AP Bio Chapter 23

Systematics

The study of evolutionary relationships among organisms by examining their similarities and differences.

Phylogeny

The evolutionary history and genealogical pathway of life that documents patterns of ancestry and descent.

Cladistics

A method of biological classification (phylogenetic systematics) that groups organisms based on shared derived characteristics rather than overall similarity.

Derived Characteristics

Traits inherited from the most recent common ancestor of a specific group (e.g., hair in mammals).

Ancestral Characteristics

Traits that evolved earlier in evolutionary history and are shared with more distant relatives (e.g., lungs in mammals).

Cladogram

A branching diagram or evolutionary tree used to illustrate hypothesized relationships based on shared character states.

Homoplasy

A shared characteristic that was not inherited from a common ancestor, often caused by convergent evolution (e.g., tail loss in frogs and humans).

Convergent Evolution

The independent evolution of similar traits in unrelated organisms due to similar environmental pressures (e.g., bat and bird wings, or shark and dolphin bodies).

Parsimony

The principle that if multiple evolutionary pathways are possible, the simplest pathway requiring the fewest changes is most likely correct.

Monophyletic Group

A natural group (clade) that includes the most recent common ancestor and ALL of its descendants.

Paraphyletic Group

A group that includes a common ancestor but excludes one or more descendant lineages.

Polyphyletic Group

An unnatural group composed of organisms that do not share a recent common ancestor within the group.

Node

A branch point on a cladogram representing a divergence event from a common ancestor.

Root

The base of a phylogenetic tree representing the most recent common ancestor of all taxa shown.

Outgroup

A closely related taxon used to root a cladogram and determine which traits are ancestral or derived.

Polytomy

A node with three or more branches, indicating uncertain evolutionary relationships.

Synapomorphy

A shared derived characteristic used to define a monophyletic group.

Symplesiomorphy

A shared ancestral characteristic found in both ancestral and descendant organisms.

Autapomorphy

A unique derived trait found in only one lineage.

Reading a Cladogram

Determine relatedness by tracing backward from organisms until their branches meet at a common ancestor. The more recent the common ancestor, the more closely related the organisms are.

Cladogram Construction

The process of organizing organisms based on shared derived characteristics, placing organisms with fewer derived traits closer to the base and those with more derived traits farther from the base.

Classical System of Classification

Kingdom → Phylum → Class → Order → Family → Genus → Species.