School › Classification › Ages 15–16
Taxonomy, binomials and evolutionary trees
The three domains and the taxonomic hierarchy, binomial nomenclature, why classification follows evolutionary relationships rather than looks, how kingdom systems changed, and designing an efficient dichotomous key.
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Modern classification first divides life into three domains: Bacteria, Archaea and Eukarya. Below them come kingdom, phylum, class, order, family, genus and species.[1]
The dog, for example, is Eukarya, Animalia, Chordata, Mammalia, Carnivora, Canidae, Canis, Canis lupus; the domestic dog is the subspecies Canis lupus familiaris.[2][1]
Binomial nomenclature, from Linnaeus, gives each species a unique two-part name. Taxon names are capitalised except the species name, genus and species are written in italics, and each named species has a type specimen as its reference example.[2][3]
At each lower level, organisms are more closely related. Members of a taxon should share a common ancestor, and closely related organisms do not always look alike, so classification follows evolutionary relationships rather than appearance alone.[1][2]
A phylogenetic tree shows these evolutionary relationships. Trees are hypotheses: they change when new data, such as DNA sequences, are found.[1]
Fungi show how classification changes. They were once grouped with plants because of their rigid cell walls. Whittaker's five-kingdom system (1969) separated them, and molecular evidence now shows that fungi are more closely related to animals than to plants.[5][4]
Try it: Designing an efficient dichotomous key
You need
- Cards with pictures of 16 different organisms (mix of invertebrates, vertebrates, plants and fungi)
- Large sheet of paper
- A stopwatch
- A partner group
Safety
Picture cards only; no specimens are needed.
What to do
- Build a dichotomous key that identifies all 16 organisms. Use only features you can observe in the pictures.
- Count the number of steps needed to reach each organism. Calculate the mean number of steps and note the longest path.
- Compare with the minimum possible: with two choices per step, n steps can separate at most 2ⁿ organisms, so 16 organisms need at least 4 steps.
- Give your key to a partner group. Record for each organism whether they identify it correctly and how long it takes.
- Calculate the percentage of correct identifications and the mean time.
- Redesign one branch to make the key shorter or clearer, and test again.
What happens, and why?
A dichotomous key splits a group in two at each step using observable traits. Because each step at most halves the group, a perfectly balanced key needs log₂(n) steps; real keys are longer because features rarely split groups exactly in half. Testing on another group checks that the questions are unambiguous. Note that a key is a tool for identification: organisms that end up next to each other in a key need not be closely related.[7][1]
Play: Correct binomial or not?
Is the scientific name written correctly (genus with a capital, species in lower case, two parts)?
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Quiz
Other ages: Ages 6–7 · Ages 10–11