GCSE · Biology · AQA · Spec 8461

Classification of living organisms

Down a microscope, two microbes can look like identical twins. Biologists still put them in completely different groups of life. By the end of this lesson, you'll know why.

Biology · Classification

Sort it once. Then sort it again.

Tap an organism, then tap the group it belongs in. Finish Sort 1, then switch to Sort 2 and sort the same seven again.

The traditional way: group organisms by their structure and characteristics. Here that means what a microscope shows you: how many cells, and whether each cell has a nucleus.

Still to sort

One cell, no nucleus (0)

A single tiny cell with no nucleus inside.

One cell with a nucleus (0)

Just one cell, but it has a nucleus.

Many cells with nuclei (0)

Built from lots of cells that have a nucleus.

7 of 7 still to sort.

Done both sorts? Now look at what moved. The three microbes with no nucleus were one group in Sort 1 and got split in Sort 2. Everything with a nucleus ended up together. So what changed: the organisms, or the evidence?

Exam line: The three domains are archaea, bacteria and eukaryota. Eukaryota includes protists, fungi, plants and animals.

Watch out: Looking alike is not the same as being closely related. Sulfolobus and E. coli look alike down a microscope, but they sit in different domains.

Why the groups changed

Reason it through

Why do scientists now use a three-domain system instead of grouping living things only by how they look?

Link 1 of 4

First link · your turn

The traditional system sorted living things by structure and characteristics. What evidence was that based on?

Locked — reveal the link above first

Biology · The Linnaean system

Groups inside groups

Put Linnaeus's seven ranks in order, from the biggest group to the smallest.

1 · Biggest group: holds the most kinds of organism7 · Smallest group: one kind of organism

Genus
Kingdom
Order
Species
Phylum
Family
Class

Watch out: Easy mistake: class and order. Class is the bigger group, so a class contains orders.

Biology · Evolutionary trees

Reading an evolutionary tree

Showing 1 layer: Evolutionary tree

Explore

Tap a part of the tree to see what it means.

Biology · Reading the tree

Who is the tiger's closest relative?

Look back at the tree above. The tips, from left to right, are: domestic cat, lion, tiger, the extinct sabre-toothed cat, and wolf.

Which of these is closest to what you think right now?

The sabre-toothed cat. It's drawn right next to the tiger.The domestic cat. Tigers evolved from small cats like it.The lion. The tiger and lion branches meet at the most recent branch point.You can't tell from the tree. You'd need to see which one looks most like a tiger.

How sure are you?

I'm sureI think soI'm guessing

The sabre-toothed cat. It's drawn right next to the tiger.

This is a common misconception

Where a tip is drawn tells you nothing. You could spin the branches round any branch point, like a hanging mobile, and the tree would mean exactly the same thing. Trace the tiger and the sabre-toothed cat back down the tree: their branches only meet much further back, so they are more distant relatives than the tiger and the lion.

The domestic cat. Tigers evolved from small cats like it.

This is a common misconception

The domestic cat is alive today, just like the tiger. On an evolutionary tree, the organisms at the tips are not each other's ancestors: the ancestors sit at the branch points. The tiger and the domestic cat share a common ancestor further back in time, so they are more like distant cousins.

The lion. The tiger and lion branches meet at the most recent branch point.

This matches the evidence

Exactly. Follow any two branches back until they meet. The more recent that meeting point, the more closely related the two organisms are, and the tiger meets the lion before it meets anything else.

You can't tell from the tree. You'd need to see which one looks most like a tiger.

This is a common misconception

Looks can mislead. Remember the microbes at the top of this lesson that looked identical but sat in different domains. The tree is exactly the tool for this job: read the branch points, not the faces.

