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An ancestral character or trait state shared by two or more taxa
Figure 1: Phylogenies showing the terminology used to describe different patterns of ancestral and derived trait states.
In phylogenetics, a plesiomorphy ("near form") is an ancestral character state. A symplesiomorphy ( from syn- "together") is a plesiomorphy shared by two or more taxa (including taxa earlier in the clade). Pseudoplesiomorphy is any trait that can neither be identified as a plesiomorphy nor as an apomorphy.
Alternative terms are plesiomorphic character and symplesiomorphic character respectively, or plesiomorphic trait and symplesiomorphic trait respectively.
Figure 2: Imaginary cladogram. The yellow mask is a plesiomorphy for each living masked species, because it is ancestral. It is also a symplesiomorphy for them. But for the four living species as a whole, it is a apomorphy because it is not ancestral for all of them. The yellow tail is a plesiomorphy and symplesiomorphy for all living species.
Plesiomorphy and symplesiomorphy correspond to apomorphy and synapomorphy which are derived trait ("evolutionary novelty") and a shared derived trait.
All of these terms are by definition relative in that a trait can be a plesiomorphy in one context and an apomorphy in another (see example in Figure 2).
Apomorphic and synapomorphic characteristics convey much information about the evolutionary clades and can be used to define taxa. However, plesiomorphic and symplesiomorphic characteristics cannot. Species should not be grouped purely by morphologic or genetic similarity. Because a plesiomorphic character inherited from a common ancestor can appear anywhere in a phylogenetic tree, its presence does not reveal anything about the relationships within the tree. Thus grouping species requires distinguishing ancestral from derived character states.
Another example is thermo-regulation in reptilia and birds. Reptilia are ectothermic (coldblooded) and birds are endothermic (warmblooded). This is plesiomorphic for birds and plesiomorphic for reptiles. Being coldblooded is symplesiomorphic for lizards, turtles and crocodiles, but they do not form a clade without birds, as crocodiles are more related to birds than to lizards and turtles.
^Patterson, Colin (1982), "Morphological characters and homology", in Joysey, Kenneth A; Friday, A. E. (eds.), Problems in Phylogenetic Reconstruction, Systematics Association Special Volume 21, London: Academic Press, ISBN0-12-391250-4.
^Futuyma, Douglas J. (1998), Evolutionary Biology (3rd ed.), Sunderland, Massachusetts: Sinauer Associates, Inc., p. 95, ISBN0-87893-189-9
^Cracraft, Joel; Donoghue, Michael J. (2004), Assembling the Tree of Life, USA: Oxford University Press, p. 367, ISBN0-19-517234-5
^Archibald, J. David (2014-08-19). Aristotle's ladder, Darwin's tree : the evolution of visual metaphors for biological order. New York. ISBN9780231537667. OCLC884645828.