by Aaron G. Filler, MD, PhD
Figure 5-3 – Hennig’s Methods for Cladistic Description of Lineages
A- A branching tree that represent the relationship among species is a cladogram. When character state a’ occurs in species B and D, where their common ancestor had character state a for that character, then we can call B and D sister species and call A the stem species. In this system we call a the original or plesiomorphic state and we call a’ the derived or apomorphic state.
B – Species can be classified into higher groups such as genus, family, order, etc. (for instance calling the group of apes and human species “hominoids”). Ideally 1 - every group name is made of species that arise uniquely from a common ancestor and 2 – the group includes all the species in that branch or “clade”. Hennig calls such a group monophyletic.
Redrawn and modified by AGF after Hennig (1966), Phylogenetic Systematics. Urbana, University of Illinois Press, pages 89 and 148 .