Preview
IB DP Biology — Unit D: Continuity and Change | SL | D4.1 Natural Selection
📋 What You Will Learn
- Identify the four conditions required for natural selection to occur in a population
- Explain how heritable variation, differential survival, and reproductive success drive allele frequency change over generations
- Analyse population genetics data including allele frequencies, Hardy-Weinberg equilibrium, and evidence for selection
- Apply the Hardy-Weinberg principle to calculate expected genotype and allele frequencies in a population
- Evaluate evidence for natural selection from real biological examples including antibiotic resistance and industrial melanism
- Distinguish between directional, stabilising, and disruptive selection and predict their effects on phenotype distributions
🔍 Guiding Question How does the process of natural selection acting on heritable variation within populations lead to evolutionary change over time, and what mathematical tools allow us to detect and measure that change?
🌍 Why This Matters In 1928, Alexander Fleming noticed that a mould was killing bacteria on his petri dish. Less than a century later, antibiotic-resistant bacteria kill over 1.2 million people annually — a direct, measurable consequence of natural selection acting on bacterial populations in real time. Understanding natural selection is not just an academic exercise; it is the framework that explains why medicine, agriculture, and conservation biology work the way they do.
The Problem the Cell — and the Species — Had to Solve
Here is the fundamental tension at the heart of biology: environments change, but organisms are built for the environments of their ancestors. Every generation, some individuals survive and reproduce; others do not. The question evolution answers is deceptively simple — which individuals? And why?
Charles Darwin and Alfred Russel Wallace independently arrived at the same answer in the 1850s. The mechanism they described — natural selection — requires no foresight, no goal, and no designer. It requires only four conditions, all of which are observable and measurable. When those conditions are met, evolution is not just possible. It is inevitable.
📖 Key Definition — Evolution A change in the heritable characteristics of a population over successive generations, resulting from changes in allele frequencies within the gene pool.
📖 Key Definition — Natural Selection The process by which individuals with heritable traits that increase their fitness in a given environment survive and reproduce at higher rates than those without such traits, causing those traits to become more common in subsequent generations.
📖 Key Definition — Fitness The relative ability of an organism to survive and reproduce in its environment, measured by its contribution of alleles to the next generation's gene pool.
The Four Conditions for Natural Selection
Natural selection is not a single event — it is a logical argument with four premises. Remove any one premise, and the conclusion (evolutionary change) does not follow. Darwin's genius was recognising that all four premises are true of virtually every population on Earth.
Condition 1 — Overproduction (Struggle for Existence)
Every population has the potential to produce far more offspring than the environment can support. A single bacterium dividing every 20 minutes would, in theory, produce a mass exceeding the Earth's mass within days. Resources — food, space, mates, light — are finite. The inevitable result is competition: not every individual that is born will survive to reproduce.
Condition 2 — Heritable Variation
Individuals within a population differ from one another in measurable ways — in body size, colouration, enzyme efficiency, immune response, behaviour. Crucially, at least some of this variation must be heritable: encoded in the genome and passed from parent to offspring. Variation that arises purely from