Natural Selection and Evolution — AP Biology
1. Evidence for Evolution ★★☆☆☆ ⏱ 3 min
Four core lines of evidence for evolution consistently tested on the AP exam all directly support the theory of common ancestry for all living organisms:
- **Fossil records**: Preserved remains of extinct organisms show transitional species bridging gaps between modern groups. For example, *Archaeopteryx* fossils have both dinosaurian and bird traits, confirming birds evolved from theropod dinosaurs.
- **Homologous structures**: Anatomical traits that share a common evolutionary origin but serve different functions across species. Human, bat, and whale forelimbs have identical bone arrangements, confirming descent from a common tetrapod ancestor.
- **Molecular homology**: Shared DNA or amino acid sequences between species indicate recent common ancestry. Humans share 98.8% coding DNA with chimpanzees, and 98.4% with gorillas, confirming closer relatedness to chimps.
- **Biogeography**: Geographic distribution of species matches evolutionary history. Galapagos finches are more closely related to South American mainland finches than African finches, confirming their colonization and local adaptation.
Exam tip: Examiners frequently ask you to distinguish between homologous and analogous traits; only homologous traits count as evidence for shared ancestry.
2. Hardy-Weinberg Equilibrium ★★★☆☆ ⏱ 4 min
- No new mutations introducing new alleles
- Random mating (no sexual selection for specific traits)
- No natural selection acting on any traits
- Extremely large population size (eliminates genetic drift)
- No gene flow (no immigration/emigration adding/removing alleles)
p + q = 1
Where $p$ = frequency of the dominant allele, $q$ = frequency of the recessive allele. For genotype frequencies:
p^2 + 2pq + q^2 = 1
Where $p^2$ = frequency of homozygous dominant individuals, $2pq$ = frequency of heterozygous individuals, $q^2$ = frequency of homozygous recessive individuals.
Exam tip: Always start Hardy-Weinberg calculations with the recessive phenotype, since you can directly calculate $q$ from its frequency.
3. Types of Natural Selection ★★☆☆☆ ⏱ 3 min
Natural selection acts on phenotypic variation in populations, and can be categorized into three types based on which segment of the phenotypic distribution is favored:
- **Directional selection**: Favors one extreme of the phenotypic range, shifting the entire distribution toward that extreme. Example: 19th century peppered moths, where dark color was favored on soot-covered tree bark.
- **Stabilizing selection**: Favors the intermediate phenotype, selects against both extremes, and reduces overall phenotypic variation. Example: Human birth weight, where very low and very high birth weight both have higher mortality.
- **Disruptive selection**: Favors both extreme phenotypes, selects against the intermediate, increases variation, and often leads to speciation. Example: African seedcracker finches, where only small or large beaks are efficient for feeding.
Exam tip: When given a before/after graph of phenotypic frequency: directional selection shifts the peak left/right, stabilizing selection narrows the peak, disruptive selection creates two distinct peaks.
4. Speciation ★★☆☆☆ ⏱ 3 min
- **Allopatric speciation**: Occurs when a geographic barrier (river, mountain range) separates a population, stopping gene flow. Genetic differences accumulate over time leading to reproductive isolation. Example: Grand Canyon antelope squirrels, now separate species on opposite rims.
- **Sympatric speciation**: Occurs without geographic isolation, most commonly via polyploidy (extra chromosome sets from cell division errors) in plants. Polyploid individuals are immediately reproductively isolated from the parent population. Example: Modern bread wheat, a hexaploid that cannot interbreed with its parent species.
Reproductive barriers that maintain speciation are divided into two categories:
- **Prezygotic barriers**: Prevent fertilization: temporal isolation (different mating seasons), behavioral isolation (different mating rituals), mechanical isolation (anatomical incompatibility), gametic isolation (sperm cannot fertilize eggs)
- **Postzygotic barriers**: Act after fertilization: hybrid inviability (hybrid embryo dies before maturity), hybrid sterility (hybrid is alive but cannot reproduce, e.g. mules are sterile)
5. Interpreting Phylogenetic Trees ★★★☆☆ ⏱ 3 min
- Nodes: Represent the most recent common ancestor (MRCA) of all groups branching from the node
- Sister taxa: Two groups that share an immediate common ancestor, making them each other's closest relatives
- Root: The common ancestor of all taxa on the tree
- Branch length: In scaled trees, indicates amount of evolutionary change or time; in unscaled cladograms, branch length has no meaning
The most commonly tested rule for the AP exam is that the left-to-right order of terminal taxa is arbitrary. Relatedness is determined only by how recent the most recent common ancestor is between two groups, not by their position on the tree.
Exam tip: Examiners often rearrange terminal nodes to trick you. Always trace branches back to the most recent common ancestor to determine relatedness, ignore label order at the ends of branches.
Common Pitfalls
Why: Students mix up definitions, confusing 'same function' with 'same origin'
Why: Students forget that dominant phenotypes include both homozygous dominant and heterozygous individuals
Why: Students confuse selection on traits with selection on underlying alleles
Why: Students learn allopatric speciation first and overlook common sympatric cases
Why: Students assume leftmost taxa are more primitive, or adjacent terminal taxa are closely related