Zoology · Ch 1 — The Living World
Systematics and Phylogenetic Classification (Cladistics)
Systematics and Phylogenetic Classification (Cladistics)
As biologists began studying evolutionary and genetic relationships more closely, a further method emerged: phylogenetic classification, commonly called cladistics. Instead of grouping organisms by overall similarity alone, cladistics classifies them by how recently they shared a common ancestor — the more recently two lineages diverged, the more closely related cladistics considers them.
Cladistic analysis compares two kinds of traits: ancestral characters, which belong to the basic body plan shared across an entire group, and derived characters, whose structure or function has changed relative to that ancestral condition. Each time one or more derived characters appear in a lineage, it marks a new evolutionary branch — potentially leading, over time, to new subspecies or higher taxa. These relationships are summarised visually as a phylogenetic tree, or cladogram, a method of representation introduced by Ernst Haeckel. …
What this figure shows. A branching phylogenetic tree (cladogram) of vertebrate groups. Starting from a common ancestral stock, the diagram branches successively at points marked by shared derived characters — the acquisition of a vertebral column and bony skeleton (leading to bony fishes such as ray-finned fish), amniotic eggs with shells (leading to reptiles/crocodiles and birds), and hair as a derived character marking the branch leading to mammals such as primates and rodents/rabbits. Each branch point represents the origin of a new derived trait that is shared by every lineage above that point but absent below it, so the tree visually summarises which groups (sharks, ray-finned fish, amphibians, crocodiles, birds, primates, rodents/r …