Apomorphy is a trait that characterizes an ancestral species and its descendants, which is used to indicate shared evolutionary history among different species. It is a trait unique to a particular species and its descendants, and can be used as a defining characteristic for a species or group in phylogenetic terms. Apomorphy restricted to a single species is referred to as autapomorphy, which can be used to indicate a degree of divergence of a species from its relatives. Apomorphy shared by two or more species is referred to as synapomorphy. These traits define the strictly monophyletic groups.

info from www.palaeos.com

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