Classification Topic 2. Consider our goals. We need to know vertebrate phylogeny, if we are to understand how vertebrates have evolved and how the developmental process evolved. If fins did indeed evolve into legs, and legs into wings, as we shall argue later on, we need to understand that fins are primitive, that legs are derived, and wings yet more highly derived. We need to be able to deduce the polarity of the evolutionary changes.
Polarity is embedded in classification.
Polarity, synapomorphy, and clade are among the important terms we need to deal with in this business of classifying the vertebrates. Including a single ancestor and all of the descendants. The MA hypothesis is that vertebrates comprise a single clade. A hodgepodge grouping that arises from more than a single ancestor. Including a single ancestor but not all of the descendants.