Integrative Biology 200A "PRINCIPLES OF PHYLOGENETICS" Spring 2008
University of California, Berkeley
Jan. 29, 2008.
The Hennig Principle: Homology, Synapomorphy, Rooting issues
The fundamental idea that has driven recent advances in phylogenetics is known as the
Hennig Principle, and is as elegant and fundamental in its way as was Darwin's principle of
natural selection. It is indeed simple, yet profound in its implications. It is based on the idea of
homology, one of the most important concepts in systematics, but also one of the most
controversial. What does it mean to say that two organisms share the same characteristic? The
modern concept is based on evidence for historical continuity of information; homology would
then be defined as a feature shared by two organisms because of descent from a common
ancestor that had that feature.
1. The Hennig Principle
Hennig's seminal contribution was to note that in a system evolving via descent with
modification and splitting of lineages, characters that changed state along a particular lineage can
serve to indicate the prior existence of that lineage, even after further splitting occurs. The
"Hennig Principle" follows from this: homologous similarities among organisms come in two
basic kinds, synapomorphies due to immediate shared ancestry (i.e., a common ancestor at a
specific phylogenetic level), and symplesiomorphies due to more distant ancestry (fig. 1,
below). Only the former are useful for reconstructing the relative order of branching events in
phylogeny -- "special similarities" (synapomorphies) are the key to reconstructing truly natural
relationships of organisms, rather than overall similarity (which is an incoherent mixture of
synapomorphy, symplesiomorphy, and non-homology).
We must pay close attention to both ontology and epistemology, and the feedback
relationship between the two: A given method makes sense only if the world really is a certain
way, yet the view we have of how the world is organized is dependent on the methods we have
used. For example, if species on earth are related genealogically and evolution is mainly by
descent with modification (in a primarily diverging mode), then the Hennig Principle is the best
method for reconstructing the history of life. Yet, the discovery of hierarchically nested
characters is the best evidence we have on how evolution has occurred.
2. Homology, or When are two things the same?