CORE IDEAS, Topic 4 - Biological Evolution

Q: What does CORE IDEAS, Topic 4 - Biological Evolution cover? A: Develop a rigorous understanding of variation, natural selection, speciation, and phylogenetics so you can write high-mark evolution essays and analyse unfamiliar data in the 2026 H2 Biology papers.

Why evolution closes the core sequence

• Synthesis requirement: Evolution questions integrate genetics, variation, ecology, and classification.
• Data-heavy: Papers 2 and 3 often present unfamiliar fossil, molecular, or statistical data sets for interpretation.
• Conceptual rigour: You must distinguish microevolution vs macroevolution, species concepts, and evidence sources with precision.

Core content highlights

• Variation sources (mutation, meiosis, reproduction)
• Natural selection mechanics and evolutionary outcomes
• Evidence lines: fossil record, anatomical and molecular homologies, biogeography
• Species concepts and speciation routes
• Phylogeny, cladistics, and modern classification
• Preservation of genetic variation and population-level thinking

Concept 1: Variation as evolution’s raw material

• Genetic mutation: Point mutations, insertions/deletions, gene duplication. Emphasise that mutation is random with respect to fitness; environment selects, not causes.
• Meiosis: Crossing-over, independent assortment; illustrate with diagrams showing recombinant chromatids.
• Sexual reproduction: Random fertilisation increases allele combinations.
• Environmental influence: Phenotypic plasticity (e.g. Himalayan rabbit fur colour) vs genetic adaptation-link to exam tasks requiring distinction.

Discuss how harmful recessive alleles can persist in heterozygotes and how balanced polymorphism (e.g. sickle cell allele vs malaria) maintains variation.

Concept 2: Natural selection and population thinking

State Darwin’s postulates: variation, heredity, differential survival, and non-random survival. Explain selection types:

• Directional: Peppered moth melanism, antibiotic resistance.
• Stabilising: Human birth mass.
• Disruptive: African finches with distinct beak sizes.

Define fitness contextually (relative reproductive success). Emphasise population-level change: allele frequency shifts over generations.

Worked exam-style analysis