Three domains and five kingdoms classification

Biology · Classification

Sort the same living things twice

Pick an organism, then pick its group. Do it under the kingdom rule first. Then switch to the domain rule and sort the very same organisms again.

Which of the five kingdoms is it in?

7 of 7 still to sort.

First by kingdom, then switch the rule and sort by domain. Notice what moves, and what splits.

Predict, then check

The numbers are invented, but the rule they test is real.

Gene sequences from three species are compared. Species P and Q differ in 2 places. Species P and R differ in 12 places. Which pair is more closely related?

What you need to know

• Groups sit within larger groups. The Linnaean levels run largest to smallest: kingdom, phylum, class, order, family, genus, species.
• Have a goA classmate swears 'species' must be the biggest level, because it sounds like it covers everything. Are they right?

  No. Species is the smallest level. Kingdom is the largest.

  Each level is a group within the larger group above it, so the levels shrink from kingdom down to species.

• Linnaeus (18th century) classified organisms by observable, measurable physical characteristics: their phenotype.
• The five kingdoms are animals, plants, fungi, protists and prokaryotes.
• Looks can mislead. Some close relatives share few physical traits, and some look-alikes are quite unrelated.
• For bacteria it is worse: different species can look very similar, so physical features are very hard to use.
• Have a goTwo bacterial species look almost identical under a microscope. Does that alone tell you they are close relatives?

  No. Looks alone cannot tell you.

  Different bacterial species can look very similar, and look-alikes can be quite unrelated, so appearance is a poor guide.

• In the 1970s Carl Woese compared gene sequences. Some genes change very slowly, so close relatives show fewer differences.
• Woese's work gave us the three-domain system, a higher level above the kingdoms: Bacteria, Archaea and Eukaryota.
• Eukaryota includes all animals, plants, fungi and protists. Their cells have membrane-bound structures such as a nucleus.
• Bacteria and archaea are both prokaryotes, yet separate domains. Archaea live in extreme places, such as high temperature or high acidity.
• Have a goBacteria and archaea together: how many kingdoms do they fill in the five-kingdom list, and how many domains?

  One kingdom (prokaryotes), but two domains.

  Both are prokaryotes, so one kingdom holds them, but they sit in the separate domains Bacteria and Archaea.

• Better methods, such as microscopy and gene sequencing, can move organisms into new groups, so the system is not complete.

The big picture

Living things are sorted into groups within larger groups, and the Linnaean levels run from kingdom down to species. Linnaeus classified organisms by their physical characteristics, but looks can mislead and are very hard to use for bacteria. In the 1970s Carl Woese compared gene sequences, which change very slowly, and introduced the three-domain system: Bacteria, Archaea and Eukaryota, above the kingdoms. New evidence can still move organisms into different groups, so the classification system is not complete.

⚠ Watch out

Reading the rule backwards. In slowly changing gene sequences, FEWER differences means MORE closely related. More differences means more distantly related.

Memory hook

King Philip Came Over For Good Soup: kingdom, phylum, class, order, family, genus, species. Then add the big idea: looks can deceive, gene sequences tell the story.

Check yourself

Without looking back: name the three domains, say which one holds all the animals, plants, fungi and protists, and explain why the bacteria and the archaea are not the same domain.