Replicator — anything in the universe of which copies are made (Dawkins). Qualities of good replicators are longevity, fecundity and fidelity.

Interactors — an entity that directly interacts as a cohesive whole with its environment in such a way that replication is differential (Hull). Dawkins prefers to call these "vehicles": any relatively discrete entity, such as an individual organism, which houses replicators and which can be regarded as a machine programmed to preserve and propagate the replicators that ride inside it.

Mendelian Gene — A stretch of chromosome that is associated with phenotypic difference.

Molecular Gene — A stretch of DNA that codes for a single polypeptide chain (classical); a DNA sequences that stores the information which specifies the order of monomers in a final functional polypeptide or RNA molecule, or a set of closely related isoforms.

Evolutionary Gene — Any stretch of DNA that seqregates and recombines with appreciable frequency and can be replaced by an alternative sequence in future generations. This has largely been succeeded by: any heritable change in an input to development that can cause a difference in phenotypic adaptation. Alternatively, a heritable potential for a phenotypic trait.

Ontogeny - The origin and development of the individual organism.

Lineage — a single line of direct ancestry and descent.

Tree — A representation of the branching pattern of relationships among a set of biological entitites that are related by descent from a common ancestor.

Clade — A part of a tree consisting of an ancestral species and all of its descendants.

Clone — An asexual counterpart, applied to organelles as well as whole organisms.

Taxon — Any named group of organisms. Plural: taxa. In phylogenetic studies some workers prefer the term OTU (operational taxonomic unit) for the units under study.

Sister taxa —Taxa that are each other’s closest relatives.

Cladistics — Methodology for the construction of phylogenetic hypotheses represented in trees using only synapomorphies.

Apomorphy — Derived character; a trait that differs from its state in an ancestor.

Synapomorphy — Derived character shared by two or more taxa.

Plesiomorphy — Character that retains the same state as in ancestors at least two generations in the past. Also sometimes called a primitive character or an ancestral trait.

Monophyletic Group — A clade. An ancestor and all its descendants. Founded on synapomorphies.

Paraphyletic Group — A grade. An ancestor and some of its descendants. Founded on symplesiomorphis, or shared ancestral traits.

Polyphyletic Group — Often a grade. A group that includes lineages that are each more closely related to lineages not included in the group.

Convergence — Independently derived similarity.

Polarity — The direction of character change, from one state of a character or trait, to another. For example, from ancestral to derived.