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At a glance
Apomorphy: A derived or specialised character.
Plesiomorphy: An ancestral or primitive character.
Synapomorphy: An apomorphy (derived or specialised character) shared by two or more groups which originated in their last common ancestor.
Symplesiomorphy: A character shared by a number of groups, but inherited from ancestors older than the last common ancestor.
Keywords: genetic divergence, genetic divergence rate, evolution rate, cladistics
Relationships between taxa are defined in terms of common ancestral species. Two species are more closely related to one another than to a third species if they share a more recent common ancestor that either does with the third. These two taxa are considered as sister groups.
A trait which characterises an ancestral species and its descendants is called an apomophy. This is an evolutionary novelty for the group under consideration; hence it is a concept which has meaning only in a particular context. These are evidence for the existence of a group. Put another way, attributes shared in common are taken to indicate a shared evolutionary history.
A novel evolutionary trait that is unique to a particular species and all its descendants and which can be used as a defining character for a species or group in phylogenetic terms. Hence, the possession of feathers is unique to birds and defines all members of the class Aves. An apomorphy that is restricted to a single species is termed an autapomorphy. It alone cannot provide any information about the phylogenetic relations of that species, although it can indicate the degree of divergence of a species from its nearest relatives. An example is speech, which is found solely in humans (Homo sapiens) and not in other primates. An apomorphy that is shared by two or more species or groups is termed a synapomorphy. Such traits define the strictly monophyletic groups, or clades, which are the basis of cladistic classification systems (see cladistics). Compare plesiomorphy (Dictionary of Biology, Oxford University Press, © Market House Books Ltd 2000).
The possession of apomorphic features by two or more taxa in common (i.e. the features are shared, derived). If the two groups share a character state that is not the primitive one, it is plausible that they are related evolutionarily, and only synapomorphic character states can be used as evidence that taxa are related. Phylogenetic trees are built by discovering groups united by synapomorphies (Dictionary of Earth Sciences, © Oxford University Press 1999).
A character which is derived, and because it is shared by the taxa under consideration, is used to infer common ancestry (UCMP Glossary of Natural History Terms, Volume 1, Phylogenetic Terms).
Features shared more widely than in a group of interest are plesiomorphies. These are primitive for the group in question and cannot provide evidence for the group.
An evolutionary trait that is homologous within a particular group of organisms but is not unique to members of that group (compare apomorphy) and therefore cannot be used as a diagnostic or defining character for the group. For example, vertebrae are found in zebras, cheetahs, and orang-utans, but the common ancestor in which this trait first evolved is so distant that the trait is shared by many other animals. Therefore, possession of vertebrae sheds no light on the phylogenetic relations of these three species (Dictionary of Biology, Oxford University Press, © Market House Books Ltd 2000).
A primitive character state for the taxa under consideration (UCMP Glossary of Natural History Terms, Volume 1, Phylogenetic Terms).
The possession of a character state that is primitive (plesiomorphic) and shared between two or more taxa. Shared possession of a symplesiomorph character state is not evidence that the taxa in question are related (Dictionary of Earth Sciences, © Oxford University Press 1999).
Note that apomorphy and plesiomorphy are relative concepts. Their status depends on their position in the phylogeny. A character is an apomorphy at one branch of the tree, but is a plesiomorphy relative to all the branches after that. For example, hair is a mammalian feature (an apomorphy of mammals), but is primitive in squirrels (a plesiomorphy). We can use presence of hair as evidence for the existence of Mammalia, but not for the existence of Sciuridae.
Nielsen, C. 2001: Animal evolution: Interrelationships of the living phyla (second edition). Oxford University Press: 1-378.
Tudge, C. 2000: The variety of life. Oxford University Press: 1-684.
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