Autapomorphy
Encyclopedia
In cladistics
, an autapomorphy is a distinctive anatomical feature, known as a derived
trait, that is unique to a given terminal group. That is, it is found only in one member of a clade
, but not found in any others or outgroup
taxa, not even those most closely related to the group (which may be a species
, family
or in general any clade). It can therefore be considered an apomorphy in relation to a single taxon. The word "autapomorphy" is derived from the Greek
words αὐτός, aut = self; ἀπό, apo = away from; and μορφή, morphe = shape.
An autapomorphy is not present in the closest relative of the terminal group and also was not present in their common ancestor. An example of an autapomorphy can be described in modern snakes. Snakes have lost the two pairs of legs that characterize all of Tetrapoda, and the closest taxa to Ophidia - as well as their common ancestors - all have two pairs of legs. Therefore, the Ophidia taxon presents an autapomorphy with respect to its absence of legs. The words autapomorphy and synapomorphy technically describe the same derived character-state, but they differ in context: Synapomorphy describes the trait as it typifies the taxon and contrasts it with a different condition in outgroups, while autapomorphy should be correctly used when the component taxa are not mentioned. An autapomorphy at a given taxonomic level may therefore also be a synapomorphy at a less-inclusive level.
The autapomorphic species concept is one of many methods that scientists might use to define and distinguish species
from one another. This definition assigns species on the basis of amount of divergence associated with reproductive incompatibility, which is measured essentially by number of autapomorphies. This grouping method is often referred to as the "monophyletic species concept" or the "phylospecies" concept and was popularized by D.E. Rosen in 1979. Within this definition, a species is seen as "the least inclusive monophyletic group definable by at least one autapomorphy." While this model of speciation is useful in that it avoids non-monophyletic groupings, it has its criticisms as well. N.I. Platnick, for example, believes the autapomorphic species concept to be inadequate because it allows for the possibility of reproductive isolation and speciation while revoking the "species" status of the mother population. In other words, if a peripheral population breaks away and becomes reproductively isolated, it would conceivably need to develop at least one autapomorphy to be recognized as a different species. If this can happen without the larger mother population also developing a new autapomorphy, then the mother population cannot remain a species under the autapomorphic species concept: it would no longer have any apomorphies not also shared by the daughter species.
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...
, an autapomorphy is a distinctive anatomical feature, known as a derived
Derived
In phylogenetics, a derived trait is a trait that is present in an organism, but was absent in the last common ancestor of the group being considered. This may also refer to structures that are not present in an organism, but were present in its ancestors, i.e. traits that have undergone secondary...
trait, that is unique to a given terminal group. That is, it is found only in one member of a 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...
, but not found in any others or outgroup
Outgroup
In cladistics or phylogenetics, an outgroup is a group of organisms that serves as a reference group for determination of the evolutionary relationship among three or more monophyletic groups of organisms....
taxa, not even those most closely related to the group (which may be a species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
, family
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...
or in general any clade). It can therefore be considered an apomorphy in relation to a single taxon. The word "autapomorphy" is derived from the Greek
Greek language
Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...
words αὐτός, aut = self; ἀπό, apo = away from; and μορφή, morphe = shape.
An autapomorphy is not present in the closest relative of the terminal group and also was not present in their common ancestor. An example of an autapomorphy can be described in modern snakes. Snakes have lost the two pairs of legs that characterize all of Tetrapoda, and the closest taxa to Ophidia - as well as their common ancestors - all have two pairs of legs. Therefore, the Ophidia taxon presents an autapomorphy with respect to its absence of legs. The words autapomorphy and synapomorphy technically describe the same derived character-state, but they differ in context: Synapomorphy describes the trait as it typifies the taxon and contrasts it with a different condition in outgroups, while autapomorphy should be correctly used when the component taxa are not mentioned. An autapomorphy at a given taxonomic level may therefore also be a synapomorphy at a less-inclusive level.
The autapomorphic species concept is one of many methods that scientists might use to define and distinguish species
Species problem
The species problem or species concept is a mixture of difficult, related questions that often come up when biologists identify species and when they define the word "species"....
from one another. This definition assigns species on the basis of amount of divergence associated with reproductive incompatibility, which is measured essentially by number of autapomorphies. This grouping method is often referred to as the "monophyletic species concept" or the "phylospecies" concept and was popularized by D.E. Rosen in 1979. Within this definition, a species is seen as "the least inclusive monophyletic group definable by at least one autapomorphy." While this model of speciation is useful in that it avoids non-monophyletic groupings, it has its criticisms as well. N.I. Platnick, for example, believes the autapomorphic species concept to be inadequate because it allows for the possibility of reproductive isolation and speciation while revoking the "species" status of the mother population. In other words, if a peripheral population breaks away and becomes reproductively isolated, it would conceivably need to develop at least one autapomorphy to be recognized as a different species. If this can happen without the larger mother population also developing a new autapomorphy, then the mother population cannot remain a species under the autapomorphic species concept: it would no longer have any apomorphies not also shared by the daughter species.
See also
- Homoplasy – a trait that is found in several terminal groups but evolved independently (i.e., was not present in their common ancestor)
- SynapomorphySynapomorphyIn 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...
– a trait that is found in some or all terminal groups of a clade, and inherited from a common ancestor, for which it was an autapomorphy (i.e., not present in its immediate ancestor).- Underlying synapomorphy – a synapomorphy that has been lost again in many members of the clade. If lost in all but one, it can be hard to distinguish from an autapomorphy.
- Apomorphy – a synapomorphy discussed in reference to a less derived ancestral state.
- SymplesiomorphySymplesiomorphyIn cladistics, a symplesiomorphy or symplesiomorphic character is a trait which is shared 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...
– an ancestral trait shared by two or more taxa.- Plesiomorphy – a symplesiomorphy discussed in reference to a more derived state.