CoachA character is any heritable feature with two or more states (limb present or absent; a given base in a gene A, C, G or T). Two organisms’ characters are homologous when inherited from a common ancestor, homoplastic when similar by convergence or reversal (the wings of birds and bats; the streamlined bodies of dolphins and sharks). A character state is ancestral (plesiomorphic) if present in the group’s ancestor, derived (apomorphic) if it arose within the group; the derived states shared by a clade, its synapomorphies, are the only evidence for it. Polarity is determined by comparison with an outgroup, a taxon known to lie outside the group being studied: the state it shows is taken as ancestral.
Examples
Example 29.8 (Two trees, counted)
Four taxa: lizard, bird, mammal, frog (outgroup). Characters: amniotic egg (lizard, bird, mammal: 1), diapsid skull (lizard, bird), endothermy (bird, mammal). Tree A, (lizard, bird) then mammal: egg 1 change, skull 1, endothermy 2 (arising twice) — length 4. Tree B, (bird, mammal) then lizard: egg 1, endothermy 1, skull 2 — length 4. A tie; add feathers on the leg scales and epidermal scales (lizard, bird): tree A length 5, tree B length 6. Tree A is preferred, and endothermy is a homoplasy: it evolved twice.