Evolution and Population Genetics
Mechanism and pattern
Evolution questions test whether you can keep the mechanism (selection, drift, gene flow, mutation) apart from the pattern (a change in allele frequency), and whether you apply Hardy–Weinberg only where its assumptions hold.
Core rules
- Natural selection needs heritable variation and differential reproduction; the environment filters phenotypes, it does not create needs-directed mutations.
- Evidence: transitional fossils, homologous versus analogous structures, embryology, molecular homology, and observed resistance to antibiotics or pesticides.
- Hardy–Weinberg: p + q = 1 and p² + 2pq + q² = 1; assumptions are no selection, no mutation, no migration, random mating and a large population. Apply it to the whole population, never to a group already screened by selection.
- Drift: bottleneck and founder effects, strongest in small populations; gene flow between populations reduces divergence.
- Selection modes: directional, stabilising, disruptive, plus sexual selection.
- Speciation: allopatric (geographic barrier) or sympatric; prezygotic isolation (habitat, timing, behaviour, mechanics) and postzygotic isolation (hybrid inviability or sterility).
Worked example
An autosomal recessive disease affects 1 in 10,000. Then q² = 0.0001, so q = 0.01 and p = 0.99. Heterozygote frequency is 2pq = 2 × 0.99 × 0.01 ≈ 0.0198, about 1 in 50 people. Reasoning: carriers hide the recessive allele, so they far outnumber affected individuals - which is why a rare disease can persist.
Marks lost
Writing that individuals evolve; using Hardy–Weinberg on a gene under strong selection; confusing allele frequency with genotype frequency; treating analogous structures as evidence of close relatedness; forgetting that mutation is the source of new alleles but selection changes their frequency.