Studying phylogeny: How cladistics works
When grouping related organisms, biologists first identify a number of variable characteristics found in members of the group under study. The process of merely comparing which members share which characteristics is called phenetics. Cladistics is more sophisticated than phenetics because it traces not just similarity, but evolutionary relationships.
A group of organisms sharing a common ancestor is called a clade, from Greek klados, or "branch," and this term leads to the name cladistics, the study of clades. To trace evolutionary history and distinguish clades, biologists must determine whether each characteristic under study represents an older, more ancestral characteristic (plesiomorphy) or a more recently evolved characteristic (apomorphy). These could be respectively called "primitive" and "advanced," but because those terms imply value judgments, cladists prefer the terms "basal" and "derived."
To determine whether characteristics are basal or derived, they are compared to one or more outgroups, that is, groups of organisms outside the group under study. An apomorphy shared by all members of the group but not by outgroups is considered a derived characteristic of the last common ancestor. This type of characteristic is called a synapomorphy.
After identifying apomorphies, synapomorphies, and plesiomorphies, researchers can apply an analysis (typically using a computer) to determine clades. Smaller clades nest within larger clades. In another way of looking at things, smaller clades are always branches from larger clades. The result of a cladistic analysis is a tree diagram called a cladogram.