Handout of terms useful in discussing systematics

Taxonomy: the science of ordering of things.  Taxonomy is a general term for use in classifying or ordering any number of entities by any chosen criterion.

Systematics: the ordering of things into more inclusive groups whose existence depend on some natural process through which their parts are related.

A character is a particular attribute of a specimen that can take two or more different forms.  In evolutionary terms a character is an original form plus all of its subsequent modifications.

Divisions of characters are called character states.

Homologous similarities are shared because of inheritance from a common ancestor.

In contrast with analogous similarities which are shared, not because of common ancestry but rather convergence on an external function (similarity of engineering solutions to the same problem).

A synapomorphy simply stated is a shared, derived homologous feature.  A shared derived characteristic.

An ingroup is the monophyletic group of interest.

An outgroup is any group used for comparative purposes in a phylogenetic analysis which is not the ingroup.

The sister group is the group most closely related, genealogically, to the ingroup.  In the general case, two groups are sister groups if they share a node.

The terminal groups are the labeled tips of the branches on a particular phylogenetic tree.

A symplesiomorphy is a shared, old, character state.  It then could be considered a synapomorphy incorrectly used at a higher level in the tree.

An autapomorphy is a new character seen only in a terminal taxon in a particular tree.  If the terminal group is split up and analyzed in greater detail, this autapomorphy is recognized as a synapomorphy for the constituent taxa.

A monophyletic group includes a common ancestor and all of its descendants.

A paraphyletic group includes a common ancestor and some but not all of its descendants.

In a polyphyletic group the most recent common ancestor of the entities included is not itself included in the group.

Parsimony is the idea that given two or more conflicting solutions to a given problem, the simplest should be chosen.

Homoplasy is:

1. Error in determining homology.  In this case analogous features are judged incorrectly to be homologous.
2. Parallelism, where a feature appears several times on the cladogram.
3. Reversal, where a trait originates then is lost, or the condition reverts to the ancestral condition.

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