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analogy
Similarity between two species that is due to convergent evolution rather than to descent from a common ancestor with the same trait.
basal taxon
In a specified group of organisms, a taxon whose evolutionary lineage diverged early in the history of the group.
binomial
A common term for the two-part, latinized format for naming a species, consisting of the genus and specific epithet; also called a binomen.
branch point
The representation on a phylogenetic tree of the divergence of two or more taxa from a common ancestor. A branch point is usually shown as a dichotomy in which a branch representing the ancestral lineage splits (at the branch point) into two branches, one for each of the two descendant lineages.
clade
A group of species that includes an ancestral species and all of its descendants.
cladistics
An approach to systematics in which organisms are placed into groups called clades based primarily on common descent.
classes
In Linnaean classification, the taxonomic category above the level of order
domain
A taxonomic category above the kingdom level. The three domains are Archaea, Bacteria, and Eukarya. (2) A discrete structural and functional region of a protein.
evolutionary lineage
The sequence of ancestral organisms leading to a particular taxon; represented by a branch (line) in a phylogenetic tree.
family
In Linnaean classification, the taxonomic category above genus.
genus
A taxonomic category above the species level, designated by the first word of a species' two-part scientific name.
horizontal gene transfer
The transfer of genes from one genome to another through mechanisms such as transposable elements, plasmid exchange, viral activity, and perhaps fusions of different organisms.
ingroup
A species or group of species whose evolutionary relationships are being examined in a given analysis.
kingdom
A taxonomic category, the second broadest after domain.
maximum parsimony
A principle that states that when considering multiple explanations for an observation, one should first investigate the simplest explanation that is consistent with the facts.
molecular clock
A method for estimating the time required for a given amount of evolutionary change, based on the observation that some regions of genomes evolve at constant rates.
monophyletic
Pertaining to a group of taxa that consists of a common ancestor and all of its descendants. A monophyletic taxon is equivalent to a clade
order
In Linnaean classification, the taxonomic category above the level of family.
outgroup
A species or group of species from an evolutionary lineage that is known to have diverged before the lineage that contains the group of species being studied. An outgroup is selected so that its members are closely related to the group of species being studied, but not as closely related as any study-group members are to each other.
paraphyletic
Pertaining to a group of taxa that consists of a common ancestor and some, but not all, of its descendants.
phyla
In Linnaean classification, the taxonomic category above class.
phylogenetic tree
A branching diagram that represents a hypothesis about the evolutionary history of a group of organisms.
phylogeny
The evolutionary history of a species or group of related species.
polyphyletic
Pertaining to a group of taxa that includes distantly related organisms but does not include their most recent common ancestor.
rooted
Describing a phylogenetic tree that contains a branch point (often, the one farthest to the left) representing the most recent common ancestor of all taxa in the tree.
shared ancestral character
A character that is shared by members of a particular clade but that originated in an ancestor that is not a member of that clade.
shared derived character
An evolutionary novelty that is unique to a particular clade.
sister taxa
Groups of organisms that share an immediate common ancestor and hence are each other's closest relatives.
systematics
A scientific discipline focused on classifying organisms and determining their evolutionary relationships.
taxon
A named taxonomic unit at any given level of classification.
taxonomy
A scientific discipline concerned with naming and classifying the diverse forms of life.
