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IB DP Biology — A: Unity and Diversity | SL | A4.1 Evolution and Speciation
📋 What You Will Learn
- Identify the four conditions required for natural selection to operate on a population
- Explain how heritable variation, differential survival, and reproduction drive allele frequency change over generations
- Analyse evidence for natural selection from real populations, including antibiotic resistance and industrial melanism
- Apply the concept of adaptation to explain how phenotypic traits increase fitness in specific environments
- Evaluate the relationship between natural selection acting on individuals and evolutionary change occurring at the population level
🔍 Guiding Question How does natural selection operating on individuals within a population lead to evolutionary change across generations?
🌍 Why This Matters Every time a patient fails to finish a course of antibiotics, natural selection quietly goes to work — the bacteria that survived the partial treatment are, by definition, the ones most resistant to the drug. Within weeks, a new population has emerged that the original antibiotic can no longer kill. Understanding natural selection is not just an academic exercise; it is the framework that explains one of the most urgent medical crises of the 21st century.
The Problem Evolution Had to Solve
Imagine a world where every offspring was an identical copy of its parent. Any environmental change — a new pathogen, a shift in climate, a new predator — would either kill every individual or kill none. There would be no survivors to carry forward a better-suited design. Life, in that world, would be extraordinarily fragile.
The solution that emerged over billions of years was not a plan — it was a consequence. Variation exists. Some variants survive better. Those variants reproduce more. Their traits become more common. This is natural selection: not a force that pushes life toward perfection, but a filter that removes what does not work well enough.
Before we can follow the mechanism step by step, we need precise definitions. These are not just vocabulary — they are the characters in the story.
📖 Key Definition — Evolution Evolution is a change in the heritable characteristics of a population over successive generations. It operates at the population level, not the individual level — an individual organism does not evolve during its lifetime.
📖 Key Definition — Natural Selection Natural selection is the process by which individuals with heritable traits that increase their fitness in a given environment tend to survive and reproduce at higher rates than individuals with less favourable traits, leading to those traits becoming more common in subsequent generations.
📖 Key Definition — Fitness Fitness is the relative ability of an organism to survive and reproduce in its environment, passing its alleles to the next generation. Fitness is always relative — it is measured against other individuals in the same population under the same environmental conditions.
📖 Key Definition — Adaptation An adaptation is an inherited characteristic — structural, physiological, or behavioural — that increases the fitness of an organism in its environment. Adaptations arise through natural selection acting on heritable variation over many generations.
The Four Conditions for Natural Selection
Natural selection is not magic, and it is not random. It is a logical consequence of four conditions that, when present simultaneously, make evolutionary change inevitable. Remove any