What you'll learn
How populations evolve (~13–20% of the exam — one of the largest units).
Natural selection
Variation + heritability + differential reproductive success → adaptation. Types: directional, stabilizing, disruptive. Selection acts on phenotypes.
Mechanisms of evolution
- Mutation (source of new alleles), gene flow, genetic drift (random; strong in small populations — bottleneck/founder effects), nonrandom mating.
Hardy-Weinberg equilibrium
A non-evolving baseline. p + q = 1 and p² + 2pq + q² = 1. Conditions: no mutation, no selection, no migration, random mating, large population. Deviations indicate evolution.
Evidence for evolution
Fossils, homologous/vestigial structures, embryology, molecular/DNA similarity, biogeography.
Speciation & phylogeny
Reproductive isolation (pre-/post-zygotic) → new species; allopatric (geographic) vs sympatric. Phylogenetic trees show shared ancestry; shared derived traits define clades.
Exam tips
- Do Hardy-Weinberg calculations (find q from q², then p).
- Distinguish drift (random) from selection (non-random).
Common mistakes
- Saying individuals evolve (populations do).
- Forgetting selection acts on phenotype, not directly on genotype.