Chapter 3 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

S4.3.3 Chapter 3

New species and extinction

How can one kind of animal become two kinds, and why do some kinds disappear for ever?

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 how isolation can lead to a new species
  • Give causes of extinction, past and present

A species is a group of living things that can breed together to give fertile young. Fertile young can have young of their own. A horse and a donkey can breed, but their young, a mule, is almost always sterile. So a horse and a donkey are two species.

This idea fits animals and plants that breed by sexual reproduction. Scientists use other tests for bacteria, which do not breed in this way.

How can one species become two? It begins with isolation. This means that two groups of one species are kept apart, so they cannot breed together. A barrier can do it, such as the sea round an island, a range of mountains, a desert or a new river. We call it geographic isolation.

Each side of the barrier has its own conditions. The food, the weather or the enemies may differ. So natural selection favours different features on each side. Each side also gets its own mutations, by chance.

Draw it first

One species becomes two

  1. A population of animals on land is shown in three pictures. In the first picture there is one population on one piece of land. In the second picture water cuts the land in two. The left side is labelled population 1 and the right side is labelled population 2. In the third picture the left side is labelled species A and the right side is labelled species B. The animals of species B are hatched. A crossed arrow between them is labelled cannot breed together. The word water marks the barrier in pictures two and three.one population

    1 One population lives on one piece of land and breeds together.

  2. A population of animals on land is shown in three pictures. In the first picture there is one population on one piece of land. In the second picture water cuts the land in two. The left side is labelled population 1 and the right side is labelled population 2. In the third picture the left side is labelled species A and the right side is labelled species B. The animals of species B are hatched. A crossed arrow between them is labelled cannot breed together. The word water marks the barrier in pictures two and three.waterpopulation 1population 2

    2 Water cuts the land in two. The two groups can no longer meet. They are still alike.

  3. A population of animals on land is shown in three pictures. In the first picture there is one population on one piece of land. In the second picture water cuts the land in two. The left side is labelled population 1 and the right side is labelled population 2. In the third picture the left side is labelled species A and the right side is labelled species B. The animals of species B are hatched. A crossed arrow between them is labelled cannot breed together. The word water marks the barrier in pictures two and three.waterspecies Aspecies Bcannot breed together

    3 After many generations the two groups have changed in different ways. If they met, they could not breed to give fertile young. They are now two species.

This takes a long time, usually many thousands of generations. The hatched animals in the last picture show that one group has changed. The changes may be in colour, in shape or in the sex cells. When the groups can no longer breed together to give fertile young, they are two species.

Notice what does not happen. The animals do not choose to change. Each group is only selected by its own conditions, over many generations.

Now think about the opposite: extinction. A species is extinct when the last living member has died. Extinction is normal. Most species that ever lived are now extinct. But sometimes many species die out in a short time. This is a mass extinction.

About 66 million years ago a large asteroid hit the Earth. Dust and soot filled the air and blocked sunlight. Plants could not make food. Plant eaters starved, and then the animals that ate them starved. Many species died out, including the dinosaurs that were not birds. Birds are descended from dinosaurs, and they survived.

Five boxes joined by arrows, from top to bottom. The boxes read: A large asteroid hits the Earth; Dust in the air blocks sunlight for a time; Many plants cannot make food and die; Animals that eat the plants starve; Animals that eat those animals starve.A large asteroid hits theEarthDust in the air blockssunlight for a timeMany plants cannot make foodand dieAnimals that eat the plantsstarveAnimals that eat thoseanimals starve
One account of the extinction about 66 million years ago. A food chain broke from the bottom.

Other past causes include huge volcanic eruptions, which put ash and gases into the air and changed the climate. Climate change from natural causes, such as an ice age, also made some species die out.

Today there are new causes, and many come from people. The first is habitat loss: a forest is cleared or a wetland is drained. The second is hunting that takes more animals than can be replaced. The third is an invasive species, a living thing that people carry to a new place, where it harms the local species. Rats and cats carried on ships eat the eggs and young of birds that nest on the ground on islands.

The fourth cause is pollution, such as oil or chemicals in the sea. The fifth is climate change caused by people, for example warmer seas that kill corals. Often several causes act together on the same species.

A table with one column headed Causes today. The rows read: habitat loss; hunting; invasive species; pollution; climate change.Causes todayhabitat losshuntinginvasive speciespollutionclimate change
Causes of extinction today. Several of them often act together on the same species.
A bar chart of kinds of fossil shell. The axis is labelled kinds of shell, with a scale from 0 to 40 in steps of 10. The bar for below reaches 40. The bar for above reaches 10.010203040belowabovekinds of shell
Example numbers: the kinds of shell found in the rock below a layer, and the same kinds found above it.

