The reconstruction of phylogeny - How do we distinguish ancestral from derived homologies?
Only shared derived homologies consistently reveal phylogenetic groups, but this leaves the problem of how to distinguish derived from ancestral homologies. This is called the question of character polarity, which has the following form:
A character has two states, which we can call a and a' : we need to know whether a evolved from a' , or the other way round.
We shall examine the three most important methods of calculating character polarity:
• Outgroup comparison
• Embryological criterion
• The fossil record