Concept preview

What you will be able to do

1. 01Construct and use a dichotomous key to identify organisms.
2. 02Read a phylogenetic tree or cladogram: tips, branches, nodes, common ancestors and clades.
3. 03Explain what shared derived characters are and how molecular data build trees.
4. 04Interpret relatedness from a tree and avoid the trap of convergent similarity.

Once organisms have been classified, biologists need a practical way to identify an unknown specimen — to work out which known species the creature in front of them actually is. The classic tool for exactly this job is the dichotomous key. The word dichotomous means 'divided into two', and that is precisely how the key works: it presents a series of paired statements, and at each step you simply choose whichever of the two fits your organism. Each choice either names the organism outright or sends you on to another pair of choices, steadily narrowing the possibilities until only a single identity remains. A good key is rather like a game of twenty questions played with nature: every either/or answer eliminates roughly half of what is left, so even a large group of species can be identified reliably in just a handful of steps.