Key Idea: Evolution is the change in the heritable characteristics of a population over generations — at the DNA level, a change in allele frequencies in the gene pool. The driver is natural selection: there is heritable variation, more offspring are produced than can survive, the best-suited individuals survive and reproduce more, and their alleles become more common. When two populations are kept apart and diverge until they can no longer interbreed, you get a new species (speciation). This topic is heavily tested on Paper 1A (quick MCQs on definitions, evidence and outcomes) and on Paper 2 (explain natural selection, evidence, or how speciation produced two species).
🧬 What evolution is
Evolution acts on populations, not individuals. A single organism does not evolve — a population changes across generations as some alleles become more common and others rarer. The change must be heritable (passed on through genes). A feature an organism gains during its life (e.g. bigger muscles from exercise) is not inherited and does not drive evolution.
- Evolution
- The change in the heritable characteristics of a population over generations (a change in allele frequency).
- Population
- All the members of one species in the same area that can interbreed.
- Allele frequency
- How common a particular allele is in the gene pool (its proportion).
An individual does not evolve, and only heritable change counts. Define evolution as a change in heritable characteristics of a population over generations.
🔎 Evidence for evolution
We cannot watch most of evolution happen, so we use several independent lines of evidence that all point to common ancestry with change. The strongest is molecular — comparing DNA base sequences (or proteins): the more similar the sequences, the more recently two species shared an ancestor.
| Evidence | What it shows |
|---|---|
| Homologous structures | same basic plan, different function (e.g. limb bones) → divergent evolution from a common ancestor |
| Selective (artificial) breeding | humans rapidly changing crops/animals shows traits can shift over generations |
| Fossils | show forms changing through time / intermediate (transitional) forms |
| Biogeography | related species cluster where ancestors lived and got isolated |
| DNA / molecular homology | similarity of base sequences measures how closely species are related — strongest evidence |
Homologous = same plan, different job → divergent evolution. Analogous = similar job, different plan → convergent evolution (NOT evidence of close relatedness).
🎯 Variation and natural selection
Individuals of a species vary, but only heritable variation matters for evolution. The sources of heritable variation are mutation, meiosis and sexual reproduction. Populations overproduce offspring, so individuals compete for limited resources. Those best suited survive and reproduce more (differential reproductive success), passing on their alleles — so favourable allele frequencies rise. The environment selects the helpful variation; it does not create it.
| Step of natural selection | What happens |
|---|---|
| Variation | heritable variation exists (from mutation, meiosis, sexual reproduction) |
| Overproduction & competition | more offspring are made than can survive; they compete for resources |
| Selection | individuals with favourable alleles survive and reproduce more |
| Allele-frequency change | favourable alleles become more common over generations → evolution |
The environment does not make the helpful variation appear — mutation does, at random. Selection only acts on variation that is already there.
🧩 Speciation and reproductive isolation
A species is a group that can interbreed and produce fertile offspring. Speciation is the formation of a new species from an existing one, and it needs two steps: (1) reproductive isolation stops gene flow between two populations, then (2) differential selection + mutation make their gene pools diverge until they can no longer interbreed to produce fertile offspring.
| Type | How the populations are separated |
|---|---|
| Allopatric speciation | a geographic barrier (river, mountain, sea) physically splits the populations |
| Sympatric speciation | populations diverge without a geographic barrier (e.g. behavioural / timing isolation) while living in the same area |
Barrier → reproductive isolation (no gene flow) → mutation + different selection → divergence → can no longer interbreed = two species.
🌿 Patterns of speciation and diversification
Repeated speciation leaves recognisable patterns. In adaptive radiation, one ancestral species spreads into many empty niches (different foods/habitats); natural selection favours different features in each niche, and the populations become separate species — a rapid burst of biodiversity (e.g. one finch → many island finches).
| Pattern | What it describes |
|---|---|
| Adaptive radiation | one ancestor rapidly produces many species filling different niches (a source of biodiversity) |
| Gradual speciation | a new species appears by the slow build-up of small heritable changes over a long time |
| Abrupt speciation | a new species appears suddenly (a short timescale) |
Gradual vs abrupt describes how fast a species forms — both still rely on natural selection acting on variation, not a different process.
