relationship: defined in terms of genealogy--common ancestral species. Two species are more closely related to one another than to a third species if they share a more recent common ancestor that either does with the third. These two taxa are considered as sister groups.

monophyletic: a group which includes an ancestral species and all its descendants. Aves, Mammalia. This group is a complete branch of the tree of life, the phylogeny of life. Such a branch is called a clade.

paraphyletic: a group lacking some of the descendant species. The old "Reptilia" and "fishes."

apomorphy: a trait which characterizes an ancestral species and its descendants. This is an evolutionary novelty for the group. These are evidence for the existence of a group. Put another way, attributes shared in common are taken to indicate a shared evolutionary history.

plesiomorphy: features shared more widely than in a group of interest. These are primitive for the group in question and cannot provide evidence for the group.

Note that apomorphy and plesiomorphy are relative concepts. Their status depends on their position in the phylogeny. A character is an apomorphy at one branch of the tree, but is a plesiomorphy relative to all the branches after that. For example, hair is a mammalian feature (an apomorphy of mammals), but is primitive in squirrels (a plesiomorphy). We can use presence of hair as evidence for the existence of Mammalia, but not for the existence of Sciuridae.

outgroup: one or, preferably, several taxa that lie outside the group of interest, that is the group in which we are trying to detect relationships. For example, if we are interested in determining the relationships of minnows, we would use the closest relatives of minnows as our outgroups. Outgroup comparison is the way we determine how widespread a particular feature may be, whether it is found only within the group of interest, or beyond that group.