProposed natural selection in 1858; Darwin published On the Origin of Species in 1859.
ExtinctionNo individuals of a species left; variation gives a better chance to survive change.
Antibiotic resistanceA chance mutation; the antibiotic kills non-resistant bacteria; resistant survivors reproduce.

Can you…?

  • I can explain genetic, environmental and combined variation, with examples
  • I can tell continuous from discontinuous variation and read a bar chart of it
  • I can put the stages of natural selection in order, and link each one to an example
  • I can use fossils, Darwin and Wallace as evidence for evolution
  • I can explain antibiotic resistance by natural selection, with worked numbers

Key terms

variation
the differences between individuals of the same species
genetic variation
variation caused by different alleles
environmental variation
variation caused by surroundings such as light and food
continuous variation
variation with values across a range, such as height
discontinuous variation
variation in separate groups, such as blood group
natural selection
the process by which individuals with advantageous alleles survive and reproduce more
fossil
the remains or traces of a living thing preserved in rock
extinction
when no individuals of a species are left alive
biodiversity
the variety of living things
gene bank
a store of seeds or hereditary material kept for the future
mutation
a sudden, random change in DNA
antibiotic resistance
when a bacterium is no longer killed by an antibiotic

Exercise 3Z  Check-up

1
Is the variation in blood group continuous or discontinuous? Write the word.
2
A plant grown in shade has smaller leaves than the same plant in full sun. What type of variation is this? Write the word.
3
What is the first stage of natural selection? Write the word.
4
Which scientists proposed natural selection in 1858? Write the names.
5
In an undisturbed rock sequence, which layer is the oldest? Write the word.
6
A population of 80 beetles has 60 green and 20 brown. How many beetles in total? Write the number.
7
A species with no individuals left has become what? Write the word.
8
What is a gene bank used for? Write one sentence. Accept any sentence saying it stores seeds or hereditary material to keep alleles for the future.
9
A population of 1000 bacteria has 10 resistant ones. What percentage is resistant? Write the number.
10
Explain how antibiotic resistance develops by natural selection. Write one sentence. Accept any sentence saying resistant bacteria survive the antibiotic, reproduce and pass on the allele.

Total 18 marks. Allow about 18 minutes.

Exercise 3X  Practice test questions

1
FName the type of variation that is caused only by genes. Write the word.[1 mark]
2
FWhich bar in the height bar chart is tallest? Write the band.[1 mark]
3
FWrite the stage of natural selection that comes immediately before survival. Write the word.[1 mark]
4
RGive two reasons why fossils are good evidence for evolution. Accept any two sensible reasons, such as that fossils show older and younger forms in layers, and that changes appear over time.[2 marks]
5
PSA population of 200 plants has 40 plants die in a drought. How many survive? Write the number.[2 marks]
6
PSFind the percentage of bacteria that are resistant if 5 out of 500 are resistant. Write the number.[2 marks]
7
FExplain why antibiotics do not cure a cold. Write one sentence. Accept any sentence saying a cold is caused by a virus, and antibiotics work only on bacteria.[2 marks]
8
RExplain how a population of beetles with green and brown colours could change over generations in a brown field. Write two sentences. Accept any answer that describes birds eating more visible beetles, the survivors reproducing, and the advantageous allele becoming more common.[3 marks]
9
RA plant population is reduced to a few individuals after a sudden change. Explain why a species with more variation is less likely to become extinct. Accept any sentence that says variation gives some individuals alleles that help them survive the change.[2 marks]
10
FName the scientist who published On the Origin of Species, and give the year. Write the name and the year.[2 marks]
← Chapter 2: Inheritance, DNA and genesChapter 4: Earth's structure and the rock cycle →

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