Some Terms Used in Systematics

The following are definitions of some terms commonly encountered in the more recent systematic literature. You should be aware that several of these words may have alternative definitions. I have taken many of these definitions from: 1) Holmes, E. B. 1980. Reconsideration of some systematic concepts and terms. Evolutionary Theory 5:35-87; 2) Wiley, E. O. 1981.Phylogenetics: The Theory and Practice of Phylogenetic Systematics. John Wiley & Sons, NY. and 3) Futuyma, D. J. 1998.Evolutionary Biology, 3rd Ed. Sinauer Associates.

Anagenetic (Anagenesis) = pertaining to evolutionary change through time.

Analogous = pertaining to similarity of function, structure or behavior due to convergence rather than common ancestry.

Apomorphy (Autapomorphy) = unique derived character.

Clade = the entire portion of a phylogeny that is descended from a single ancestral species and is therefore a monophyletic group.

Cladistic = pertaining to monophyletic groups; pertaining to the branching sequence in evolution.

Cladogenetic = pertaining to the branching sequence in evolution.

Convergence = the independent development of similar characters in two or more lineages that is not based on inherited genotypic similarity.

Derived (Advanced) = evolved later in time.

Homologous = pertaining to a relationship, existing only at the level of the ancestral feature, between features (in two or more organisms) that have evolved from the same feature in the most recent common ancestral species of these organisms.

Homoplastic (Homoplasy) = pertaining to nonhomologous similarity; includes convergence, parallelism and evolutionary reversals.

Monophyletic = pertaining to a group of species that includes the most recent common ancestral species of the entire group plus all more recent justifiably inferred ancestors of each species of the group; pertaining to a group of species comprising a single ancestral species and all its descendants.

Out-group = a species or higher monophyletic taxon that is examined in the course of a phylogenetic study to determine which of two homologous characters may be inferred to be apomorphic.

Parallelism = the separate development of similar characters in two or more relatively closely related lineages on the basis of genotypic similarity inherited from a common ancestor.

Paraphyletic = pertaining to a monophyletic group that excludes one or more discrete groups descended from the most recent common ancestral species of the entire group.

Parsimony = The principle that an explanation requiring the fewest undocumented assumptions should be preferred over more complicated hypotheses that require more assumptions for which evidence is lacking; the principle of accounting for observations by that hypothesis requiring the fewest or simplest assumptions that lack evidence. In systematics, the principle of invoking the minimal number of evolutionary changes to infer phylogenetic relationships.

Phenetic = pertaining to a measure of overall similarity derived from the use of many equally (or differentially) weighted characters selected (and weighted) without regard to evolutionary history.

Phylogenetic (Phylogeny) = pertaining to evolutionary history.

Polyphyletic = pertaining to a group of species that does not include both the most recent common ancestral species of the entire group and all more recent inferred ancestors of each species of the group.

Primitive (Ancestral) = evolved earlier in time.

Sister group = a species or higher monophyletic taxon that is hypothesized to be the closest genealogical relative of a given taxon exclusive of the ancestral species of both taxa.

Symplesiomorphy = shared primitive character.

Synapomorphy = shared derived character.

Taxon (Taxa) = a grouping of organisms given a proper name, or, a group that could be given a proper name but is not named as a matter of convention.