Symplesiomorphy
Encyclopedia
In cladistics
, a symplesiomorphy or symplesiomorphic character is a trait which is shared (a symmorphy) between two or more taxa
, but which is also shared with other taxa which 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, 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.
The concept of the symplesiomorphy shows the danger of grouping species together on the basis of general morphologic
or genetic
similarity, without distinguishing between resemblances caused by either primitive or derived traits. This phenetic method of analysis was common before cladistics became popular in the 1980s.
A famous example is pharyngeal
gill
breathing in bony and cartilaginous fishes. The former are more closely related to Tetrapoda (terrestrial vertebrate
s, which evolved out of a clade of bony fishes) that breathe via their skin or lungs, rather than to the shark
s, rays
, et al.. Their kind of gill respiration is shared by the "fishes" because it was present in their common ancestor and lost in the other living vertebrate
s.
Cladistics
Cladistics is a method of classifying species of organisms into groups called clades, which consist of an ancestor organism and all its descendants . For example, birds, dinosaurs, crocodiles, and all descendants of their most recent common ancestor form a clade...
, a symplesiomorphy or symplesiomorphic character is a trait which is shared (a symmorphy) between two or more taxa
Taxon
|thumb|270px|[[African elephants]] form a widely-accepted taxon, the [[genus]] LoxodontaA taxon is a group of organisms, which a taxonomist adjudges to be a unit. Usually a taxon is given a name and a rank, although neither is a requirement...
, but which is also shared with other taxa which 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, that the taxa form a certain clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
to the exclusion of certain other taxa, can only be shown by the discovery of synapomorphies
Synapomorphy
In cladistics, a synapomorphy or synapomorphic character is a trait that is shared by two or more taxa and their most recent common ancestor, whose ancestor in turn does not possess the trait. A synapomorphy is thus an apomorphy visible in multiple taxa, where the trait in question originates in...
: 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.
The concept of the symplesiomorphy shows the danger of grouping species together on the basis of general morphologic
Morphology (biology)
In biology, morphology is a branch of bioscience dealing with the study of the form and structure of organisms and their specific structural features....
or genetic
Genetics
Genetics , a discipline of biology, is the science of genes, heredity, and variation in living organisms....
similarity, without distinguishing between resemblances caused by either primitive or derived traits. This phenetic method of analysis was common before cladistics became popular in the 1980s.
A famous example is pharyngeal
Pharynx
The human pharynx is the part of the throat situated immediately posterior to the mouth and nasal cavity, and anterior to the esophagus and larynx. The human pharynx is conventionally divided into three sections: the nasopharynx , the oropharynx , and the laryngopharynx...
gill
Gill
A gill is a respiratory organ found in many aquatic organisms that extracts dissolved oxygen from water, afterward excreting carbon dioxide. The gills of some species such as hermit crabs have adapted to allow respiration on land provided they are kept moist...
breathing in bony and cartilaginous fishes. The former are more closely related to Tetrapoda (terrestrial vertebrate
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...
s, which evolved out of a clade of bony fishes) that breathe via their skin or lungs, rather than to the shark
Shark
Sharks are a type of fish with a full cartilaginous skeleton and a highly streamlined body. The earliest known sharks date from more than 420 million years ago....
s, rays
Batoidea
Batoidea is a superorder of cartilaginous fish commonly known as rays and skates, containing more than 500 described species in thirteen families...
, et al.. Their kind of gill respiration is shared by the "fishes" because it was present in their common ancestor and lost in the other living vertebrate
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...
s.