Plesiomorphy and symplesiomorphy
In phylogenetics, a plesiomorphy (“near form”), is a primitive or ancestral character state and can therefore be called a plesiomorphic character. A symplesiomorphy ( from syn- “together”) or symplesiomorphic character is a shared ancestral character (a shared plesiomorphy), shared by two or more taxa - but also with other taxa linked earlier in the clade. The term symplesiomorphy was first introduced in 1950 by German entomologist Willi Hennig.
A plesiomorphy or a plesiomorphic character can be seen as the opposite of an apomorphy, which is a derived character, an “evolutionairy novelty”. Both of these terms are by definition relative, because a trait character can be a plesiomorphy in one context, but an apomorphy in another. Figure 2 shows this more clearly and this will be explained in the next alinea. Synapomorphy, a shared derived character, is the contrast of a symplesiomorphy (a shared ancestral character). In figure 2, an imaginary cladogram with 4 living birdspecies is shown. The yellow mask is a derived character for the four living bird species, and therefore an apomorphy. But the mask is a plesiomorphic character in the context of the three masked species, because for these species it is an ancestral character. A symplesiomorphy is a shared plesiomorphy, so for these yellow tailed birds the yellow tail is plesiomorphy as an ancestral trait, but this character is shared by all yellow tailed birds, so it is also a symplesiomorphy. Another example of a plesiomorphy and symplesiomorphy is in primates. Primates have more recently evolved from mammals. Hair is an ancestral character (plesiomorphy) for primates. But because hair, a plesiomorphic character, is present in all primates, it is a symplesiomorphy for them in general as well. Pseudoplesiomorphy – is a trait that cannot be identified as a plesiomorphy nor as an apomorphy.
The concept of plesiomorphy addresses 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.. Apomorphic characters and synapomorphic characters carry a lot of information about the evolutionary clades and can be used to define taxa. However the use of only plesiomorphic characters and symplesiomorphic characters can lead to incorrect conclusions about the evolutionary lines. The question of taxa are closely related to one another should be answered with more evidence. To illustrate this two examples are given:
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 sharks, rays, and 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. The shared trait cannot treated as evidence that bony fish are more closely related to sharks and rays than they are to terrestrial vertebrates.
Another example to show that the symplesiomorphic characters do not mean that the taxa harbouring this character are necessarily closely related is the one of Reptilia. Reptilia are ectothermic (coldblooded) and birds are endothermic (warmblooded). This is a plesiomorphic character inherited from a common ancestor for both birds and reptiles. Being coldblooded is thus a symplesiomorphic character for lizards, turtles an crocodiles. However this symplesiomorphy does not mean that lizards, turtles and crocodiles form a group that is more closely related and excludes the endothermic birds.
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