homology was probably first popularized by Huxley in 1863 to argue for human evolution.
Refers to features of two or more organisms sharing common ancestry.
Apomorphy: A derived or specialised character. mor
Plesiomorphy: An ancestral or primitive character. more
Synapomorphy: An apomorphy (derived or specialised character) shared by two or more groups which originated in their last common ancestor. more
Symplesiomorphy: A character shared by a number of groups, but inherited from ancestors older than the last common ancesto
Within this taxon set, hair is a shared derived character (synapomorphy) of therians
that indicates a close relationship of marsupial and placental mammals.
Within this taxon set, hair is a unique character (autapomorphy) of mammals
The Carnassial Pair as a synapomorphy of Cats & Dogs The diagram indicates that a particular character state (presence of a carnassial pair formed from the 4th upper premolar and the 1st lower molar) occurs Cats and Dogs. Eutherian insectivores (such as shrews) and marsupials (such as kangaroos) retain the ancestral, unspecialized form of the molariform teeth.
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.