Natural selection — IB Diploma Biology HL
IB Biology natural selection: variation, overproduction, selection pressures, fitness, sexual selection, Endler's guppies and Hardy–Weinberg (HL).
Revision notes
7 short notes, in the order of the specification. Each one in short:
Natural selection is the mechanism that drives evolutionary change. Individuals with heritable traits that suit their environment survive and reproduce more, so those traits become commoner over generations. It has operated for billions of years, producing the biodiversity of life. Characteristics acquired during a lifetime are not encoded in genes, so they are not inherited.
Variation is the raw material of natural selection. Mutation generates new alleles by changing the base sequence of a gene. Sexual reproduction generates new combinations of alleles through crossing over and random orientation in meiosis, and random fertilisation. Without variation, all individuals would be equally fit and there would be nothing to select.
Natural selection is promoted by overproduction of offspring and competition for limited resources. Populations produce more offspring than the carrying capacity allows, so individuals compete for food, water and space. Abiotic factors such as extreme temperatures also act as selection pressures. Individuals with higher fitness survive and reproduce more, passing on their alleles.
Sexual selection is a selection pressure caused by competition for mates. Physical and behavioural traits that signal overall fitness, such as the plumage of male birds of paradise, increase mating success. Endler's guppy experiments controlled predation to show the balance between sexual selection for bright colour and natural selection for camouflage.
A gene pool consists of all the genes and their different alleles present in a population. Geographically isolated populations have no gene flow between them, so their allele frequencies diverge. Natural selection changes allele frequencies because individuals with fitter heritable traits reproduce more. Combining Darwin's theory with genetics is called neo-Darwinism.
Directional selection favours one extreme of a trait, stabilizing selection favours the intermediate and disruptive selection favours both extremes. All three result in a change in allele frequency. Artificial selection is the deliberate choice by humans of individuals with desirable traits for breeding. Antibiotic resistance is natural selection, not artificial selection.
The Hardy–Weinberg equation, p² + 2pq + q² = 1, predicts genotype frequencies from the two allele frequencies p and q, where p + q = 1. It holds only when a population is in genetic equilibrium: large, randomly mating, with no mutation, migration or selection. Frequencies that do not fit show a condition is not met.
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Quick check questions
What generates new alleles?
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Mutation: a random change in the base sequence of a gene.
Why are acquired characteristics not inherited?
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They are not encoded in the base sequence of genes.
What is fitness?
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The survival value and reproductive potential of a genotype.
HL only What does 2pq represent in the Hardy–Weinberg equation?
Show answer
The frequency of heterozygotes.
HL only Is antibiotic resistance an example of artificial selection?
Show answer
No, it is natural selection; nobody chose which bacteria would reproduce.
Frequently asked questions
How does natural selection lead to evolution?
Natural selection leads to evolution by making useful heritable traits more common over generations. Individuals vary, more offspring are produced than can survive, and those with better-adapted traits survive and reproduce more. They pass their alleles to offspring, so the allele frequencies of the population change over many generations.
What is the difference between Darwinism and Lamarckism?
Darwinism explains evolution by natural selection acting on heritable variation, while Lamarckism claimed that characteristics acquired during life were passed on. Acquired characteristics are not encoded in the base sequence of genes, so they cannot be inherited. Darwin's theory replaced Lamarckism, an example of a paradigm shift in biology.
What did Endler's guppy experiment show?
Endler's experiment showed that male guppy colour is a balance between sexual and natural selection. In ponds with no predator or only the weak predator Rivulus, males evolved more spots because females prefer colourful mates. In ponds with the dangerous pike cichlid, males evolved fewer spots because bright males were eaten.
What are the five Hardy–Weinberg conditions?
HL only The five Hardy–Weinberg conditions are a very large population, random mating, no mutation, no migration and no natural selection. When all five hold, allele and genotype frequencies stay constant between generations. If genotype frequencies do not fit p² + 2pq + q² = 1, at least one condition is not being met.
Written and checked against the IB Biology HL specification · Updated October 2026