Cladistics is an approach to biological classification that categorizes organisms into groups ("clades") based on their most recent common ancestor. It was developed in the 1950s by the German taxonomist Willi Hennig. Cladistics analyzes morphological, physiological, behavioral, and DNA characters to construct cladograms that illustrate evolutionary relationships. Characters are determined to be ancestral or derived traits, and clades are formed based on shared derived characters called synapomorphies. Conflicts among characters are resolved using parsimony, which favors the hypothesis requiring the fewest evolutionary changes.

1. Cladistics
Philosophy
FRANCIS D.
HEMPESAO and
CHARIEL FLORO

2. Cladistics
 From the Greek word cládos is an approach
to biological classification in which
organisms are categorized in groups
("clades") based on the most recent
common ancestor.

3. History
 Darwin
- envisioned that all species were
descended from a single common
ancestor.
-He depicted this history of life as
a branching tree.
-Darwin called this process
“descent with modification”
cladogram

4. History
 Willi Hennig
- A German taxonomist who developed cladistics in
1950.

5. Cladistics
 Characters can be any aspect of the phenotype
-Morphology
- Physiology
-Behavior
- DNA
 Characters should exist in recognizable
character states
 Example: Teeth in amniote vertebrates has two states,
present in most mammals and reptiles and absence in
birds and turtles

6. Examples of ancestral versus
derived characters
Presence of hair is a shared derived
feature of mammals
Presence of lungs in mammals is an
ancestral feature; also present in
amphibians and reptiles

7. Determination of ancestral versus
derived
 First step in a manual cladistic analysis is
to polarize the characters (are they
ancestral or derived)
 Example: polarize “teeth” means to
determine presence or absence in the
most recent common ancestor

8. Construction of a cladogram
 Polarize characteristics
 Clade: species that share a common ancestor as
indicated by the possession of shared derived
characters
 Clades are evolutionary units and refer to a common
ancestor and all descendants
 Synapomorphy: a derived character shared by clade
members

9. Construction of a cladogram
A simple cladogram is a nested set of
clades
Plesiomorphies: ancestral states
Symplesiomorphies: shared
ancestral states, not informative about
phylogenetics

10.  Homoplasy: a shared character state that has not
been inherited from a common ancestor
 Results from convergent evolution
 Results from evolutionary reversal
 If there are conflicts among characters, use the
principle of parsimony which favors the hypothesis
that requires the fewest assumptions

11. Parsimony and Homoplasy

12. A Cladogram; DNA