Evolution of animal body plans: the role of metazoan phylogeny at the interface between pattern and process.

Comprehensive integrative studies are the hallmark of evolutionary developmental biology. A properly defined phylogenetic framework takes a central place in such analyses as the meeting ground for observation and inference. Molecular phylogenies take this place in many current studies on animal body plan evolution. In particular, 18S rRNA/DNA sequence analyses have yielded a new view of animal evolution that is often contrasted with a presumed traditional or classical view. First, I expose this traditional view to be a simplified historical abstraction that became textbook dogma. Second, I discuss how two recent important studies of animal body plan evolution, examining the evolution of the platyhelminth body plan and the evolutionary significance of indirect development and set-aside cells, have actively incorporated two problematic aspects of the newly emerging molecular view of animal evolution: incomplete and unresolved phylogenies.