Groups of organisms are descended from a common ancestor. At each node (divergence of a population), there are two branching lines of descendants. Evolution results in modifications of characteristics over time.

MONOPHYLETIC A monophyletic group consists of organisms that share recent common ancestors (and therefore have similar features). The members of a monophyletic group are closely related to each other. A monophyletic group is called a clade. An example of a monophyletic group is mammals.

PARAPHYLETIC A paraphyletic group consists of a common ancestor and some, but not all, of its descendants. These are incomplete groups based primarily on physical characteristics rather than directly on evolutionary relationships. An example of a paraphyletic group is the dinosaurs (without including the birds).

POLYPHYLETIC A polyphyletic group consists of organisms but not their common ancestors. This is an artificial group which is based primarily on physical characteristics rather than on evolutionary relationships. An example is "flying vertebrates" which includes birds, pteranodons and bats.

Ghost Lineages
A ghost lineage is a group of organisms that are thought to exist because of cladistic analysis, but for which there is as yet no fossil evidence of their existence.

Stratocladistics
Stratocladistics is a method of classifying organisms based upon both cladistics (considering common ancestors with shared anatomical characteristics) together with stratiography (information from the fossil record which lets you know which animals lived earlier or later than others; older fossils are deeper than more recent fossils). In stratocladistics, cladograms are generated in which ancestors precede their descendants. (see Science, June 11, Vol 284, 1999)