Evolution
This domain tests the mechanisms of evolutionary change and the evidence supporting them. Expect natural selection, adaptation, speciation, Hardy-Weinberg calculations and interpretation of antibiotic resistance.
Natural selection
Variation arises from mutation and meiosis. Selection acts on phenotypes, not directly on genotypes. Individuals with advantageous alleles survive and reproduce more, so allele frequencies change over generations. Evolution is a change in population allele frequencies, not a change in an individual during its lifetime. Evidence includes fossils, homologous structures, molecular sequence comparisons and observed resistance to antibiotics or pesticides.
Speciation
Speciation occurs when populations become reproductively isolated, so gene flow stops and divergence accumulates. Geographic isolation may lead to allopatric speciation; polyploidy can cause instant sympatric speciation in plants. Reproductive isolating mechanisms include behavioural, temporal, mechanical and post-zygotic barriers.
Hardy-Weinberg
For a gene with alleles A and a, p + q = 1 and p² + 2pq + q² = 1. The assumptions are a large population, random mating, no mutation, no migration and no selection.
Worked example
A recessive condition affects 1 in 10,000 people. Then q² = 0.0001, so q = 0.01 and p = 0.99. The carrier frequency is 2pq = 2 × 0.99 × 0.01 = 0.0198, about 1 in 50. This shows why a harmful recessive allele can persist at a relatively high carrier frequency while affected individuals are rare.
Traps
Individuals do not evolve; populations do. Dominant alleles do not automatically become more common. Use 2pq for carriers, not q². Remember that Hardy-Weinberg describes an idealised, non-evolving population.