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出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2015/05/26 10:51:58」(JST)

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A cladogram showing the terminology used to describe different patterns of ancestral and derived character states.^[1]

In cladistics, a symplesiomorphy or symplesiomorphic character is an ancestral trait shared by two or more taxa. A plesiomorphy refers to the ancestral trait on its own, usually in reference to another, more derived trait. A symplesiomorphic trait is also shared with other taxa that have an earlier last common ancestor with the taxa under consideration. They are therefore not an indication that the taxa be considered more closely related to each other than to the more distant taxa, as all share the more primitive trait. A close phylogenetic relationship (meaning that the taxa form a certain clade to the exclusion of certain other taxa) can only be shown by the discovery of synapomorphies: shared traits that have originated with the last common ancestor of the taxa considered, or at least in the branch, not including the taxa to be excluded, leading to it.^[2]

The concept of the symplesiomorphy shows the danger of grouping species together purely on the basis of morphologic or genetic similarity without distinguishing primitive from derived traits. This phenetic method of analysis was common before cladistics became popular in the 1980s. Since a plesiomorphic character inherited from a common ancestor can appear anywhere in a phylogenetic tree, its presence cannot reveal anything about the relationships within that tree.^[3]

A famous example is the trait of breathing via gills in bony fish and cartilaginous fish. Bony fish are more closely related to terrestrial vertebrates, which evolved out of a clade of bony fishes that breathe through their skin or lungs, than they are to the sharks, rays, and the other cartilaginous fish. Their kind of gill respiration is shared by the "fishes" because it was present in their common ancestor and lost in the other living vertebrates. But based on this shared trait, we cannot infer that bony fish are more closely related to sharks and rays than they are to terrestrial vertebrates.^[4]

The term "symplesiomorphy" was first introduced in 1950 by German entomologist Willi Hennig, who is widely regarded as the father of modern cladistics.^[2]

References

^ Page, Roderic D.M. and Holmes, Edward C. Molecular evolution: a phylogenetic approach. Wiley-Blackwell, 1st edition, 1998.
^ ^a ^b Futuyma, Douglas J. (1998), Evolutionary Biology (3rd ed.), Sunderland, Massachusetts: Sinauer Associates, Inc., p. 95, ISBN 0-87893-189-9
^ Patterson, Colin (1982), "Morphological characters and homology", in Joysey, Kenneth A; Friday, A. E., Problems in Phylogenetic Reconstruction, Systematics Association Special Volume 21, London: Academic Press, ISBN 0-12-391250-4 .
^ Cracraft, Joel; Donoghue, Michael J. (2004), Assembling the Tree of Life, USA: Oxford University Press, p. 367, ISBN 0-19-517234-5

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English Journal

New Sensiava species (Copepoda: Calanoida: Diaixidae) from the deep South Atlantic and first description of the female.

Markhaseva EL.
Zootaxa.Zootaxa.2014 May 26;3802(2):217-39. doi: 10.11646/zootaxa.3802.2.5.
Females are described for the first time in the genus Sensiava Markhaseva & Schulz, 2006, that was known previously only from males. Four species of this genus are identified from samples collected in the vicinity of the abyssal sea bed. Two species, Sensiava secunda sp. nov. and S. peculiaris s
PMID 24871004

The presence of the recurrent veinlet in the Middle Jurassic Nymphidae (Neuroptera): a unique character condition in Myrmeleontoidea.

Makarkin VN1, Yang Q, Shi C, Ren D.
ZooKeys.Zookeys.2013 Aug 20;(325):1-20. doi: 10.3897/zookeys.325.5453. eCollection 2013.
A well-developed recurrent veinlet is found in the forewing of two species of Nymphidae from the Middle Jurassic locality of Daohugou (Inner Mongolia, China), Liminympha makarkini Ren & Engel and Daonymphes bisulca gen. et sp. n. This is the first record of this trait in the clade comprised of t
PMID 24003318

Divergences of MPF2-like MADS-domain proteins have an association with the evolution of the inflated calyx syndrome within Solanaceae.

Zhang J1, Khan MR, Tian Y, Li Z, Riss S, He C.
Planta.Planta.2012 Oct;236(4):1247-60. Epub 2012 Jun 19.
The inflated calyx syndrome (ICS) is a post-floral novelty within Solanaceae. Previous work has shown that MPF2-like MADS-box genes have been recruited for the development and evolution of ICS through heterotopic expression from vegetative to floral organs. ICS seems to be a plesiomorphic trait in P
PMID 22711285

Japanese Journal

Some Characters of the Lower Brachycera (Diptera) and Their Plesiomorphy and Apomorphy

NAGATOMI Akira
昆蟲 49(3), 397-407, 1981-09-25
… The characters of the face, antenna, palpus, metapleural hairs, halter, wing venation, and tibial spurs, in the lower Brachycera are categorized, presented as to their distribution in the genera and families, and discussed as to their plesiomorphy or apomorphy. …
NAID 110003377643