Inphylogenetics, anautapomorphy is a distinctive feature, known as aderived trait, that is unique to a given taxon. That is, it is found only in onetaxon, but not found in any others oroutgrouptaxa, not even those most closely related to the focal taxon (which may be aspecies,family or in general any clade).^[2] It can therefore be considered as anapomorphy in relation to a single taxon.^[3] The wordautapomorphy, introduced in 1950 by GermanentomologistWilli Hennig, is derived from theGreek words αὐτός,autos "self"; ἀπό,apo "away from"; and μορφή,morphḗ = "shape".^[4]

Because autapomorphies are only present in a single taxon, they do not convey information about relationship. Therefore, autapomorphies are not useful to infer phylogenetic relationships. However, autapomorphy, likesynapomorphy andplesiomorphy is a relative concept depending on the taxon in question. An autapomorphy at a given level may well be a synapomorphy at a less-inclusive level.^[5] An example of an autapomorphy can be described in modern snakes. Snakes have lost the two pairs of legs that characterize all ofTetrapoda, and the closest taxa toOphidia – 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.^[3]

Theautapomorphic species concept is one of many methods that scientists might use to define and distinguishspecies 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.^[6] 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".^[7] 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 populationalso 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.^[8]

Phylogenetic similarities: These phylogenetic terms are used to describe different patterns of ancestral and derived character or trait states as stated in the above diagram in association with synapomorphies.^[1]

• Homoplasy inbiological systematics is when atrait has been gained or lost independently in separate lineages during evolution. Thisconvergent evolution leads to species independently sharing a trait that is different from the trait inferred to have been present in their common ancestor.^[9][10][11]
  • ParallelHomoplasy – derived trait present in two groups or species without a common ancestor due toconvergent evolution.^[12]
  • ReverseHomoplasy – trait present in an ancestor but not in direct descendants that reappears in later descendants.^[13]
• Apomorphy – a derived trait. Apomorphy shared by two or more taxa and inherited from a common ancestor is synapomorphy. Apomorphy unique to a given taxon is autapomorphy.^{[14][15][16][17]}
  • Synapomorphy/Homology – a derived 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 inits 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.
  • Autapomorphy – a distinctive derived trait that is unique to a given taxon or group.^[3]
• Symplesiomorphy – an ancestral trait shared by two or more taxa.
  • Plesiomorphy – a symplesiomorphy discussed in reference to a more derived state.
  • Pseudoplesiomorphy – is a trait that cannot be identified as neither a plesiomorphy nor an apomorphy that is a reversal.^[18]
• Reversal – is a loss of derived trait present in ancestor and the reestablishment of a plesiomorphic trait.
• Convergence – independent evolution of a similar trait in two or more taxa.
• Hemiplasy^[19][20]

• Phylogenetics
• Evolutionary biology terminology

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