Worked example

A scientist finds 40 kinds of fossil shell in the rock below a layer. Only 10 of those kinds are in the rock above it. What percentage of the kinds are missing above the layer?

  1. The number of kinds that are missing is 40 − 10 = 30.
  2. 30 out of 40 is 30 ÷ 40 = 0.75.
  3. As a percentage, 0.75 × 100 = 75%.

Answer: 75%

A bar chart of kinds of fish in a river. The axis is labelled kinds of fish, with a scale from 0 to 30 in steps of 10. The bar for long ago reaches 24. The bar for today reaches 18.0102030long agotodaykinds of fish
Example numbers for one river.

Worked example

A river held 24 kinds of fish long ago. Today it holds 18 kinds. What percentage of the kinds have been lost?

  1. The number of kinds lost is 24 − 18 = 6.
  2. 6 out of 24 is 6 ÷ 24 = 0.25.
  3. As a percentage, 0.25 × 100 = 25%.

Answer: 25%

Try it

Find a barrier near you

You need: a pencil and paper, a safe place with a clear barrier, such as a hill, a fence or a stream, with an adult.

Be safe: Take an adult with you. Stay away from roads and deep water. Do not touch wild animals or plants you do not know.

  1. Draw a plan of the place, with the barrier as a thick line.
  2. On each side, write the names of three living things you can see, such as plants, insects or snails.
  3. For each one, write whether it could cross the barrier. A bird could. A snail could only rarely cross a wide stream or a busy road.
  4. Write which living things the barrier keeps apart.

What you will see: A barrier only isolates the things that cannot cross it. A road or a wide stream keeps most snails apart, but not birds. Different living things are isolated by different barriers.

If you cannot try it, this is what happens.
Two pieces of land with water between them. The word water marks the barrier. The left land is labelled population 1 and the right land is labelled population 2. Each holds animals of the same kind, and they cannot meet across the water.waterpopulation 1population 2

A wrong idea many people have

Two groups that look different must be two species.

Why it feels right: We often sort living things by how they look.

The test that breaks it: A small dog and a large dog look very different. Yet the young of different breeds of dog are fertile, so all dogs are one species.

What is true: Looks are not the test. Two groups are separate species when they can no longer breed together to give fertile young.

Key words

species
a group of living things that can breed together to give fertile young.
isolation
keeping two groups of one species apart, so that they cannot breed together.
extinction
the death of every living member of a species.
mass extinction
many species dying out in a short time.
invasive species
a living thing carried to a new place, where it harms the local living things.

Back to your guess

The answer is A: A barrier splits a group. The two sides have different conditions, and after many generations the two groups can no longer breed together.

A barrier is only the start. Each side is selected by its own conditions, and the groups change over many generations. When they can no longer breed to give fertile young, they are two species.

Check yourself

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

  1. Which is the best definition of a species?

    Tap your answer.

    1. Yes, that is right. The test is whether they can breed together and have fertile young. This applies to animals and plants that breed sexually.

    2. Not this one. Looks are not the test. Dogs of different breeds look different, but their young are fertile, so they are one species. Try another.

    3. Not this one. A horse and a donkey can mate, but their young is a mule, which is almost always sterile. So they are two species. Try another.

    See the answer

    A group of living things that can breed together to give fertile young

    The test is whether they can breed together and have fertile young. This applies to animals and plants that breed sexually.

    • If you chose B: Looks are not the test. Dogs of different breeds look different, but their young are fertile, so they are one species.
    • If you chose C: A horse and a donkey can mate, but their young is a mule, which is almost always sterile. So they are two species.

  2. Look at the picture. Two groups of beetles live on the two sides of a range of mountains. Why might they become two species?

    Two pieces of land with mountains between them. The word mountains marks the barrier. The left land is labelled population 1 and the right land is labelled population 2. Each holds animals of the same kind.mountainspopulation 1population 2
    Tap your answer.

    1. Not this one. The mountains do not change the beetles. The conditions on each side select different features over many generations. Go to S4.3.1. Try another.

    2. Not this one. Habits learned in life are not passed on in the alleles. Features change through selection and mutation. Go to S4.3.1. Try another.

    3. Yes, that is right. The barrier keeps the groups apart. Each group is selected by its own conditions, and each gets its own mutations.

    See the answer

    The two sides have different conditions, so selection acts differently on each. They change in different ways over many generations.

    The barrier keeps the groups apart. Each group is selected by its own conditions, and each gets its own mutations.

    • If you chose A: The mountains do not change the beetles. The conditions on each side select different features over many generations. Go to S4.3.1.
    • If you chose B: Habits learned in life are not passed on in the alleles. Features change through selection and mutation. Go to S4.3.1.

  3. Two groups of beetles live on opposite sides of a wide river and cannot cross it. What must happen before they count as two species?

    Tap your answer.

