Classification and cladistics

Millions of species need organizing. A single binomial name works in every language, groups make identification easier, and a species' group lets us predict its features.

The ranks from kingdom to species are fixed levels, but evolution produces continuous branching. Ranks are not equivalent between groups, convergent evolution can mislead, and some traditional groups, such as reptiles, leave out descendants of their ancestor.

If every member of a group comes from one common ancestor, members share inherited features, so we can predict the features of species that have not been studied, and the classification shows evolutionary history.

A clade is a common ancestor and all its descendants. The most objective evidence for a clade comes from base sequences of genes and amino acid sequences of proteins; morphology can also be used.

Sequence differences build up at a roughly steady rate, so the number of differences estimates how long ago two clades diverged. It is only an estimate because mutation rates vary with generation time, population size and selection.

Count the differences between each pair of sequences. Join the most similar pair first, then add the others in order of similarity.

The root is the common ancestor of all the taxa; each node is a hypothetical common ancestor; each terminal branch leads to one taxon. Taxa that share a more recent node are more closely related.

DNA comparisons showed that the figwort family was not a clade, so many of its genera, such as foxgloves and speedwells, were moved to other families, mostly the plantain family.

In 1977, ribosomal RNA sequences showed that prokaryotes form two very different groups. A new rank, the domain, now divides life into Bacteria, Archaea and Eukarya.