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

• Paper 1 (1 h, 15%): MCQs on variation sources, selection, and species concepts.
• Paper 2 (2 h, 30%) and Paper 3 (2 h, 35%): Data-based and essay prompts link genetics, variation, evidence for evolution, and classification.
• Paper 4 (2 h 30 min, 20% split across Planning/MMO/PDO/ACE): Evolution datasets can appear in data-handling tasks; population-level reasoning is expected.
• Core Idea 4 scope (SEAB 9477, first exam 2026): Variation from mutation/meiosis/sexual reproduction, natural selection as a population process, evidence lines, species concepts, allopatric/sympatric speciation, phylogeny and classification using molecular sequences.

Core content highlights

• Variation sources and why populations-not individuals-evolve
• Natural selection driven by environmental factors
• Evidence lines: fossils, anatomical and molecular homologies, biogeography
• Species concepts and allopatric/sympatric speciation routes
• Phylogeny, classification, and use of genome sequences
• Preservation of genetic variation (including harmful recessive alleles)

Concept 1: Variation as evolution’s raw material

• Genetic mutation: Point mutations, insertions/deletions, and gene duplication generate new alleles. Mutation is random with respect to fitness; selection acts on the outcomes.
• Meiosis: Crossing-over and independent assortment create recombinant gametes.
• Sexual reproduction: Random fertilisation increases allele combinations.
• Environmental influence: Phenotypic plasticity (e.g. Himalayan rabbit fur colour) vs inherited adaptation-distinguish clearly in data-based questions.

Harmful recessive alleles can persist in heterozygotes; balancing situations (e.g. heterozygote advantage) help maintain variation in populations.

Concept 2: Natural selection and population thinking

State Darwin’s postulates: variation, heredity, differential survival, and non-random survival. Selection acts on individuals, but allele frequencies shift at the population level (the smallest unit that can evolve). Explain selection types: