Chapter 1 of 8

All 8 chapters
  1. 1Natural selection, readunderstood
  2. 2Evidence for evolution, readunderstood
  3. 3New species and extinction, readunderstood
  4. 4Classification, readunderstood
  5. 5The carbon and nitrogen cycles, readunderstood
  6. 6Measuring populations, readunderstood
  7. 7Interdependence, readunderstood
  8. 8Biodiversity and people, readunderstood

S4.3.1 Chapter 1

Natural selection

Why do so many animals look just right for the place where they live?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

  • You picked A. Read on. At the end, the chapter comes back to your guess.

  • You picked B. Read on. At the end, the chapter comes back to your guess.

  • You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

  • Explain natural selection in steps: variation, competition, survival and inheritance
  • Use natural selection to explain antibiotic resistance in bacteria

Animals and plants often seem made for the place where they live. A sand coloured beetle is hard to see on sand. A cactus keeps water in a dry land. Natural selection is the process that explains this.

It begins with a population. This is all the living things of one kind that live in one place, such as the moths of one wood. The first step is that the moths vary. Some are dark and some are light.

You know where variation comes from. A mutation makes a new allele by chance. Sexual reproduction shuffles alleles into new sets, so no two young are exactly alike.

The second step is competition. Most kinds of living thing make far more young than can live. There is not enough food, space or safe shelter for all of them. So the young compete, and many die.

The third step is survival. Some of the young have features that suit the surroundings a little better. They are more likely to live long enough to have young. Dark moths on dark bark are hard for birds to see. Light moths on dark bark stand out.

The fourth step is inheritance. The survivors pass on the alleles for their features. Their young are more likely to have those features too. Repeat this for many generations and the features become more common. This change in a population over many generations is evolution.

Draw it first

Natural selection in steps

  1. Boxes joined by arrows, from top to bottom. The strip grows by one box in each picture. The boxes read: The animals in a population vary; More young are born than can survive; The best suited survive and breed more; They pass on their alleles; The population changes over many generations.The animals in a populationvaryMore young are born than cansurvive

    1 Living things of one kind vary, and they make more young than can live.

  2. Boxes joined by arrows, from top to bottom. The strip grows by one box in each picture. The boxes read: The animals in a population vary; More young are born than can survive; The best suited survive and breed more; They pass on their alleles; The population changes over many generations.The animals in a populationvaryMore young are born than cansurviveThe best suited survive andbreed more

    2 The young with the best suited features are more likely to survive and breed.

  3. Boxes joined by arrows, from top to bottom. The strip grows by one box in each picture. The boxes read: The animals in a population vary; More young are born than can survive; The best suited survive and breed more; They pass on their alleles; The population changes over many generations.The animals in a populationvaryMore young are born than cansurviveThe best suited survive andbreed moreThey pass on their alleles

    3 They pass on their alleles to the next generation.

  4. Boxes joined by arrows, from top to bottom. The strip grows by one box in each picture. The boxes read: The animals in a population vary; More young are born than can survive; The best suited survive and breed more; They pass on their alleles; The population changes over many generations.The animals in a populationvaryMore young are born than cansurviveThe best suited survive andbreed moreThey pass on their allelesThe population changes overmany generations

    4 Over many generations the population is no longer the same.

Four panels joined by arrows, from top to bottom. A key names the two kinds: dark moth, drawn black, and light moth, drawn white. Generation 1 shows 2 dark moths and 6 light moths. A dashed box says Birds eat the moths they can see against the bark. Survivors shows 2 dark moths and 2 light moths. An arrow marked breed leads to Generation 2, which shows 6 dark moths and 2 light moths.dark mothlight mothGeneration 1Birds eat the moths they cansee against the barkSurvivorsbreedGeneration 2
The numbers are made up to show the steps. Each moth is drawn as a simple shape. The bark is dark. The dark moths are hard to see, so more of them live and breed.

Look at what changed. No moth turned from light to dark. The change is in the population. The share of dark moths grew, because dark moths lived and bred more often than light ones.

