Palaeos Systematics: Systematics Glossary

analogous, analogy
opposite to homologous: correspondence in function between anatomical parts of different structure and origin; analogous: exhibit analogy (synonym: similar)

autapomorphy
derived trait or apomorphy that is unique to only one group

binomial nomenclature
System of naming types of organisms developed by the 18th century Swedish naturalist Carl von Linne. Every organism has a generic or "family" name (written with a capital) and a trivial or species name (always in low case, and following the generic name). For example, Homo sapiens, Homo is the generic name (or genus), sapiens the species name.

character state
the state or value of a character; for example some character states for the character 'color' are red, green and blue

clade
a group of organisms that share a common ancestor; lineage; a monophyletic group

cladist
one who uses the phylogenetic systematic (cladistic) methodology as a way of determining the relationship between organisms whether living or extinct

cladogram
a dichotomous phylogenetic tree that branches repeatedly, suggesting a classification of organisms based on the sequence in which evolutionary branches arise; a nested diagram of synapomorphies indicating relations between groups; each point of branching represents divergence from a common ancestor
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classification
Associating organisms together into various groupings which reflect, as best we know, the relationships between them. The Linnean hierarchy organizes such groups or taxa from the most general grouping (kingdom) to the most specific (species).

convergent evolution, convergence
the independent development of similar (analogous) structures in different groups; convergent evolution is thought to be the result of similar environmental selection pressures on different groups

derived trait
same as apomorphy; a derived character / trait is inferred to be a modified version of a more primitive condition of that character and therefore inferred to have arisen later in the evolution of the clade

evolutionary grade
same as paraphyletic group showing similarities in morphology, ecology or life history
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evolutionary systematics
same as synthetic systematics; a way to determine natural relationships of organisms by studying a group in detail and comparing degree of similarity; evolutionary systematics does not have an explicit methodology, but rather relies on the expertise of authorities very familiar with the group in question; compare to phenetics or numerical taxonomy and phylogenetic systematics or cladistics
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form taxon
binomial name (genus and species) given to a fossilized plant organ when it is found in isolation, i.e. when the taxonomic affinities of the organ are not known with certainty; for example spore and pollen taxa have their own binomial names, since it is rarely known which fossil genus may have produced them

homologous, homology
likeness and correspondence in structure between parts of different organisms, due to common ancestry of the organisms; a similar character state present in two or more species of organisms that is derived from a single character in their common ancestor.
(compare to analogy)

identification
To identify an organism is to determine which taxon it belongs to. An “accurate” identification is not only correct, but will identify an organism with a particular species. However, it is not at all unusual, in practice, that an identification can only be made to genus or even higher level. There are many possible reasons. Perhaps the organism being identified is incomplete; some part (e.g. a flower) which is necessary for a completely accurate identification is not present. This problem is particularly acute when it comes to identifying fossils, which are more commonly fragmentary than not. In some cases, the species may not have been previously recognized, or even if recognized, not described. In such cases, a relationship to a similar species which has been described might be indicated with an aff. indicating “affinity to,” or the less confident cf. meaning “compare with.”

index fossil
a geographically widespread fossil, that is diagnostic of a particular time period and therefore useful in correlating the age of rock formations from different geographic areas

Linnean system, Linnean hierarchy
System of binomial nomenclature and hierarchical classification developed by the 18th century Swedish naturalist Carl von Linné. Although still in wide use today, it has fallen out of favour among cladists.
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monophyly, monophyletic group
terms applied to a group of organisms that includes an ancestral species and all of its descendants; e.g. Aves, Mammalia. This group is a complete branch of the tree of life, the phylogeny of life. Such a branch is called a clade.
Compare to paraphyletic and polyphyletic
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most recent common ancestor
in cladistics, the presumed organism at the base of a clade, from which that clade arose, and which was the first to evolve the synapomorphies that define that clade.

numerical taxonomy
same as phenetics; a method of generating phylogenies that is based on large numbers of quantifiable (measurable) characters which groups organisms with respect to overall similarity

outgroup
one or, preferably, several taxa that lie outside the group of interest, that is the group in which we are trying to detect relationships. For example, if we are interested in determining the relationships of minnows, we would use the closest relatives of minnows as our outgroups. Outgroup comparison is the way we determine how widespread a particular feature may be, whether it is found only within the group of interest, or beyond that group. The sister taxon to the ingroup.

paraphyly, paraphyletic group
terms applied to a group of organisms that include an ancestor and some, but not all of its descendants; a group lacking some of the descendant species. The old "Reptilia" and "fishes."
compare to monophyletic and polyphyletic
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parsimony
In cladistics, where there are a number of alternative ways of portraying phylogenetic relationships, the simplest or most parsimonious arrangement is considered the one that best represents the evolutionary branching of that group

phylogeny
the evolutionary relationships among organisms; the "family tree" of organisms as they evolve through geological time, the pattern of lineage branching produced by the evolutionary history of the organisms considered

plesiomorphy, plesiomorphic trait
An ancestral or primitive character. Also called a primitive trait
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polyotomy
multiple branches stemming from the same node, resulting from uncertainty in or inability to determine the relationship between three or more terminal taxa.

primitive trait
same as plesiomorphy; a character that is present in the common ancestor of a clade; a primitive trait is inferred to be the original character state of that character within the clade under consideration; compare to derived trait

relationship
defined in terms of genealogy--common ancestral species. Two species are more closely related to one another than to a third species if they share a more recent common ancestor that either does with the third. These two taxa are considered as sister groups.

sister group, sister taxa
In cladistics two taxa or clades which meet at a node; or in other words two lineages which diverged from that particular common ancestor. Because sister groups share a common ancestor, they are each other's closest relatives.

strict consensus
a method for choosing among several most parsimonious trees generated by a phylogenetic analysis; strict consensus means that only clades that show up in all the most parsimonious trees are recognized

symplesiomorphy
In cladistics a character shared by a number of groups, but inherited from ancestors older than the last common ancestor; a shared primitive trait (plesiomorphy)
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synapomorphy
In cladistics a derived or specialized character (apomorphy) shared by two or more groups which originated in their last common ancestor. Compare to autapomorphy
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"systematic"
Biological or paleontological reviews which organize their material in a taxonomic or phylogenetic manner

systematics
field of biology that deals with the grouping and organizing of organisms.

taxon, taxa (pl.)
a group or category of living organisms; the published groups within each of the divisions in the Linnean hierarchy. Thus, Animalia, Chordata, Vertebrata, Carnivora, etc. are each taxa, as is the great Siberian Tiger, Felis tigris, figuratively lying in the shade, at the bottom of the tree.

taxonomy
the naming of organisms and groups of organisms (literally, the naming of taxa); the systematic classification of living, or once living, things, which is encouraged to be consistent with their presumed evolutionary relationships (see phylogeny.

terminal taxa
the taxa along the top line of a cladogram (does not mean in terminal in time).

page organized by M. Alan Kazlev
content by xxx , Chris Clowes, Thomas R. Holtz, Jr. (Lecture 15: Phylogeny, the Tree of Life) and M. Alan Kazlev
page uploaded 28 April 2002, last modified 26 May
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