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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 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 ...
Plesiomorphy, symplesiomorphy, apomorphy, and synapomorphy all mean a trait shared between species because they share an ancestral species. [a] Apomorphic and synapomorphic characteristics convey much information about evolutionary clades and can be used to define taxa. However, plesiomorphic and symplesiomorphic characteristics cannot.
Secondary homology is implied by parsimony analysis, where a character state that arises only once on a tree is taken to be homologous. [20] [21] As implied in this definition, many cladists consider secondary homology to be synonymous with synapomorphy, a shared derived character or trait state that distinguishes a clade from other organisms.
Researchers must decide which character states are "ancestral" (plesiomorphies) and which are derived (synapomorphies), because only synapomorphic character states provide evidence of grouping. [10] This determination is usually done by comparison to the character states of one or more outgroups. States shared between the outgroup and some ...
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 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 ...
Willi Hennig 1972 Peter Chalmers Mitchell in 1920 Robert John Tillyard. The original methods used in cladistic analysis and the school of taxonomy derived from the work of the German entomologist Willi Hennig, who referred to it as phylogenetic systematics (also the title of his 1966 book); but the terms "cladistics" and "clade" were popularized by other researchers.
The terms "plesiomorphy" and "apomorphy" are typically used in the technical literature: for example, when a plesiomorphic trait is shared by more than one member of a clade, the trait is called a symplesiomorphy, that is, a shared primitive trait; a shared derived trait is a synapomorphy.
This is different from homology, which is the term used to characterize the similarity of features that can be parsimoniously explained by common ancestry. [1] Homoplasy can arise from both similar selection pressures acting on adapting species, and the effects of genetic drift .