Phylogenetic tree (or cladogram)-is a graphic representation of the genealogic relationships between taxa as these relationships are understood by a particular investigator. In other words a phylogenetic tree is a hypothesis (not a fact) of genealogic relationships on the taxon level.

Tree branch- a line connecting a branch point to a terminal taxon in a phylogenetic tree.

Node-represents a speciation event on the tree

Internode-a line connecting two speciation events and represents at least one ancestral species.

Plesiomorphic "primitive"-structures that are ancestral within a taxon

Symplesiomorphic-shared ancestral structures between two taxa

Apomorphic "derived"-a descendent character

Synapomorphic-a shared descendent character between two taxa

Autapomorphic-a descendent character unique to a branch.

Homology - structures with same evolutionary origin

Analogy - structures with same function

Homoplasy - structures which appear similar

Natural taxon- A group of organisms that exists in nature as a result of evolution. Example: mammals

Artificial taxon-is a grouping that does not correspond to a unit involved in the evolutionary process or to a unit of evolutionary history. Example: Birds not being grouped with reptiles, grouping all worm-like animals as "Vermes".

Cladogenesis-branching or divergent evolution caused by speciation.

Anagenesis-a change within a species that does not involve branching. Example: Homo erectus evolving into Homo sapiens.

Gradualism-evolution occurring at a constant and slow rate over time.

Quantum evolution (punctuated equilibrium)-evolution occurring rapidly followed by long periods of little change over time.

Convergent evolution- an evolutionary change producing similar characteristics in two or more distantly related forms as a result of their separate adaptation to common ecological conditions.

Parallel evolution-the development of similar characteristics in species or other taxonomic groups not directly related in evolutionary descent.

Ingroup-the group actually studied by the investigator. The group of interest.

Outgroup-any group used in an analysiss that is not included in the taxon under study. It is used for comparative purposes.

Sister group-the taxon that is genealogically most closely related to the ingroup. The ancestor of the ingroup cannot be its sister because the ancestor is a member of the group.

Traditional Systematics-places together organisms with similar characteristics that may or may not be homologous

Phylogenetic systematics (Cladistics)- organisms grouped strictly on homology. Trees are based on genealogy not simply similarity.

Monophyletic- a group of species that includes an ancestral species and all of its descendants. Example: Mammalia

Polyphyletic-are artificial groups where the common ancestor is placed in another taxon

Paraphyletic-are artificial groups where one or more descendants of an ancestor are excluded from the group. Example: Reptilia.

Grade-an artificial taxon. Grade taxa are frequently paraphyletic and sometimes polyphyletic but are supposed to represent some level of evolutionary progress.

Clade-synonymous with monophyletic

An example of grades and clades: birds and crocodilia represent separate GRADES-birds and reptilia respectively. However, evolutionarily they are more closely related to each other than either is to modern reptiles. Therefore, modern birds, crocodilians, and extinct species of archosauria (ancestors of both modern birds and crocodilians) make up a CLADE or monophyletic group.