Closest relatives share the most recent branch point. Ignore how near the tips are drawn, and remember that the tips are relatives, not ancestors: ancestors sit at the branch points.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

How scientists sort life into groups, and why the groups change when the evidence does

What you need to know

Classification is a model: when better evidence arrives, the groups can change.
Linnaeus sorted organisms by structure and characteristics into seven nested ranks, and gave each a two-part name: genus, then species.
Chemical analysis gave us the three domains: archaea, bacteria and eukaryota.
On an evolutionary tree, the closest relatives share the most recent branch point.

The big picture

Classification is how biologists sort living things into groups. Carl Linnaeus built the traditional system, grouping organisms by their structure and characteristics into seven ranks from kingdom down to species, and naming each organism by its genus and species. As microscopes improved and the understanding of biochemical processes grew, new models of classification were proposed. Evidence from chemical analysis led Carl Woese to the three-domain system: archaea, bacteria and eukaryota. Evolutionary trees show how scientists believe organisms are related.

Key points

1Linnaeus's traditional system grouped living things by their structure and characteristics.

2The seven Linnaean ranks, biggest to smallest: kingdom, phylum, class, order, family, genus, species.

3The binomial system names an organism by its genus and species, for example Homo sapiens.

4Two organisms that share a lower rank share every rank above it; the lower the shared rank, the more closely related they are.

5Improved microscopes revealed internal structures, and understanding of biochemical processes progressed, so new models of classification were proposed.

6Evidence from chemical analysis led Carl Woese to the three-domain system: archaea (primitive bacteria usually living in extreme environments), bacteria (true bacteria) and eukaryota (protists, fungi, plants and animals).

7Evolutionary trees show how scientists believe organisms are related, using current classification data for living organisms and fossil data for extinct ones.

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Worked example

Problem

Here are three bears: the brown bear (Ursus arctos), the polar bear (Ursus maritimus) and the giant panda (Ailuropoda melanoleuca). All three are in the family Ursidae. Which two are the most closely related? Explain your answer.

⚠ Watch out

Judging how closely related organisms are by how alike they look. Appearance can mislead: use the lowest rank two organisms share or, on an evolutionary tree, the most recent branch point they share.

🧠

Memory hook

King Philip Came Over For Good Soup: Kingdom, Phylum, Class, Order, Family, Genus, Species. For the three domains, think ABE: Archaea, Bacteria, Eukaryota.

✓

Check yourself

A friend says E. coli and Sulfolobus must belong together because they look identical under a microscope. What would you tell them, and what evidence would you mention?

Flashcards

(13)

Who developed the traditional system of classification, and what did it group living things by?

Carl Linnaeus. It grouped living things by their structure and characteristics.

List the seven Linnaean ranks, from biggest to smallest.

Kingdom, phylum, class, order, family, genus, species.

What is the binomial system?

Naming an organism with two words: its genus, then its species. For example, Homo sapiens.

The golden eagle's binomial name is Aquila chrysaetos. Which word is its genus, and which is its species?

Aquila is the genus; chrysaetos is the species. The genus always comes first.

How can you use classification information to judge how closely related two organisms are?

Find the lowest rank they share. The lower the shared rank, the more closely related they are.

Which two developments in biology led to new models of classification?

Improved microscopes, which gave evidence of internal structures, and progress in understanding biochemical processes.

What evidence led to the three-domain system, and who developed it?

Evidence from chemical analysis. The system was developed by Carl Woese.

Name the three domains.

Archaea, bacteria and eukaryota.

What are archaea?

Primitive bacteria that usually live in extreme environments.

Which groups of organisms does the eukaryota domain include?

Protists, fungi, plants and animals.

What is an evolutionary tree?

A method scientists use to show how they believe organisms are related.

What data do scientists use to place living organisms and extinct organisms on an evolutionary tree?

Current classification data for living organisms; fossil data for extinct organisms.

On an evolutionary tree, which organisms are the most closely related?

The ones that share the most recent branch point (the most recent common ancestor).

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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Classification of living organisms

Sort it once. Then sort it again.

Groups inside groups

Who is the lion's closest relative?

Who is the tiger's closest relative?

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