. A cladistic approach.
1. Modern practice of inferring phylogenetic relationships - German entomologist, Willi Hennig (1966).
2. Hennig pointed out that taxa may be similar because they share: a. Uniquely derived character states
b. Ancestral character states.
c. Homoplasious character states.
3. When inferring a phylogenetic tree, we can rule out homoplasious characters.
4. Ancestral character states - more problematic.
a. Hennig said that evidence that species share a more recent ancestor with each other than
they do with any other species is provided only by shared derived (advanced) characters
that evolved in the species' common ancestor.
b. Thus, they form a monophyletic group.
c. E.g. the placenta is a derived character that provides evidence of the common ancestry of
horses, humans, and other eutherian mammals;
d. But the primitive character state (lack of a placenta) does not tell us that animals without a
placenta (birds, reptiles, fishes, and for that matter, insects and sponges) are more closely related to each other than they are to mammals (as they indeed, are not).
5. To make a phylogenetic tree, we can only consider the similarity due to uniquely derived character states. But this presents a problem.
a. How can we tell which state of a character is derived?
b. How can we tell whether it is uniquely derived or homoplasious?
6. Hennig's approach is the root of cladistics.