Synapomorphy

Synapomorphy

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In cladistics
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 synapomorphy or synapomorphic character is a trait that is shared ("symmorphy") by 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...

 and their most recent common ancestor
Most recent common ancestor
In genetics, the most recent common ancestor of any set of organisms is the most recent individual from which all organisms in the group are directly descended...

, 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 their last common ancestor. The word "synapomorphy" 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 , syn = with, in company with, together with; , apo = away from; and , morphe = shape.

True synapomorphies usually uniquely characterise a given set of terminal groups, but this is not essential to the concept. Thus, if some descendants of a last common ancestor possess a synapomorphic trait, it is not strictly necessary that all of its descendants must possess the same trait.

Comparisons with other shared traits


A synapomorphy should not be confused with other types of shared traits:
  • A synapomorphy is a shared trait found among two or more taxa and their most recent common ancestor, whose ancestor in turn does not possess the trait. An example is the dipteran
    Fly
    True flies are insects of the order Diptera . They possess a pair of wings on the mesothorax and a pair of halteres, derived from the hind wings, on the metathorax...

     halteres
    Halteres
    Halteres are small knobbed structures modified from the hind wings in some two-winged insects. They are flapped rapidly and function as gyroscopes, informing the insect about rotation of the body during flight....

    , the uniquely modified hind wings found in all families of winged flies. No other group of insect
    Insect
    Insects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...

    s possesses similar structures (in place of hind wings—insects in the order Strepsiptera
    Strepsiptera
    The Strepsiptera are an order of insects with ten families making up about 600 species...

     have convergently-evolved
    Convergent evolution
    Convergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action. Although their last common ancestor did not have wings, both birds and bats do, and are capable of powered flight. The wings are...

     halteres in place of fore wings). However, the fact that the trait is found exclusively in Diptera, to the exclusion of all other groups, is not essential in identifying the trait as a synapomorphy; rather, this fact makes its determination easier. A synapomorphy is a great way to help distinguish between 2 parsimonious phylogenetic trees.

  • A symplesiomorphy
    Symplesiomorphy
    In 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...

     is a shared trait found among two or more taxa, but which is also found in taxa with an earlier common ancestor. An example of this is the five toes seen on the hind legs of rats and apes. This character-state originated very early in Tetrapod
    Tetrapod
    Tetrapods are vertebrate animals having four limbs. Amphibians, reptiles, birds and mammals are all tetrapods; even snakes and other limbless reptiles and amphibians are tetrapods by descent. The earliest tetrapods evolved from the lobe-finned fishes in the Devonian...

    a and occurs in other tetrapod groups, e.g. in lizards. There is thus no indication that the group formed of rats and apes is a clade to the exclusion of these other groups.

  • A homoplasy
    Convergent evolution
    Convergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action. Although their last common ancestor did not have wings, both birds and bats do, and are capable of powered flight. The wings are...

     is a shared trait found among different taxa but not in their common ancestor (i.e., the same trait emerged in different taxa independently of each other). An example of this is homeothermy
    Warm-blooded
    The term warm-blooded is a colloquial term to describe animal species which have a relatively higher blood temperature, and maintain thermal homeostasis primarily through internal metabolic processes...

     in birds and mammals. This trait is a derived character-state (in relation to poikilotherm
    Poikilotherm
    A poikilotherm is an organism whose internal temperature varies considerably. It is the opposite of a homeotherm, an organism which maintains thermal homeostasis. Usually the variation is a consequence of variation in the ambient environmental temperature...

    y, the character-state of the last common ancestor of both groups), which evolved independently in these two groups (or at least in the larger clades to which these groups belong).

Cladistic analyses


Synapomorphies are used to establish phylogenies in cladistic analyses. As such they are empirical data which can support a certain hypothesis that terminal groups form 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...

 (monophyletic
Monophyly
In common cladistic usage, a monophyletic group is a taxon which forms a clade, meaning that it contains all the descendants of the possibly hypothetical closest common ancestor of the members of the group. The term is synonymous with the uncommon term holophyly...

 group) together to the exclusion of certain other groups – whereas character-states that are shared, but also shared by other terminal groups descending from an earlier common ancestor, cannot be used to exclude these other groups. The latter character-states can consist of symplesiomorphies
Symplesiomorphy
In 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...

 ("primitive" character-states having originated in the earlier common ancestor) or homoplasies
Convergent evolution
Convergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action. Although their last common ancestor did not have wings, both birds and bats do, and are capable of powered flight. The wings are...


(superficially similar but independently evolved derived character-states).

The key problem is to identify the polarity of the transformation series to which several character-states belong, i.e. to tell which character-state is apomorphic and which is plesiomorphic. Various criteria were used to polarise the transformation series in earlier cladistics; however in the recent two decades pattern criteria based on outgroup comparison have dominated the field.

The concepts of apomorphy and plesiomorphy are relative to a certain level of generality. What counts as an apomorphy at one level of generality may well be a plesiomorphy at the other. For example, for rats and apes, the presence of mammary glands is a symplesiomorphy, but it is a synapomorphy for mammals in relation to tetrapods more broadly.

It is not essential to a synapomorphy that all members of a clade possess it; even if some would have secondarily lost the trait it could still be a synapomorphy of the clade as a whole. A character state that is a synapomorphy for a clade, but for lineages in this clade is a plesiomorphy that is altered in some lineages, is called underlying synapomorphy. If no crown group
Crown group
A crown group is a group consisting of living representatives, their ancestors back to the most recent common ancestor of that group, and all of that ancestor's descendants. The name was given by Willi Hennig, the formulator of phylogenetic systematics, as a way of classifying living organisms...

 taxa are known, it is sometimes difficult to decide which character state is the underlying synapomorphy and which the autapomorphy
Autapomorphy
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...

 that overlies it.

Clades are not defined by synapomorphies as such, though it is possible to define them by apomorphies in general.

Relative apparent synapomorphy analysis
Relative apparent synapomorphy analysis
Relative apparent synapomorphy analysis, or RASA, is a method that aims to determine whether a given character is shared between taxa due to shared ancestry or due to convergence. A synapomorphy is a shared trait found among two or more taxa and their most recent common ancestor, whose ancestor in...

is a method used to determine whether a given character is shared between taxa due to shared ancestry or due to convergence.