Some common terms in systematics
Biol. 425
CLASSIFICATION: The process and practice of describing, defining and ranking taxa within a hierarchical
series of groups; permits organizing and ordering knowledge about organisms, and creates a simplified
language for conveying information about order in the diversity of life. In biology, we use the Linnean
classification hierarchy (Kingdom, Phylum, Class, Order, Family, Genus, Species).
SYSTEMATICS: The field of biology that deals with the diversity of living organisms, usually divided into
the two subdisciplines of taxonomy and phylogenetics.
TAXONOMY: The science of describing, naming, and classifying (grouping) organisms.
TAXON (PLURAL: TAXA): Any named group of organisms (not necessarily a clade). A subspecies
represents a "taxon," as does every hierarchical level up to Kingdom.
PHYLOGENETICS: The field of biology that deals with the relationships between organisms, including the
discovery of these relationships and the study of the causes behind these patterns.
PHYLOGENY: The evolutionary relationships among organisms; the patterns of lineage branching produced
by the evolutionary history of organisms.
INGROUP: The set of taxa which are hypothesized to be more closely related to each other than any are to the
outgroup(s).
OUTGROUP: Any taxon (or taxa) hypothesized to be less closely related to each of the taxa under
consideration (the ingroup) than any members of the ingroup are to each other. Outgroups are typically used to
"root" trees and to infer the direction (polarity) of character change.
SISTER GROUP: The closest outgroup (living or extinct) to a given ingroup. "Sister taxa" refers to the two
lineages or clades resulting from the splitting of a single lineage.
OPERATIONAL TAXONOMIC UNITS (OTUS): The taxa included in a phylogenetic analysis representing
the terminal nodes.
NODE: A branch point representing lineage splitting (=cladogenesis), ultimately the result of speciation.
BRANCH: A line, representing a lineage, connecting two internal nodes or an internal node with a terminal
node (OTU).
INTERNODE: An internal branch connecting two internal nodes.
ROOT: The internode at the bottom of a tree.
BASAL GROUP: The earliest diverging group within a clade. However, as both lineages are basal to each
other, this term is often used when referring to one or the other lineage/clade. E.g., gorillas are basal to chimps,
bonobos, and humans.
DENDROGRAM: A branching diagram with nodes and branches (a "tree"). Does not have to depict
organismal relationships or even be limited to biological depictions.
PHYLOGENETIC TREE or CLADOGRAM: A dendrogram representing the genealogic relationships
between taxa with ancestry and relative time implied.
PHYLOGRAM: A phylogenetic tree with branch lengths drawn to represent the degree of genetic divergence.
EVOLUTIONARY TREE: A phylogenetic tree that includes a time axis and (generally) all members, living
and extinct.
MONOPHYLETIC GROUP (MONOPHYLY): A group of organisms/taxa that includes ALL known
descendants of that group's common ancestor (including the common ancestor). Monophyletic groups (clades)
are grouped by shared derived characters (synapomorphies) not present in earlier ancestors or other groups.
Monophyletic groups are also called "natural" groups. Mammalia is an example of a monophyletic group.
CLADE: A monophyletic or "natural" group.
SUBCLADE: A clade nested within a larger, more inclusive clade.
PARAPHYLETIC GROUP (PARAPHYLY): An artificial group of organisms/taxa that includes SOME, but
NOT ALL, of the descendants of a common ancestor. Paraphyletic groups are typically recognized by the
sharing of primitive (plesiomorphic) characters. Reptiles are an example of a paraphyletic group. A "grade" is
also, by definition, paraphyletic.
GRADE: An artificial (non-monophyletic) taxon, frequently paraphyletic and sometimes polyphyletic. Grades
are supposed to represent some level of evolutionary progress.
POLYPHYLETIC GROUP (POLYPHYLY): An artificial group of organisms/taxa derived from two or
more independent, distinct ancestral taxa. The common ancestor, and many closely related taxa, are excluded
from the group. Polyphyletic groups are generally recognized by superficial similarities resulting from
convergence that do not reflect close evolutionary relationships. An example is "Homeothermia," the taxon
once proposed to include birds and mammals (which are both homeothermic). Polyphyly is a special case of
paraphyly.
TREE THINKING: A conceptual framework for considering and exploring biological and evolutionary
patterns and processes that incorporates hypotheses of descent from common ancestors.