Evolutionary Classification

What is the goal of evolutionary classification?

The concept of descent with modification led to the study of phylogeny (fy LAHJ uh nee)—the evolutionary history of lineages. Advances in phylogeny, in turn, led to phylogenetic systematics. The goal of phylogenetic systematics, or evolutionary classification, is to group species into larger categories that reflect lines of evolutionary descent, rather than overall similarities and differences.

Common Ancestors Phylogenetic systematics places organisms into higher taxa whose members are more closely related to one another than they are to members of any other group. The larger a taxon is, the farther back in time all of its members shared a common ancestor. This is true all the way up to the largest taxa.

Clades Classifying organisms according to these rules places them into groups called clades. A clade is a group of species that includes a single common ancestor and all descendants of that ancestor—living and extinct. How are clades different from Linnaean taxa? A clade must be a monophyletic (mahn oh fy LET ik) group. A monophyletic group includes a single common ancestor and all of its descendants.

Some groups of organisms defined before the advent of evolutionary classification are monophyletic. Some, however, are paraphyletic, meaning that the group includes a common ancestor but excludes one or more groups of descendants. These groups are invalid under evolutionary classification.

In Your Notebook In your own words, explain what makes a clade monophyletic or paraphyletic.