Chapter 4 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

S4.3.4 Chapter 4

Classification

Why does every species get a two part name, such as Homo sapiens?

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 the two part naming system, such as Homo sapiens
  • Explain how DNA evidence changed the way we group living things

There are millions of kinds of living thing. To study them, scientists sort them into groups. This is classification. A good classification shows how closely living things are related.

In the 1700s a scientist called Carl Linnaeus built the system we still build on. Later scientists added more levels. Today it has seven levels, from the biggest group to the smallest. The biggest is the kingdom. Below it come the phylum, the class, the order and the family. Next comes the genus, and last comes the species. The plural of genus is genera.

Each group sits inside a bigger one, like boxes inside boxes. A genus is a group of very closely related species.

Seven boxes, each inside the next, from the biggest to the smallest. Each is named with its level, and the example is a human. Kingdom is Animalia. Phylum is Chordata. Class is Mammalia. Order is Primates. Family is Hominidae. Genus is Homo. Species is Homo sapiens.kingdomAnimaliaphylumChordataclassMammaliaorderPrimatesfamilyHominidaegenusHomospeciesHomo sapiens
How a human is classified. Each level is a smaller group inside the one before.

Linnaeus also gave every species a two part name. The first word is its genus and starts with a capital letter. The second word is its species name and starts with a small letter. Our own name is Homo sapiens. In print the name is written in italics. By hand, you underline each word. The text of this book prints the names upright, but the pictures show them in italics.

Draw it first

Write the name of a species

  1. The two part name of the human is built in three pictures. The first picture shows the word genus over the word Homo. The second adds the word species over the word sapiens. The third shows the whole name, Homo sapiens.genusHomo

    1 Write the genus first, with a capital letter. For us it is Homo.

  2. The two part name of the human is built in three pictures. The first picture shows the word genus over the word Homo. The second adds the word species over the word sapiens. The third shows the whole name, Homo sapiens.genusHomospeciessapiens

    2 Write the species name after it, with a small letter. For us it is sapiens.

  3. The two part name of the human is built in three pictures. The first picture shows the word genus over the word Homo. The second adds the word species over the word sapiens. The third shows the whole name, Homo sapiens.genusHomospeciessapiensHomo sapiens

    3 Together they are the name of the species. In print, it is in italics.

Two species in one genus share the first word. A lion is Panthera leo. A tiger is Panthera tigris. They are in the same genus, so they are close relatives. A domestic cat is Felis catus. It is in the same family as the lion, but in a different genus.

Seven boxes, each inside the next, from the biggest to the smallest. The example is the lion. Kingdom is Animalia. Phylum is Chordata. Class is Mammalia. Order is Carnivora. Family is Felidae. Genus is Panthera. Species is Panthera leo.kingdomAnimaliaphylumChordataclassMammaliaorderCarnivorafamilyFelidaegenusPantheraspeciesPanthera leo
How a lion is classified.
Seven boxes, each inside the next, from the biggest to the smallest. The example is the domestic cat. Kingdom is Animalia. Phylum is Chordata. Class is Mammalia. Order is Carnivora. Family is Felidae. Genus is Felis. Species is Felis catus.kingdomAnimaliaphylumChordataclassMammaliaorderCarnivorafamilyFelidaegenusFelisspeciesFelis catus
How a domestic cat is classified. It differs from the lion at the genus.

Worked example

A lion and a domestic cat are classified as in the two pictures. How many levels do they share, counting from the kingdom?

  1. Compare the two pictures level by level, from the kingdom down.
  2. The kingdom, phylum, class, order and family are the same.
  3. The genus is different, so they stop sharing there. They share 5 levels.

Answer: 5 levels

Linnaeus sorted living things by how they look. Later, scientists could compare DNA. DNA shows how closely living things are related, as you saw in the chapter on evidence for evolution. Sometimes it showed that the old groups were wrong.

Here are two examples. Fungi look like plants, because they grow from the ground and do not move. But DNA shows that fungi are closer relatives of animals than of plants. And DNA showed that the giant panda belongs with the bears, in the same family as the brown bear.

A tree of descent with time running to the right. The common ancestor, A, splits into Bacteria and branch point B. B splits into Archaea and Eukarya. The arrow is labelled time.ABacteriaBArchaeaEukaryatimecommon ancestor
The three domains. Archaea are more closely related to Eukarya than to Bacteria.

