Phylogenetic Systematics (Cladistics) reconstructs evolutionary relationships among organisms.
Willi Hennig developed this method in the mid-20th century.
He called it phylogenetic systematics.
Researchers often shorten it to cladistics.
Cladistics focuses on common ancestry.
It builds hypotheses of relationships first.
Then, it represents them in branching diagrams.
These diagrams are called cladograms.
Cladograms show clades only.
A clade includes an ancestor and all its descendants.
Clades must be monophyletic.
Thus, cladistics rejects paraphyletic and polyphyletic groups.
Scientists gather character data next.
Characters include anatomical traits, behaviors, or genetic sequences.
Not all traits help equally.
Shared ancestral characters (plesiomorphies) provide no useful grouping information.
However, shared derived characters (synapomorphies) do.
These synapomorphies indicate evolutionary innovations.
They support close relationships accordingly.
Researchers identify synapomorphies carefully.
Then, they group taxa that share them.
Cladistics applies the principle of parsimony.
It selects the tree requiring the fewest evolutionary changes.
This approach minimizes assumptions.
Therefore, it produces the most defensible hypothesis.
Outgroups help root the tree.
An outgroup is a related taxon outside the study group.
Finally, cladistics tests hypotheses rigorously.
It uses new data to refine or challenge trees.
This method dominates modern systematics today.
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