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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.

The dog, for example, is Eukarya, Animalia, Chordata, Mammalia, Carnivora, Canidae, Canis, Canis lupus; the domestic dog is the subspecies Canis lupus familiaris.

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.

Experiment: Designing an efficient dichotomous key

1. Build a dichotomous key that identifies all 16 organisms. Use only features you can observe in the pictures.
2. Count the number of steps needed to reach each organism. Calculate the mean number of steps and note the longest path.
3. 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.
4. Give your key to a partner group. Record for each organism whether they identify it correctly and how long it takes.
5. Calculate the percentage of correct identifications and the mean time.
6. Redesign one branch to make the key shorter or clearer, and test again.

Measure	First key	Improved key
Mean number of steps
Longest path (steps)
Correct identifications (%)
Mean time per organism (s)

Questions

1. Write out the full classification of the domestic dog from domain to subspecies.

2. Explain why fungi were moved out of the plant kingdom. Refer to nutrition and to molecular evidence.

3. Your key for 16 organisms has a mean of 6.5 steps. What is the minimum possible, and why is a real key usually longer?

4. Explain why a phylogenetic tree is described as a hypothesis.

Sources for every fact on this sheet: echoesof.science/school/classification/sources/

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