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出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2016/07/04 09:33:34」(JST)

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Phylogenies showing the terminology used to describe different patterns of ancestral and derived character or trait states.^[1]

In phylogenetics, a plesiomorphy, symplesiomorphy or symplesiomorphic character is an ancestral character or trait state shared by two or more taxa. A plesiomorphy refers to the ancestral trait state, usually in reference to a derived trait state. 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 ancestral character state.^[2] The term symplesiomorphy was first introduced in 1950 by German entomologist Willi Hennig.

Discussion

The concept of plesiomorphy shows the perils of grouping species together purely on the basis of morphologic or genetic similarity without distinguishing ancestral from derived character states. 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]

References

^ Roderick D.M. Page; Edward C. Holmes (14 July 2009). Molecular Evolution: A Phylogenetic Approach. John Wiley & Sons. ISBN 978-1-4443-1336-9.
^ 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

Cytogenetics, cytotaxonomy and chromosomal evolution of Chrysomelinae revisited (Coleoptera, Chrysomelidae).

Petitpierre E.
ZooKeys.Zookeys.2011;(157):67-79. doi: 10.3897/zookeys.157.1339. Epub 2011 Dec 21.
Nearly 260 taxa and chromosomal races of subfamily Chrysomelinae have been chromosomally analyzed showing a wide range of diploid numbers from 2n = 12 to 2n = 50, and four types of male sex-chromosome systems. with the parachute-like ones Xy(p) and XY(p) clearly prevailing (79.0%), but with the XO w
PMID 22303104

Conservation of the function counts: homologous neurons express sequence-related neuropeptides that originate from different genes.

Neupert S1, Huetteroth W, Schachtner J, Predel R.
Journal of neurochemistry.J Neurochem.2009 Nov;111(3):757-65. doi: 10.1111/j.1471-4159.2009.06361.x. Epub 2009 Aug 27.
By means of single-cell matrix assisted laser desorption/ionization time-of-flight mass spectrometry, we analysed neuropeptide expression in all FXPRLamide/pheromone biosynthesis activating neuropeptide synthesizing neurons of the adult tobacco hawk moth, Manduca sexta. Mass spectra clearly suggest
PMID 19712058

Deciduous teeth of the Neandertal mandible from Molare Shelter, near Scario (Salerno, Italy).

Mallegni F1, Ronchitelli AT.
American journal of physical anthropology.Am J Phys Anthropol.1989 Aug;79(4):475-82.
Excavations at hearth levels at the Molare Shelter in 1985 yielded the mandible of a 3-4-year-old child. The associated lithic artifacts recovered with it are Mousterian. Its features, although quite archaic, are within the known range of variation of Neandertals. Four deciduous molars are still pre
PMID 2672830