Patterns in Life: Diversity and Classification
From two kingdoms to three domains
Follow how classification grew from two kingdoms to three domains.
Classification has been improved again and again, each time to fit organisms that didn't fit before.
Early ideas. Aristotle (4th century BCE) grouped animals by where they live (land, water, air) and how they look. The Rigveda and the Brihat Samhita also grouped animals by habitat, behaviour and their role in nature. But these used only features you can see easily.
Building the five kingdoms, one problem at a time:
| Year | Scientist | Kingdoms | Problem solved |
|---|---|---|---|
| 1758 | Linnaeus | 2: Plantae (don't move, make food) and Animalia (move, eat others) | A first simple split. But where do Amoeba, Paramecium and bacteria go? |
| 1866 | Haeckel | 3: adds Protista | A home for one-celled, microscopic organisms |
| 1938 | Copeland | 4: adds Monera | Bacteria have no true nucleus, unlike Amoeba, so they got their own kingdom |
| 1969 | Whittaker | 5: adds Fungi | Mushrooms don't move, but don't make food either; they absorb it |
Be Whittaker's assistant: sort any organism with yes/no questions.
- Does its cell have a true nucleus? No: Monera.
- Yes. Is it one cell? Yes: Protista.
- Many cells. Do the cells have a cell wall? No: Animalia.
- Yes. Does it make its own food? Yes: Plantae. No: Fungi.
These four questions use Whittaker's criteria: cell type, cell wall, single or many cells, and mode of nutrition.
Three domains. By comparing DNA, Carl Woese (1977) showed that tiny living things are far more varied than anyone thought. He proposed three domains: Bacteria, Archaea and Eukarya (everything with a true nucleus).
"Why does classification keep changing?" Because new tools, like microscopes and DNA studies, keep showing us more. Changing your ideas when the evidence changes is exactly how science works.
Practise this lesson
Questions that get harder as you improve, a Tutor that helps without giving the answer away, and progress you can see. Free for every learner.
No account, no email address. Opens the real product.