Macroevolution

AP Biology Notes — Microevolution, Macroevolution & Patterns of Evolution

Microevolution vs. Macroevolution

Microevolution

• Small genetic changes within a species.

• Leads to variation or subspecies, not new major groups.

• Driven by natural selection, mutation, gene flow, genetic drift.

• Example: Northern house sparrows larger than southern sparrows.

Macroevolution

• Evolutionary change at or above the species level.

• Involves speciation, extinction, and major structural changes.

• Occurs over millions of years.

• Examples:

  • Evolution of birds from reptiles (Archaeopteryx).

  • Evolution of whales from land mammals.

Archaeopteryx

• Transitional fossil (“missing link”).

• Shows reptilian traits (teeth, long tail) and avian traits (feathers, wings).

• Evidence for macroevolution and common ancestry.

Patterns of Macroevolution

Adaptive Radiation

• Rapid diversification from one common ancestor.

• Occurs when organisms enter a new environment.

• Produces homologous structures.

• Example: Darwin’s finches, Cambrian explosion.

Coevolution

• Two or more species evolve in response to each other.

• Example: predator–prey, flower–pollinator relationships.

Convergent Evolution

• Unrelated species evolve similar traits.

• Driven by similar environmental pressures.

• Produces analogous structures.

• Example: wings in birds and bats.

Extinction

• Complete disappearance of a species.

• Alters evolutionary pathways of remaining organisms.

• Example: trilobites.

Punctuated Equilibrium

• Proposed by Stephen Jay Gould & Niles Eldredge (1972).

• Evolution occurs in short bursts separated by long periods of stasis.

• Pattern: stasis → rapid evolution → stasis.

• Changes still take thousands of years, but appear sudden geologically.

• Often triggered by:

  • Geographic isolation

  • Migration

  • Genetic drift

Examples:

• Dinosaurs (Judith River Formation)

• Ammonites

• Trilobite Phacops rana (eye lens change)

Phyletic Gradualism

• Supported by Charles Darwin.

• Evolution occurs slowly and continuously.

• Small changes accumulate over long periods.

• Speciation occurs via anagenesis.

• Missing links explained by incomplete fossil record.

Examples:

• Human evolution (bipedalism, brain size increase)

• Whale evolution

Whale Evolution (Gradualism Example)

• Pakicetus (52 mya): Land mammal with whale-like ear bones.

• Ambulocetus (46 mya): Semi-aquatic, walking/swimming.

• Rodhocetus (40 mya): Fully marine, reduced hind limbs.

• Modern whales: Fully aquatic, vestigial hind limbs.

Trilobites & Evolution

• Marine arthropods (540–250 mya).

• Excellent fossil record due to exoskeleton.

• Some species lost eyes after moving to deep-water habitats.

• Key evidence for punctuated equilibrium.

Punctuated Equilibrium vs. Phyletic Gradualism (AP-Style)

AP Exam Triggers

• Archaeopteryx → macroevolution, transitional fossil

• Stasis + sudden change → punctuated equilibrium

• Slow steady change → phyletic gradualism

• New species origin → speciation

• Similar traits, unrelated species → convergent evolution