Teaching Animal Molecular Phylogenetics

IV. Incorporating Molecular Phylogenies Into Teaching

The most important conclusions from animal molecular phylogenetics are that Bilateria (triploblasts) and Deuterostomia (Echinodermata + Hemichordata + Chordata) are each monophyletic, and protostomes comprise the two clades Lophotrochozoa and Ecdysozoa.

Deuterostomia does not include lophophorates and chaetognaths.
Pseudocoelomates are not monophyletic, and they are derived from coelomates.
Acoelomates may be monophyletic, but they are also derived from coelomates.
Lophotrochozoa comprises lophophorates, groups with trochophore larva, and all descendants of their most recent common ancestor. Lophotrochozoa includes Platyhelminthes and the traditional spiralians.
Ecdysozoa comprises Arthropoda and other groups of animals that molt the cuticle in one piece, including Nematoda.

The traditional approach of proceeding from the simplest animals to the more complex is pedagogically sound.

Unlike evolution, most students find it hard to deal with many groups of animals at the same time. Therefore it is probably more practical to cover the major groups in some kind of sequence rather than trigger a Cambrian explosion by attempting to branch out in all directions simultaneously.
Even though deuterostomates diverged before the protostomate phyla diversified, it might be too jarring for beginning students to jump directly from the relative simplicity of Cnidaria to the complexity of Echinodermata and Chordata.
It is reasonable, therefore, to follow the presentation of "radiata" with a relatively simple protostomate group, such as Platyhelminthes.

The practice of treating the "acoelomates" and the "pseudocoelomates" as clades outside of coelomates should be abandoned.

More natural groupings would be Lophotrochozoa and Ecdysozoa.

The following proposed sequence of topics is consistent with molecular phylogenetics without departing too radically from the traditional zoology syllabus.

Topics in the table on the next page are keyed to chapters in the following McGraw-Hill texts:
Integrated Principles of Zoology, 11th ed. Hickman, Roberts, and Larson
Biology of the Invertebrates, 4th ed. Pechenik
Zoology, 5th ed. Miller and Harley
Animal Diversity Hickman and Roberts
Biology of Animals 7th ed., Hickman, Roberts, and Larson

Table 2. A zoology sequence that is consistent with molecular phylogeny. Bold numbers refer to chapters; parentheses enclose page numbers; ~ means "except".