Evolutionary Biology: Mechanisms and Evidence
What you must understand
Evolution is change in allele frequencies in a population over generations. The mechanisms: mutation (source of new variation), recombination and gene flow (redistribute it), genetic drift (random change, strongest in small populations, including bottleneck and founder effects), and natural selection (non-random differential reproductive success). Selection acts on phenotypes, and only heritable variation responds. Types of selection: directional, stabilising, disruptive. Sexual selection explains traits that reduce survival but raise mating success.
Speciation: allopatric (geographic isolation) is the commonest route; sympatric (e.g. polyploidy in plants) occurs without isolation. Reproductive isolation - prezygotic (behavioural, temporal, mechanical) or postzygotic (hybrid inviability or sterility) - defines biological species. Evidence: fossil record, homologous structures (common ancestry) vs analogous structures (convergent evolution), vestigial features, embryology, molecular sequences, and observed evolution (antibiotic resistance, industrial melanism).
Traps
"Survival of the fittest" is misleading: fitness means reproductive success, not strength. Individuals do not evolve - populations do. Natural selection is not random, but mutation and drift are. Homology indicates common ancestry; analogy does not - a favourite distractor. Acquired traits are not inherited (Lamarckian trap). Antibiotic resistance arises from pre-existing resistant variants selected by drug use, not from bacteria "learning" to resist.
Worked example
A population has alleles A (p = 0.6) and a (q = 0.4). Hardy–Weinberg gives heterozygote frequency 2pq = 2 × 0.6 × 0.4 = 0.48. Rule: p + q = 1 and p² + 2pq + q² = 1; use it only when the question states no selection, drift, mutation or migration.