Worked example

In generation 1 there are 2 dark moths and 6 light moths. What percentage of the moths are dark?

  1. There are 2 + 6 = 8 moths in all.
  2. The dark moths are 2 out of 8, which is 2 ÷ 8 = 0.25.
  3. As a percentage, 0.25 × 100 = 25%.

Answer: 25%

See it three ways

As a picture

A table with the columns Generation, Dark and Light, counting moths. Generation 1 has 2 dark moths and 6 light moths. Generation 2 has 6 dark moths and 2 light moths.GenerationDarkLight126262

In words

Dark moths are hard for birds to see on dark bark. They survive and breed more often, so there are more of them in generation 2.

In symbols

Dark moths: 2 out of 8 is 25%, then 6 out of 8 is 75%.

Now think about bacteria. An antibiotic is a medicine that kills bacteria. It does not work on viruses. In a large group of bacteria, a few may carry an allele that makes them resistant. The antibiotic does not kill a resistant bacterium.

That allele came from a mutation, by chance, before any antibiotic was used. Resistant bacteria are rare at first. The antibiotic is the pressure. It kills the bacteria that are not resistant. The resistant ones live and divide. Bacteria can also pass a resistance allele to other bacteria.

Three panels joined by arrows, from top to bottom. A key names resistant bacterium, drawn hatched, and bacterium not resistant, drawn white. Before the antibiotic shows 12 bacteria, of which 2 are resistant. An arrow marked antibiotic added leads to After the antibiotic, which shows only the 2 resistant bacteria. An arrow marked survivors divide leads to Many generations later, which shows 12 resistant bacteria.resistant bacteriumbacterium not resistantBefore the antibioticantibiotic addedAfter the antibioticsurvivors divideMany generations later
The antibiotic does not make bacteria resistant. It kills the others and leaves the resistant ones to divide.

Bacteria divide fast. Under good conditions, some kinds of bacterium can divide in about 20 to 30 minutes. So a few resistant survivors soon become a large group, and the antibiotic no longer works on them. They can also spread to other people.

Worked example

One bacterium divides into two every 30 minutes. Food does not run out. How many bacteria are there after 3 hours?

  1. 3 hours is 6 lots of 30 minutes, so the bacteria divide 6 times.
  2. Each division doubles the number: 1, 2, 4, 8, 16, 32, 64.
  3. After 6 doublings there are 2 × 2 × 2 × 2 × 2 × 2 = 64 bacteria.

Answer: 64 bacteria

In real life food runs out, so the growth stops. But at first it is this fast. This is why a person should take an antibiotic only when a health worker prescribes it. They should take the full course as the health worker says. Antibiotics do nothing for a cold or flu, which are caused by viruses.

Try it

Be the bird

You need: 10 light dry beans and 10 dark dry beans, a dark cloth, or a dark page of an old newspaper, a sheet of white paper, a friend and a clock with a second hand.

Be safe: Use the beans only as counters and do not eat them. Keep small beans away from babies and small children.

  1. Spread the 20 beans over the dark cloth. Your friend is the bird and looks away.
  2. Your friend turns round, then picks up 10 beans in 10 seconds, taking the ones they see first.
  3. Count the light beans and the dark beans left on the cloth. Write the numbers down.
  4. Do it again on the white paper with the same 20 beans. Compare the numbers.

What you will see: On the dark cloth, more dark beans are left. On the white paper, more light beans are left. The surroundings decide which kind is picked less often. That is selection.

If you cannot try it, this is what happens.
A table with the columns Surface, Light and Dark, counting the beans left. On the cloth, 3 light beans and 7 dark beans are left. On the paper, 7 light beans and 3 dark beans are left.SurfaceLightDarkcloth37paper73

A wrong idea many people have

An animal grows a feature because it needs it, and then passes the feature to its young.

Why it feels right: It seems natural to say that an animal "needed" to hide, so it became dark.