    1. Yes, that is right. They become separate species only when they can no longer interbreed to give fertile young.

    2. Not this one. Being apart is only the start. They are still one species until they cannot breed together to give fertile young. Try another.

    3. Not this one. Looks are not the test. What counts is whether they can still breed together to give fertile young. Try another.

    See the answer

    After many generations they must have changed so that they can no longer breed together to give fertile young.

    They become separate species only when they can no longer interbreed to give fertile young.

    • If you chose B: Being apart is only the start. They are still one species until they cannot breed together to give fertile young.
    • If you chose C: Looks are not the test. What counts is whether they can still breed together to give fertile young.

  4. About 66 million years ago a large asteroid hit the Earth. Which account is best?

    Tap your answer.

    1. Not this one. Many kinds survived, including birds, small mammals and many plants and sea animals. Try another.

    2. Yes, that is right. Dust and soot cut off the light plants need. This broke food chains from the bottom, and many species died out.

    3. Not this one. Many other kinds died too, such as the ammonites and many sea animals and plants. Try another.

    See the answer

    Dust blocked the sunlight for a time. Plants died, and the animals that depended on them died too.

    Dust and soot cut off the light plants need. This broke food chains from the bottom, and many species died out.

    • If you chose A: Many kinds survived, including birds, small mammals and many plants and sea animals.
    • If you chose C: Many other kinds died too, such as the ammonites and many sea animals and plants.

  5. A forest is cleared to make farmland, and the monkeys that lived in it disappear. Which cause of extinction is this?

    Tap your answer.

    1. Not this one. Hunting means taking too many animals by killing them. Here the monkeys lost their home, not their numbers to hunters. Try another.

    2. Not this one. An invasive species is a living thing that people carry to a new place. Nothing was carried here. The forest was cleared. Try another.

    3. Yes, that is right. The monkeys lost the place that gave them food and shelter. Habitat loss is the biggest cause of extinction today.

    See the answer

    Habitat loss

    The monkeys lost the place that gave them food and shelter. Habitat loss is the biggest cause of extinction today.

    • If you chose A: Hunting means taking too many animals by killing them. Here the monkeys lost their home, not their numbers to hunters.
    • If you chose B: An invasive species is a living thing that people carry to a new place. Nothing was carried here. The forest was cleared.

  6. Which statement about extinction is true?

    Tap your answer.

    1. Not this one. Species died out long before people, for example in the asteroid impact 66 million years ago and after huge volcanic eruptions. Try another.

    2. Yes, that is right. Mass extinctions happened before people. Today habitat loss, hunting, invasive species, pollution and climate change caused by people add more.

    3. Not this one. Species die out for many reasons, such as habitat loss, pollution and climate change. Hunters are only one cause. Try another.

    See the answer

    Extinction has happened many times in the past, and people now add new causes.

    Mass extinctions happened before people. Today habitat loss, hunting, invasive species, pollution and climate change caused by people add more.

    • If you chose A: Species died out long before people, for example in the asteroid impact 66 million years ago and after huge volcanic eruptions.
    • If you chose C: Species die out for many reasons, such as habitat loss, pollution and climate change. Hunters are only one cause.

  7. From an earlier chapter

    Resistant bacteria were rare before an antibiotic was used. A few days later, almost all the bacteria are resistant. Why?

    Tap your answer.

    1. Not this one. The antibiotic does not make bacteria resistant. It kills the others. Go to S4.3.1. Try another.

    2. Yes, that is right. The antibiotic killed the bacteria that were not resistant. The few resistant ones survived and divided.

    3. Not this one. It is the bacteria that are resistant, not the person. Go to S4.3.1. Try another.

    See the answer

    The resistant survivors divided, and their descendants are resistant.

    The antibiotic killed the bacteria that were not resistant. The few resistant ones survived and divided.

    • If you chose A: The antibiotic does not make bacteria resistant. It kills the others. Go to S4.3.1.
    • If you chose C: It is the bacteria that are resistant, not the person. Go to S4.3.1.

  8. A lake held 40 kinds of fish long ago. Today 30 kinds are left. What percentage of the kinds have been lost?

    See the answer

    25%

    The number lost is 40 − 30 = 10. 10 ÷ 40 = 0.25, which is 25%.

  9. A forest had 200 kinds of bird. Over time 15% of the kinds were lost. How many kinds are left?

    See the answer

    170 kinds

    15% of 200 is 0.15 × 200 = 30 kinds lost. 200 − 30 = 170 kinds are left.

Keep it

Copy this into your exercise book.

A species breeds to give fertile young. A barrier plus different conditions can make two species. Past causes of extinction: an asteroid, volcanoes, climate. Today: habitat loss, hunting, invasive species, pollution, climate change.

Draw: A piece of land split by water, with two groups of animals and a crossed arrow between them.

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

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