As plesiomorphy (from ancient Greek πλησίος plēsíos "adjacent" and μορφή morph "shape, form") is referred to in the developmental system and Kladistik an original feature quantity that before considered lineage arose and remained unchanged in this. The opposite of plesiomorphism is a newly acquired trait, an apomorphism . The classification as “original” or “newly acquired” is relative, as it always depends on the two lines ( taxa ) considered .
For example, the quadruped feature in the development of fossil reptiles from fossil amphibians is a plesiomorphism, since all subgroups of terrestrial vertebrates originally had four limbs. In comparison of the terrestrial vertebrates with the fossil flesh- flippers, on the other hand, the quadruped is an apomorphy , since the characteristic was newly acquired compared to the parent group belonging to the bony fish . The later regression of the extremities in snakes is also an apomorphism, because the fossil reptiles originally had four extremities.
If two different stem lines (taxa) have the same origin ( homologous ) plesiomorphism, this is called a symplesiomorphism (from the Greek σύν syn "together, together"), i.e. a match in the original characteristics.
Since the ancestor (s) of a symplesiomorphism can also have further descendants in addition to two considered, symplesiomorphies between groups alone do not establish monophyly , since this must contain all descendants. Are individual subgroups excluded, such as B. in the past the birds of the actual reptiles, these only form a paraphyletic group.
For example, the trait quadruped is not suitable to justify the taxon reptiles because, on the one hand, the snakes belonging to the reptiles have lost this original trait and, on the other hand, the taxa amphibians and mammals also have the trait quadruped, but are not classified as reptiles. Quadruped is related to reptiles a symplesiomorphism, the characteristic is evolutionarily much older than the reptiles. Incidentally, the reptile taxon in its traditional composition does not represent a monophyletic group because the bird taxon is not included, although the birds are only a subgroup of the feathered predatory dinosaurs.
- Bernhard Wiesemüller, Hartmut Rothe , Winfried Henke : Phylogenetic systematics. An introduction. Springer, Berlin et al. 2003, ISBN 3-540-43643-X , Plesmiomorphie und Apomorphie; Google Books .