Cladistics (Ch. 22) Based on phylogenetics – an inferred reconstruction of evolutionary history

Goals of cladograms: Clarify evolution of a group Aid in the classification of the group

Cladistic assumptions: The group of organisms to be studied is related by descent from a common ancestor Descent follows a bifurcating pattern – ancestral form splits into 2 sister taxa and the ancestor goes extinct. This is controversial. Changes in characteristics occurs in lineages over time.

Reading a cladogram: The shared derived characters of the homologous structures are shown by solid square boxes along the branches. Common ancestors are shown by open circles. The closer the fork in the branch between 2 organisms, the closer their evolutionary relationship.

Constructing a cladogram: Choose a clade (ancestor + inferred descendants) Determine characters – this is the most difficult step. Are the similarities homologies or the result of convergent evolution? Group by shared derived characters – use a nested Venn diagram Build the cladogram

Cladogram rules: This is NOT an evolutionary tree showing which organism is descended from whom. All taxa go to the endpoints of the cladogram, never at the nodes. All nodes must include the character common to all taxa above the node. All characters must appear on the cladogram only once, unless they were derived separately by evolutionary parallelism.