Apomorphy

Determining apomorphy

Whether a character state is derived or ancestral is called character polarity. Since genealogical classifications are based on synapomorphies, there must be a way to determine which character state is derived and which is ancestral (or what is special and what is general, to use less evolutionarily freighted terminology) without reference to genealogical classifications, to avoid a circular argument. Some features have been recognized as unique to particular taxa for thousands of years (e.g., feathers for birds, or an internal bony skeleton for vertebrates), and these sorts of common-sense presence/absence characters provide a scaffold upon which the polarity of other characters can be inferred: feathered animals form a natural group; things that lack feathers are just the complement - everything else (mammals, sharks, plants, bacteria). Once a taxon called "birds" is recognized that is defined by the synapomorphy "presence of feathers", then the polarity of other characters present at greater or lesser levels of inclusiveness can be discovered and evaluated. This may identify larger clades, such as the diapsid skull that defines diapsids, or less inclusive clades, such as the syrinx that defines songbirds.