In the 1970s Carl Woese compared one gene that every living thing has, across many single celled living things. He found that one group, now called Archaea, is very different from Bacteria. Both groups have cells with no nucleus. But their genes differ greatly.

So scientists added a level above the kingdom. It is the domain. There are three: Bacteria, Archaea and Eukarya. Eukarya have cells with a nucleus. Animals, plants, fungi and many single celled living things are all in Eukarya.

A bar chart of the species in three genera of one family. The axis is labelled species, with a scale from 0 to 8 in steps of 2. The bar for genus A reaches 4. The bar for genus B reaches 7. The bar for genus C reaches 2.02468genus Agenus Bgenus Cspecies
Example numbers for the three genera of one family.

Worked example

A family has three genera. Genus A has 4 species, genus B has 7 species and genus C has 2 species. How many species are in the family?

  1. Each species belongs to one genus, so we add the species of the three genera.
  2. 4 + 7 = 11.
  3. 11 + 2 = 13 species in the family.

Answer: 13 species

Try it

Sort leaves into boxes in boxes

You need: 8 fallen leaves, from at least 3 kinds of plant, a sheet of paper and a pencil.

Be safe: Pick up only fallen leaves, and do not touch plants you do not know, in case they sting or irritate. Wash your hands afterwards.

  1. Lay the leaves out. Sort them into two or three big groups by a feature, such as a smooth or a toothed edge.
  2. Sort each big group into smaller groups by another feature, such as the shape.
  3. Draw a big box for each group, and draw the smaller groups as boxes inside it.
  4. Write what each box has in common.

What you will see: Every group sits inside a bigger one, and a smaller group shares more features than a bigger one. This is how kingdoms, families and genera work.

If you cannot try it, this is what happens.
A table with the columns Edge, Shape and Leaves. It is one example of how 8 leaves might be sorted into big groups by edge and smaller groups by shape. smooth edge, long shape: 3 leaves. smooth edge, round shape: 1 leaves. toothed edge, long shape: 2 leaves. toothed edge, round shape: 2 leaves.EdgeShapeLeavessmoothlong3smoothround1toothedlong2toothedround2

A wrong idea many people have

Living things that look alike must be closely related.

Why it feels right: Early scientists grouped living things by their looks, and looks often do show relationships.

The test that breaks it: A dolphin looks a little like a fish. But it breathes air and feeds its young on milk, so it is a mammal. DNA shows that fungi, which look like plants, are closer to animals.

What is true: Looks can mislead, because unrelated living things can come to look alike when they live in the same way. DNA is a more reliable guide to how closely they are related.

Key words

classification
sorting living things into groups that show how closely they are related.
kingdom
one of the biggest groups of living things, such as animals.
genus
a group of very closely related species. The plural is genera.
domain
the biggest group of all. There are three: Bacteria, Archaea and Eukarya.

Back to your guess

The answer is B: Every scientist, in any language, can then use the same name for the same species.

Common names change from place to place and from language to language. A two part name is the same everywhere, so scientists always know which species is meant.

Check yourself

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

  1. Which is the correct way to write the two part name of the human?

    Tap your answer.

    1. Not this one. The genus is the first word, and it starts with a capital letter. So it is Homo. Try another.

    2. Not this one. The species name is the second word, and it starts with a small letter. So it is sapiens. Try another.

    3. Yes, that is right. The genus starts with a capital letter and the species name with a small one. In print the name is in italics.

    See the answer

    Homo sapiens

    The genus starts with a capital letter and the species name with a small one. In print the name is in italics.

    • If you chose A: The genus is the first word, and it starts with a capital letter. So it is Homo.
    • If you chose B: The species name is the second word, and it starts with a small letter. So it is sapiens.

  2. Which list gives the levels of classification from the biggest group to the smallest?

    Tap your answer.

    1. Yes, that is right. Each group sits inside the one before it. A species is in a genus, a genus is in a family, and so on up to the kingdom.

    2. Not this one. That list is the wrong way round. The kingdom is the biggest group, and the species is the smallest. Try another.

    3. Not this one. Two pairs are swapped. A phylum is a bigger group than a class, and a family is bigger than a genus. Try another.

    See the answer

    kingdom, phylum, class, order, family, genus, species

    Each group sits inside the one before it. A species is in a genus, a genus is in a family, and so on up to the kingdom.