The test that breaks it: A person who lifts heavy loads gets strong arms. Their children are not born with strong arms. The change was made in the body, and it did not change the alleles in the sex cells.

What is true: Alleles come first, by chance. The environment does not make them. It selects among them, because some animals survive better and pass on their alleles more often.

Key words

natural selection
the process in which the living things best suited to their surroundings survive and breed more, and pass on their alleles.
population
all the living things of one kind in one place.
competition
living things trying to get the same food, space or shelter when there is not enough for all.
evolution
the change in the inherited features of a population over many generations.
antibiotic
a medicine that kills bacteria. It does not work on viruses.
resistant
not killed by something, such as an antibiotic.

Back to your guess

The answer is B: Animals that suit their home better survive and have more young, so their features become common over many generations.

Animals do not choose, and a change made in one lifetime is not passed on. In each generation the ones better suited to their home are more likely to live and have young. Over many generations their features spread.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

  1. Which of these must be true for natural selection to happen in a population?

    Tap your answer.

    1. Not this one. If all had the same alleles, there would be nothing to select. Natural selection needs variation. Go to S4.2.7. Try another.

    2. Not this one. If every young one lived, there would be no competition and nothing to choose between. Most kinds make more young than can live. Try another.

    3. Yes, that is right. There must be inherited variation, and competition because there are too many young. Then the best suited are more likely to live and breed.

    See the answer

    The living things vary in features that are inherited, and more young are born than can live.

    There must be inherited variation, and competition because there are too many young. Then the best suited are more likely to live and breed.

    • If you chose A: If all had the same alleles, there would be nothing to select. Natural selection needs variation. Go to S4.2.7.
    • If you chose B: If every young one lived, there would be no competition and nothing to choose between. Most kinds make more young than can live.

  2. On green leaves, some beetles are green and some are brown. Birds eat more brown beetles. After many generations most beetles are green. Which is the best explanation?

    Tap your answer.

    1. Not this one. A beetle does not change colour because it needs to. The alleles for green were already in the population. Try another.

    2. Not this one. Birds eat beetles. They do not change alleles. They only decide which beetles live to breed. Try another.

    3. Yes, that is right. Green beetles survived more often, so more of them bred. Each generation had a larger share of green beetles.

    See the answer

    Green beetles were eaten less, so they bred more and passed on their alleles.

    Green beetles survived more often, so more of them bred. Each generation had a larger share of green beetles.

    • If you chose A: A beetle does not change colour because it needs to. The alleles for green were already in the population.
    • If you chose B: Birds eat beetles. They do not change alleles. They only decide which beetles live to breed.

  3. Look at the picture. Why are the hatched bacteria the only ones left after the antibiotic?

    Three panels joined by arrows, from top to bottom. A key names resistant bacterium, drawn hatched, and bacterium not resistant, drawn white. Before the antibiotic shows 8 bacteria, of which 1 are resistant. An arrow marked antibiotic added leads to After the antibiotic, which shows only the 1 resistant bacteria. An arrow marked survivors divide leads to Many generations later, which shows 8 resistant bacteria.resistant bacteriumbacterium not resistantBefore the antibioticantibiotic addedAfter the antibioticsurvivors divideMany generations later
    Tap your answer.

    1. Not this one. The antibiotic did not change them. A mutation made them resistant by chance, before the antibiotic was added. Try another.

    2. Yes, that is right. A few bacteria already had an allele for resistance. The antibiotic killed the rest and left them to divide.

    3. Not this one. The person is not resistant. It is the bacteria that are resistant to the antibiotic. Try another.

    See the answer

    They carried an allele for resistance, so the antibiotic did not kill them.

    A few bacteria already had an allele for resistance. The antibiotic killed the rest and left them to divide.

    • If you chose A: The antibiotic did not change them. A mutation made them resistant by chance, before the antibiotic was added.
    • If you chose C: The person is not resistant. It is the bacteria that are resistant to the antibiotic.

  4. A health worker gives Tariq an antibiotic for a bacterial infection. After two days he feels better and stops taking it. Why is this a risk?

