Some common terms in systematics

Question 1

Classification

Answer 1

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).

Question 2

Systematics

Answer 2

The field of biology that deals with the diversity of living organisms usually divided into the two subdisciplines of taxonomy and phylogenetics.

Question 3

Taxonomy

Answer 3

The science of describing, naming, and classifying (grouping) organisms.

Question 4

Taxon (plural: taxa)

Answer 4

Any named group of organisms (not necessarily a clade). A subspecies represents a “taxon,” as does every hierarchical level up to Kingdom.

Question 5

Phylogenetics

Answer 5

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.

Question 6

Phylogeny

Answer 6

The evolutionary relationships among organisms; the patterns of lineage branching produced by the evolutionary history of organisms.

Question 7

Ingroup

Answer 7

The set of taxa which are hypothesized to be more closely related to each other than any are to the outgroup(s).

Question 8

Outgroup

Answer 8

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.

Question 9

Sister Group

Answer 9

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.

Question 10

Operational Taxonomic Units (OTUs)

Answer 10

The taxa included in a phylogenetic analysis representing the terminal nodes.

Question 11

Node

Answer 11

A branch point representing lineage splitting (=cladogenesis), ultimately the result of speciation.

Question 12

Branch

Answer 12

A line, representing a lineage, connecting two internal nodes or an internal node with a terminal node (OTU).

Question 13

Internode

Answer 13

An internal branch connecting two internal nodes.

Question 14

Root

Answer 14

The internode at the bottom of a tree.

Question 15

Basal

Answer 15

Literally, towards the base. In phylogenetics, when correctly used, it refers to a character state transition or ancestral lineage towards the root (base) of a given tree. However, it is often misused as a synonym for “primitive” or to incorrectly refer to an extant (living) taxon subjectively determined to somehow be primitive or ancestral.