What the exam asks
- Section I (Multiple Choice): Expect stimulus sets containing sequence-comparison matrices, fossil layer diagrams with radiometric dates, or anatomical illustrations. You will be asked to determine which species share the most recent common ancestor, distinguish homologous from analogous structures, or identify conserved cellular traits specific to eukaryotes.
- Section II (Free Response): Commonly appears in Q5 (Analyze Model or Visual Representation) or Q6 (Analyze Data). Typical prompts ask you to describe evolutionary relationships from a table of molecular differences, explain why DNA sequences can diverge when amino acid sequences do not, or justify a claim that two organisms are closely related.
Core ideas
1. Geological and geographical evidence (Fossils and biogeography)
Evolution is supported by geographical, geological, physical, biochemical, and mathematical data. Fossils document extinct species and transitional forms.
Scientists determine a fossil’s age using three main lines of evidence:
- Age of the rocks (Relative dating): Sedimentary rock forms in layers (strata). By the law of superposition, undisturbed lower strata are older than higher strata.
- Decay rate of radioisotopes (Absolute/radiometric dating): Unstable isotopes decay into daughter isotopes at a constant, known rate measured by half-life (, the time required for of the parent isotope to decay). Carbon-14 () dates recent organic remains up to about years old; isotopes with longer half-lives (such as uranium-238 or potassium-40) date older volcanic ash layers above and below a fossil.
- Geographical data (Biogeography): The global distribution of fossils and living species reflects continental drift (plate tectonics) and local adaptation from ancestral populations. Geography also helps date fossils: rock layers can be matched from one region to another, and a fossil species found on continents that were once joined is probably older than the split between them.
2. Morphological homologies vs. convergent evolution
- Morphological homologies are anatomical features shared by different species because they were inherited from a common ancestor, even if modified for different functions (e.g., the pentadactyl, or five-digit, forelimb bones of humans, bats, and whales).
- Vestigial structures are reduced, non-functional (or differently functioning) remnants of homologous structures that were functional in an ancestor (e.g., pelvic bones in whales and some snakes, or the tiny, non-working eyes of some cave-dwelling fish).
- Analogous structures evolve independently in distantly related lineages facing similar environmental selective pressures (convergent evolution), such as the streamlined bodies of sharks (fish) and dolphins (mammals). Analogies do not indicate recent common ancestry.