    Tap your answer.

    1. Yes, that is right. Bacteria that are left have survived the medicine so far. They can divide again, and the ones that are left are the ones the medicine kills least easily.

    2. Not this one. It is the bacteria that can become resistant. A person does not become resistant. Try another.

    3. Not this one. People often feel well before every bacterium is dead. The ones left can divide again. A health worker says how long to take the medicine. Try another.

    See the answer

    Feeling better does not mean all the bacteria are dead, so some may be left to divide again.

    Bacteria that are left have survived the medicine so far. They can divide again, and the ones that are left are the ones the medicine kills least easily.

    • If you chose B: It is the bacteria that can become resistant. A person does not become resistant.
    • If you chose C: People often feel well before every bacterium is dead. The ones left can divide again. A health worker says how long to take the medicine.

  5. Aisha lifts water jars every day, and her arms become strong. Will her children be born with strong arms because of this?

    Tap your answer.

    1. Not this one. Strong arms from work are a change to the body. The alleles in her sex cells are not changed by lifting jars. Try another.

    2. Yes, that is right. Muscles grow from use. This does not change the alleles in the egg cells, so her children do not inherit it.

    3. Not this one. Daughters and sons get their alleles in the same way. Lifting jars changes no alleles in either case. Try another.

    See the answer

    No. The change is in her body, and her alleles are not changed by it.

    Muscles grow from use. This does not change the alleles in the egg cells, so her children do not inherit it.

    • If you chose A: Strong arms from work are a change to the body. The alleles in her sex cells are not changed by lifting jars.
    • If you chose C: Daughters and sons get their alleles in the same way. Lifting jars changes no alleles in either case.

  6. Which sentence describes how a population of bacteria becomes resistant to an antibiotic?

    Tap your answer.

    1. Not this one. A bacterium does not learn. Survival depends on the alleles it already has. Try another.

    2. Yes, that is right. The resistant bacteria are the survivors, so they are the ones that divide. Their descendants are all resistant.

    3. Not this one. Bacteria that are killed do not divide. Resistance spreads from the survivors. Try another.

    See the answer

    The antibiotic kills most of them. The few resistant ones divide, and so most of the next generation is resistant.

    The resistant bacteria are the survivors, so they are the ones that divide. Their descendants are all resistant.

    • If you chose A: A bacterium does not learn. Survival depends on the alleles it already has.
    • If you chose C: Bacteria that are killed do not divide. Resistance spreads from the survivors.

  7. Look at the picture of beetles. What percentage of the beetles in generation 2 are green?

    Four panels joined by arrows, from top to bottom. A key names the two kinds: green beetle, drawn dotted, and brown beetle, drawn black. Generation 1 shows 4 green beetles and 4 brown beetles. A dashed box says Birds eat the beetles they can see on the leaves. Survivors shows 4 green beetles and 1 brown beetles. An arrow marked breed leads to Generation 2, which shows 8 green beetles and 2 brown beetles.green beetlebrown beetleGeneration 1Birds eat the beetles theycan see on the leavesSurvivorsbreedGeneration 2
    See the answer

    80%

    Generation 2 has 8 green beetles and 2 brown beetles, 10 in all. 8 ÷ 10 = 0.8, which is 80%.

  8. One bacterium divides into two every 30 minutes. Food does not run out. How many bacteria are there after 2 hours?

    See the answer

    16 bacteria

    2 hours is 4 lots of 30 minutes. The number doubles 4 times: 2 × 2 × 2 × 2 = 16.

Keep it

Copy this into your exercise book.

A population varies. More young are born than live. The best suited survive, breed and pass on alleles. Over generations the population changes. Resistant bacteria survive an antibiotic and divide.

Draw: Five boxes in a line, joined by arrows: vary, compete, survive, pass on, change.

Reading with a child? A short page for the adult: what this chapter teaches and three questions to ask.

No internet tomorrow? Save the whole book as one file (1.0 MB). It opens on any phone.