✍️ Worked examples
IB-style question — convergent evolution
Sharks (a fish) and dolphins (a mammal) both have a streamlined body and dorsal fin, but evolved from different ancestors. Explain why these are analogous structures. [3]
Model answer:
Each lineage had heritable variation in body shape, and both live in the same kind of environment (open water).
Natural selection favoured the more streamlined individuals in each lineage separately, so each evolved a streamlined body independently.
Because the feature was not inherited from a shared streamlined ancestor, the structures are analogous (convergent evolution), not homologous.
Both faced the same open-water selection pressure; natural selection favoured streamlined bodies independently in each lineage, so the similarity arose separately (convergent evolution) rather than from a common streamlined ancestor.
IB-style question — antibiotic resistance
A patient takes an antibiotic. At first nearly all the bacteria die, but later the infection returns and the antibiotic no longer works. Explain this using natural selection. [3]
Model answer:
Random mutation had already produced variation, so a few bacteria carried a resistance allele before the drug was used.
The antibiotic is a selection pressure: it kills the non-resistant bacteria but the resistant ones survive.
The survivors reproduce and pass on the resistance allele, so its frequency rises — the population becomes resistant.
Mutation produced a resistant allele in a few bacteria; the antibiotic kills the non-resistant ones while the resistant survive and reproduce, so the resistance allele frequency increases. The antibiotic selected for resistance — it did not create it.
IB-style question — a new river splits a population
A single population of lizards lives across a valley. A new river forms and divides it into two groups. Explain how this could lead to two separate species. [4]
Model answer:
The river is a geographic barrier, so the two groups are reproductively isolated and there is no gene flow between them (allopatric).
Mutation produces different new alleles in each group, and the two sides may face different selection pressures.
Natural selection makes the two gene pools diverge over many generations.
Eventually they have changed so much that they can no longer interbreed to produce fertile offspring — so they are now two separate species.
The river reproductively isolates the groups (no gene flow); mutation gives different alleles and different selection pressures make the gene pools diverge; eventually they can no longer interbreed to produce fertile offspring, so two species have formed.
✅ Quick self-check
Tap each card to check yourself.
Define evolution. The change in the heritable characteristics of a population over generations — a change in allele frequency in the gene pool.
Homologous vs analogous structures? Homologous = same basic plan, different function (divergent evolution, evidence of common ancestry). Analogous = similar function, different plan (convergent evolution).
What is the strongest evidence for evolution? Molecular evidence — comparing DNA base sequences (or proteins). The more similar the sequences, the more recently the species shared an ancestor.
What are the sources of heritable variation? Mutation, meiosis and sexual reproduction. The environment selects variation but does not create it.
What two steps does speciation need? Reproductive isolation (no gene flow) first, then divergence by mutation + differential selection until the populations can no longer interbreed to produce fertile offspring.
What is adaptive radiation? The rapid evolution of many new species from one ancestor that spreads into different empty niches — a major source of biodiversity.
Exam Tips
- Define evolution precisely: a change in HERITABLE characteristics of a POPULATION over generations (allele frequencies).
- Individuals do not evolve — populations do. Don't say a single organism 'evolved' a feature in its lifetime.
- Homologous = same plan/different job (divergent); analogous = same job/different plan (convergent). Don't swap them.
- Selection acts on variation; it does NOT create it. Mutation (random) makes the variation first.
- For natural selection name all the steps: variation → overproduction/competition → survival & reproduction → allele-frequency change.
- For speciation give the ORDER: barrier → reproductive isolation (no gene flow) → divergence → can no longer interbreed = two species.
- Species test = can interbreed AND produce FERTILE offspring. 'Fertile' is the word examiners look for.