Natural selection, fitness and Hardy–Weinberg population genetics

Learning objectives

Explain natural selection in terms of heritable variation, differential reproductive success and the resulting change in allele frequency in a population over generations

Distinguish directional, stabilizing and disruptive selection from a graph of phenotype frequency, and identify the selective pressure acting in each case

State the five conditions required for Hardy–Weinberg equilibrium, and explain that the model describes a population that is not evolving

Calculate allele and genotype frequencies using p + q = 1 and p² + 2pq + q² = 1, and justify a claim that a population is or is not in Hardy–Weinberg equilibrium by comparing observed with expected genotype counts

Explain how genetic drift, the founder effect, bottlenecks, gene flow and non-random mating change allele frequencies, and explain why drift has a larger effect in small populations

Explain why a recessive allele can persist in a population at low frequency even when selection acts against the recessive phenotype