The big idea: Members of a species are not identical — they vary.
Natural selection can only work if some of that variation is heritable (it can be passed to offspring through genes).
Three things create this heritable variation: mutation, meiosis and sexual reproduction.
| Source | What it does |
|---|---|
| Mutation | A random change to DNA — the ONLY source of brand-new alleles |
| Meiosis | Shuffles existing alleles into new combinations in gametes |
| Sexual reproduction | Combines alleles from two different parents in the offspring |
Heritable vs not: Only heritable variation matters for evolution.
A trait you gain during life (like a suntan or big muscles from training) is not passed on — it cannot drive natural selection.
Bigger than point mutations — whole-chromosome changes: Heritable variation does not only come from tiny one-letter changes to DNA.
Chromosome-level changes can alter many genes at once, and these can also be inherited and drive evolution:
Polyploidy — an organism ends up with one or more whole extra sets of chromosomes (for example 3 or 4 sets instead of the usual 2). This is common in plants and can instantly create a new, larger or more vigorous variety.
Large chromosome rearrangements — big chunks of a chromosome are deleted, duplicated, flipped around or moved to a different chromosome, changing how many genes are present and how they are arranged.
- Polyploidy
- Having one or more complete extra sets of chromosomes (e.g. three or four sets instead of two). It is a heritable, chromosome-level source of variation, common in plants.
- Chromosome rearrangement
- A large change to the structure of a chromosome — a big section deleted, duplicated, inverted (flipped) or moved to another chromosome — that affects many genes at once.
Why this matters for evolution: Because chromosome-level changes are heritable and can change many genes at once, they can produce big new differences for natural selection to act on — and can even help create new species (especially polyploidy in plants).
So the sources of new heritable variation are: point mutations (single DNA changes) and chromosome-level changes (polyploidy, large rearrangements).
Natural selection in three steps: a varied population (mix of dark and light beetles) → a selection pressure (a predator) removes the unfavoured light variant → the favoured dark variant survives, reproduces and becomes common. The helpful allele rises in frequency over generations.
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A population produces more offspring than can survive, so individuals must compete for limited resources such as food, space and mates.
Because individuals vary, some happen to have features that make them better suited to the environment. These individuals are more likely to survive and reproduce — so they pass on their helpful alleles.
- Variation
- Differences between individuals of the same species.
- Heritable variation
- Variation caused by genes (alleles), so it can be passed to offspring.
- Adaptation
- An inherited feature that makes an organism better suited to its environment.
- Natural selection
- The process where individuals best suited to the environment survive and reproduce more, passing on their alleles.
- Allele frequency
- How common a particular allele is in a population.
- Evolution
- A change in the heritable characteristics (allele frequencies) of a population over many generations.
The logic, step by step
- Step 1 — Variation: there is heritable variation in a population.
- Step 2 — Competition: more offspring are produced than the environment can support.
- Step 3 — Selection: individuals with favourable alleles are better suited → survive and reproduce more (differential survival).
- Step 4 — Inheritance: they pass on those favourable alleles to their offspring.
- Step 5 — Evolution: over many generations the favourable allele becomes more common.
Selection acts, variation does not appear on demand: The environment does not create the helpful variation — it only selects from variation that is already present (mostly from earlier mutations).
Organisms do not change themselves on purpose to fit the environment.
Two kinds of variation: continuous vs discontinuous: When you look at a trait across a population, the variation falls into one of two patterns:
Continuous variation — a smooth range of values with no gaps, where any value in between is possible (for example height or body mass). It is usually controlled by many genes (polygenic) and is often also affected by the environment.
Discontinuous variation — falls into distinct, separate categories with no in-between values (for example ABO blood group — A, B, AB or O). It is usually controlled by one or a few genes.
| Continuous variation | Discontinuous variation | |
|---|---|---|
| Pattern | Smooth range, any value possible | Distinct, separate categories |
| Controlled by | Many genes (polygenic), often + environment | One or a few genes |
| Example | Height, body mass | ABO blood group (A, B, AB, O) |
| Graph shape | Bell-shaped / spread of values | Separate bars for each category |
Spotting the type from the data: If the data form a smooth spread (you can measure any value in between, like shell length or height), it is continuous.
If the data fall into a few clear groups with nothing in between (like blood groups), it is discontinuous. Always justify your choice by describing the shape of the distribution.
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How this is tested: Paper 1A often asks you to identify an outcome of natural selection (a favourable allele becomes more common) or to spot a source of variation.
Paper 2 is where the marks are: an Explain question giving a real scenario (antibiotic resistance, pesticide resistance, an environmental change) and asking you to explain how the population changes by natural selection. Examiners want the full chain: variation → selection → reproduction → allele more common.
IB-style question — antibiotic resistance in bacteria
A patient is treated with an antibiotic. At first it kills almost all the bacteria, but after repeated treatments the infection no longer responds. Explain how a population of bacteria becomes resistant to the antibiotic by natural selection. [3]
How to score all three marks
- Start with variation. Random mutation produces variation in the bacteria; by chance a few already carry an allele that makes them resistant to the antibiotic.
- Apply the selection pressure. When the antibiotic is used, the non-resistant bacteria are killed, but the resistant ones survive — this is differential survival.
- Reproduce and shift the frequency. The surviving resistant bacteria reproduce and pass on the resistance allele, so over generations the resistance allele becomes more common in the population — the population is now resistant.
Final answer
Mutation produces a resistant allele in a few bacteria; the antibiotic kills the non-resistant bacteria while the resistant survive; the survivors reproduce and pass on the allele, so it becomes more common in the population.
✓ Why the antibiotic did not 'create' resistance: The antibiotic did not make the bacteria resistant. The resistant allele was already there (from an earlier random mutation). The antibiotic only selected for it by killing everything else — a perfect example of natural selection in action.