Darwinian Descent with Modications: Evolutionary Taxonomy and Cladistics
Such a statement immediately raises a problem because, for a fossil species, one can never be sure about its precise ancestor, only its approximate location on the diagram. In the case of species B in the example above, all one can say is that its parent species had to be a species whose phenotype includes property x that separated it from the inheritance line from species A. is further species is considered to be the last common ancestor (LCA) of all organisms with property x, and the historical facts are rewritten in the form:
<species F> <derives with modication from> <LCA with property x>
e cladogram for this example is shown in Figure 5.1 and the terms are of the form:
<species C> <derives with modication from> <LCA2>
<species D> <derives with modication from> <LCA3>
<species LCA3> <derives with modication from> <LCA2>
Such a cladogram can, of course, be extended to include further species (or, indeed, any taxonomic class or taxon) as the data demand. e key point, however, is that every taxon included in a branch of the hierarchy should share with the others on that branch a single LCA that is at the base of the branch (species A in this case). A hierarchy or tree that includes only the descendants of a single, root species is known as a monophyletic group.