Inside This Unit: The Full Breakdown
Natural Selection explores how evolution occurs through variation, selection, and adaptation. This unit covers Darwin's theory, evidence for evolution, mechanisms of speciation, and how populations change over time through microevolution and macroevolution.
Why it matters
Evolution is the unifying theme of all biology and appears throughout the AP exam. The College Board explicitly states that evolution is one of the four Big Ideas. You must connect evolutionary thinking to genetics, ecology, and every other unit.
Key concepts
- Natural selection requires heritable variation, differential survival and reproduction, and a connection between traits and fitness.
- Evidence for evolution includes the fossil record, comparative anatomy (homologous structures), molecular biology (DNA/protein similarities), and biogeography.
- Hardy-Weinberg equilibrium (p² + 2pq + q² = 1) describes a non-evolving population; deviations indicate evolution is occurring.
- Speciation occurs when populations become reproductively isolated through geographic (allopatric) or ecological/behavioral (sympatric) barriers.
Natural Selection and Adaptation
Charles Darwin proposed that evolution occurs through natural selection: organisms with traits better suited to their environment survive and reproduce at higher rates, passing those advantageous traits to offspring. For natural selection to operate, three conditions must be met — variation must exist in the population, that variation must be heritable, and individuals with certain variants must have a fitness advantage. Over many generations, advantageous alleles increase in frequency while disadvantageous ones decrease. Natural selection can take different forms: directional selection favors one extreme phenotype, stabilizing selection favors the average phenotype, and disruptive selection favors both extremes. Importantly, natural selection acts on individuals but evolution occurs in populations.
Evidence for Evolution and Hardy-Weinberg
Multiple independent lines of evidence support evolution. The fossil record shows transitional forms and patterns of change over time. Homologous structures (like the forelimbs of mammals) reveal common ancestry, while analogous structures (like wings of birds and insects) show convergent evolution. Molecular evidence — similarities in DNA sequences and protein structures — provides the most precise measure of evolutionary relationships. Biogeography explains why island species resemble nearby mainland species. The Hardy-Weinberg model provides a mathematical null hypothesis for evolution: if allele frequencies remain constant (p² + 2pq + q² = 1 and p + q = 1), no evolution is occurring. The five conditions for equilibrium — no mutation, no migration, large population, random mating, and no selection — are rarely met in nature, confirming that evolution is continuous.
Speciation and Macroevolution
Speciation is the formation of new species, defined biologically as populations that can no longer interbreed and produce fertile offspring. Allopatric speciation occurs when a geographic barrier (mountain range, river, ocean) physically separates a population, allowing independent evolution. Sympatric speciation occurs without geographic separation, often through polyploidy in plants or ecological niche differentiation. Reproductive isolation can be prezygotic (habitat isolation, temporal isolation, behavioral isolation, mechanical isolation, gametic isolation) or postzygotic (hybrid inviability, hybrid sterility, hybrid breakdown). Over long periods, speciation produces the branching tree of life. Macroevolutionary patterns include adaptive radiation (rapid diversification from a common ancestor) and mass extinctions followed by new radiations.
AP exam tip
Hardy-Weinberg math problems appear regularly on the AP exam. Practice calculating allele and genotype frequencies. Remember: start by finding q² (the homozygous recessive frequency), take the square root to get q, then p = 1 − q.
Connections to other units
- Unit 5 (Heredity): Genetic variation from meiosis (crossing over, independent assortment) provides the raw material for natural selection.
- Unit 6 (Gene Expression): Mutations create new alleles; gene regulation can affect which traits are expressed and selected.
- Unit 8 (Ecology): Organisms' ecological interactions — competition, predation, symbiosis — are powerful selective pressures.
