AP Biology: Phylogenetic Trees – Cladograms, Monophyletic/Paraphyletic/Polyphyletic Groups

Phylogenetic Trees – Cladograms, Monophyletic/Paraphyletic/Polyphyletic Groups

Concept Summary

• Phylogenetic tree: A branching diagram showing evolutionary relationships among species based on shared derived traits (synapomorphies).
• Cladogram: A type of phylogenetic tree that groups organisms by common ancestry, using only branching order (not time or genetic distance).
• Monophyletic group (clade): A group containing a common ancestor and all its descendants; the only valid grouping in modern systematics.
• Paraphyletic group: A group containing a common ancestor but not all its descendants (e.g., reptiles excluding birds).
• Polyphyletic group: A group with no recent common ancestor, formed by convergent traits (e.g., "warm-blooded animals" = birds + mammals).

Core Questions

WHAT (definitional)

Q: What is a synapomorphy? A: A shared derived trait (e.g., feathers in birds) used to define monophyletic groups. Trap/Clarification: Ancestral traits (plesiomorphies, e.g., vertebrae) are not synapomorphies—they don’t define clades.

Q: What distinguishes a cladogram from other phylogenetic trees? A: Cladograms show only branching order (relative relationships), not time, genetic distance, or evolutionary rates. Trap/Clarification: Branch length in cladograms is arbitrary; in phylograms, length may represent genetic change.

WHY (causal/explanatory)

Q: Why are monophyletic groups the only valid taxonomic groups? A: They reflect true evolutionary history by including all descendants of a common ancestor, avoiding artificial groupings. Trap/Clarification: Paraphyletic groups (e.g., "fish") are outdated because they exclude descendants (e.g., tetrapods).

Q: Why do polyphyletic groups misrepresent evolution? A: They group organisms by convergent traits (e.g., wings in bats and birds) rather than shared ancestry. Trap/Clarification: Polyphyletic groups often result from homoplasy (independent evolution of similar traits).

HOW (process/application)

Q: How do you identify a monophyletic group on a cladogram? A: Find a single node (ancestor) and trace all branches descending from it. Trap/Clarification: If any descendant is excluded, it’s paraphyletic (e.g., "dinosaurs" excluding birds).

Q: How is a cladogram constructed from trait data? A: 1) List traits for each species, 2) identify shared derived traits (synapomorphies), 3) group species by synapomorphies, 4) minimize homoplasy (principle of parsimony). Trap/Clarification: The simplest tree (fewest evolutionary changes) is preferred, but molecular data may override morphological traits.

CAN (conditions/possibilities)

Q: Can a monophyletic group be nested within another monophyletic group? A: Yes—clades are hierarchical (e.g., mammals are a clade within amniotes). Trap/Clarification: A subgroup (e.g., primates) is still monophyletic if it includes all descendants of its ancestor.

Q: Can a trait be both ancestral and derived? A: Yes—relative to different nodes. Feathers are derived for birds but ancestral for songbirds. Trap/Clarification: Context matters: a trait’s status depends on the clade being analyzed.

Quick Facts & Traps

• Fact: Outgroup comparison is used to determine ancestral vs. derived traits (outgroup lacks the derived trait).
• Trap: Assuming branch length = time-Reality: Only true in chronograms; cladograms ignore time.
• Fact: Homoplasy (convergent evolution) creates polyphyletic groups (e.g., "flying animals" = bats + birds + insects).
• Trap: Confusing paraphyletic with polyphyletic-Reality: Paraphyletic excludes descendants; polyphyletic lacks a recent common ancestor.
• Fact: Maximum parsimony assumes the fewest evolutionary changes, but molecular data (e.g., DNA) may override this.
• Trap: Ignoring horizontal gene transfer-Reality: Can complicate trees (e.g., bacteria/archaea).

Rapid-Fire True/False

• Statement: A clade must include all descendants of its most recent common ancestor. Answer: TRUE Why the common mistake happens: Students forget that "all" is non-negotiable (e.g., excluding birds from reptiles).

• Statement: If two species share a trait, they must be in the same clade. Answer: FALSE Why the common mistake happens: Homoplasy (convergent evolution) can create shared traits in unrelated groups (e.g., wings in bats and birds).

• Statement: The root of a cladogram represents the most recent common ancestor of all taxa in the tree. Answer: TRUE Why the common mistake happens: Students confuse the root with the "oldest" species (the root is an ancestor, not a living species).