📌Definition Table

Term	Definition
Hardy-Weinberg equilibrium	Condition where allele and genotype frequencies remain constant in a population.
Evolutionary change	Alteration of allele frequencies across generations.
Punctuated equilibrium	Long periods of stasis interrupted by rapid evolutionary events.
Gradualism	Slow, steady accumulation of evolutionary changes.
Stabilising selection	Selection favouring intermediate phenotypes.
Directional selection	Selection favouring one extreme phenotype.

📌Introduction

Populations are not static: they evolve under selective pressures, genetic drift, gene flow, and mutation. However, in the absence of these forces, populations remain in Hardy-Weinberg equilibrium. Evolutionary change can be gradual or punctuated, and selection can stabilise, diversify, or direct populations towards new adaptive peaks. These patterns provide a framework for understanding both microevolutionary changes and macroevolutionary trends in the fossil record

📌 Hardy-Weinberg Equilibrium

• States that allele frequencies remain constant if no evolutionary forces act.
• Assumptions: large population, random mating, no mutation, no migration, no selection.
• Provides a null model for studying evolution.
• Deviations reveal which evolutionary processes are at work.
• Useful in population genetics, epidemiology, and conservation biology.

🧠 Examiner Tip: Show ability to apply Hardy-Weinberg calculations. Simply defining it without calculation limits marks.

📌 Types of Natural Selection

• Stabilising selection: reduces variation, favours intermediate traits.
• Directional selection: shifts population towards one extreme (e.g., antibiotic resistance).
• Disruptive selection: favours both extremes, may lead to speciation.
• Balancing selection maintains multiple alleles (e.g., sickle-cell trait).
• Selection modes shape population equilibrium and change.

🧬 IA Tips & Guidance: Students could collect data on variation in a measurable trait (e.g., seed size) and test if observed distribution fits stabilising or directional selection.

📌 Gradualism vs Punctuated Equilibrium

• Gradualism: Darwin’s view — small changes accumulate over long time.
• Punctuated equilibrium: Eldredge & Gould — long stasis interrupted by rapid speciation.
• Fossil record supports both: some lineages show stasis, others rapid bursts.
• Environmental shifts (mass extinctions, climate change) often trigger punctuation.
• Together, they reflect multiple evolutionary tempos.

🌐 EE Focus: An EE could examine fossil evidence for tempo of evolution, e.g., comparing trilobite stasis vs mammalian radiation post-dinosaur extinction

📌 Microevolution and Macroevolution

• Microevolution: allele frequency shifts within populations.
• Macroevolution: large-scale patterns (speciation, extinction).
• Same processes drive both, but at different scales.
• Links population genetics with paleobiology.
• Demonstrates continuity of evolutionary theory.

❤️ CAS Link: Students could create interactive models showing Hardy-Weinberg dynamics or demonstrate selection types with real-life analogies for school outreach.

📌 Long-Term Evolutionary Dynamics

• Evolution is not linear but shaped by interactions of drift, selection, flow, mutation.
• Extinctions and radiations punctuate long-term patterns.
• Coevolution drives reciprocal adaptations.
• Evolutionary arms races explain rapid trait changes (predator-prey, host-pathogen).
• Stability and change are complementary features of evolution.

🔍 TOK Perspective: Models like Hardy-Weinberg and punctuated equilibrium simplify reality. TOK issue: How far can simplified models capture complex natural processes without distorting understanding?

📝 Paper 2: Questions may involve solving Hardy-Weinberg problems, analysing selection graphs, or evaluating gradualism vs punctuated equilibrium with examples.