(A Level) Variation and natural selection

Learning objectives

Explain, with examples, how phenotypic variation can be caused by genes, by the environment or by both, and distinguish discontinuous variation (one or a few genes, distinct classes) from continuous variation (many additive polygenes, a range of values).

Use the t-test to decide whether the means of two samples differ significantly, stating a null hypothesis and using standard deviations, degrees of freedom (n₁ + n₂ − 2) and the critical value at p = 0.05.

Explain natural selection as a consequence of overproduction of offspring, competition for resources, and the better-adapted individuals surviving to breed so that their alleles become commoner in the next generation, and describe stabilising, directional and disruptive selection.

Explain why allele frequencies shift under selection and by chance, comparing genetic drift, the founder effect and population bottlenecks, and why chance matters most in small populations.

Use the spread of antibiotic resistance in bacteria — random mutation, selection by the antibiotic, reproduction of survivors and horizontal gene transfer — as a worked example of natural selection.

Calculate allele, genotype and carrier frequencies with the Hardy–Weinberg equations (p + q = 1, p² + 2pq + q² = 1) and state the assumptions the model needs: a large population, random mating, and no selection, mutation or migration.