    • If you chose B: That list is the wrong way round. The kingdom is the biggest group, and the species is the smallest.
    • If you chose C: Two pairs are swapped. A phylum is a bigger group than a class, and a family is bigger than a genus.

  3. Two animals have the same first word in their two part names. What does this tell you?

    Tap your answer.

    1. Not this one. Two animals of one species share both words. Here only the first word is the same, so the species differ. Try another.

    2. Yes, that is right. The first word is the genus. Species in one genus are very closely related.

    3. Not this one. The first word is the genus. It does not say where a species lives. Try another.

    See the answer

    They are in the same genus, so they are close relatives.

    The first word is the genus. Species in one genus are very closely related.

    • If you chose A: Two animals of one species share both words. Here only the first word is the same, so the species differ.
    • If you chose C: The first word is the genus. It does not say where a species lives.

  4. What led scientists to put Bacteria and Archaea in two different domains?

    Tap your answer.

    1. Not this one. Some Archaea live in hot or salty places, but many do not. Where a living thing lives does not decide its group. Try another.

    2. Not this one. Both are tiny cells with no nucleus, and they look alike. The difference is in their genes. Try another.

    3. Yes, that is right. Carl Woese compared genes and found that Archaea differ greatly from Bacteria, though both are single celled with no nucleus.

    See the answer

    Comparing their genes showed that Archaea are very different from Bacteria.

    Carl Woese compared genes and found that Archaea differ greatly from Bacteria, though both are single celled with no nucleus.

    • If you chose A: Some Archaea live in hot or salty places, but many do not. Where a living thing lives does not decide its group.
    • If you chose B: Both are tiny cells with no nucleus, and they look alike. The difference is in their genes.

  5. Look at the tree. Which animal is the giant panda most closely related to?

    A tree of descent with time running to the right. The common ancestor, A, splits into the cat and branch point B. B splits into the dog and branch point C. C splits into the giant panda and the brown bear. The arrow is labelled time.AcatBdogCgiant pandabrown beartimecommon ancestor
    Tap your answer.

    1. Yes, that is right. The giant panda and the brown bear branch from point C, which is the latest branch point on the tree. DNA shows that the panda is a bear.

    2. Not this one. The dog shares branch point B with the panda, which is further back in time. The panda and the brown bear share the later branch point C. Try another.

    3. Not this one. The cat splits off first, at the oldest branch point A. It is the most distant relative on this tree. Try another.

    See the answer

    The brown bear

    The giant panda and the brown bear branch from point C, which is the latest branch point on the tree. DNA shows that the panda is a bear.

    • If you chose B: The dog shares branch point B with the panda, which is further back in time. The panda and the brown bear share the later branch point C.
    • If you chose C: The cat splits off first, at the oldest branch point A. It is the most distant relative on this tree.

  6. From an earlier chapter

    Two groups of snails live on opposite sides of a mountain range. Why are they not yet two species?

    Tap your answer.

    1. Yes, that is right. The barrier is only the start. Each group is selected by its own conditions, and over many generations they may stop being able to interbreed.

    2. Not this one. Any living thing that breeds can form new species. Snails have done this many times. Try another.

    3. Not this one. A name is only a label that scientists give after they find that the groups differ. It does not make the groups different. Try another.

    See the answer

    Being apart does not make a new species. They are one species until they can no longer breed together to give fertile young.

    The barrier is only the start. Each group is selected by its own conditions, and over many generations they may stop being able to interbreed.

    • If you chose B: Any living thing that breeds can form new species. Snails have done this many times.
    • If you chose C: A name is only a label that scientists give after they find that the groups differ. It does not make the groups different.

  7. Two animals are in the same family, but not in the same genus. How many of the seven levels do they share, counting from the kingdom?

    See the answer

    5 levels

    They share the kingdom, phylum, class, order and family. That is 5 levels.

  8. A family has three genera. Genus A has 3 species, genus B has 5 species and genus C has 6 species. How many species are in the family?

    See the answer

    14 species

    Each species belongs to one genus. 3 + 5 + 6 = 14 species.

Keep it

Copy this into your exercise book.

Seven levels: kingdom, phylum, class, order, family, genus, species. A species has a two part name, such as Homo sapiens. DNA led to three domains: Bacteria, Archaea and Eukarya.

Draw: Boxes inside boxes from kingdom to species, with Homo sapiens written in the smallest box.

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

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