Topic 18.3 · Core + Supplement
Selective Breeding and Artificial Selection
Selective breeding is natural selection with the environment replaced by a human decision — and the most common way to lose marks on it is describing a single cross as though that were the whole process. Cambridge wants the repeated cycle, run over many generations, not one round of breeding.
The Cambridge-required sequence — Core
Selective breeding, also called artificial selection, happens when humans decide which organisms reproduce because those organisms show a characteristic humans consider desirable. The process runs as a cycle:
- Select individuals that already show the desired feature.
- Cross, or breed, those selected individuals.
- Examine their offspring and select the ones that show the desired feature most strongly.
- Breed those selected offspring together as the next generation’s parents.
- Repeat the selection and breeding cycle over many generations.
Step five is not an optional flourish — it is the part of the definition that separates selective breeding from a single lucky cross. Gradually, repeating the cycle produces plants or animals in which the desired feature is far more strongly and consistently represented than in the original population.
Applying it to crops and livestock
Cambridge expects the process applied to crop plants, domesticated animals, and unfamiliar contexts a question might supply. Typical crop targets include higher yield, disease resistance, drought tolerance, larger edible structures, or a particular taste or quality. Typical livestock targets include greater milk yield, greater meat yield, faster growth, or another clearly stated production or husbandry trait. When a question names a specific target, use it: “select the plants producing the highest seed yield each generation” earns more credit than a vague “select the best plants,” because it shows you understand what “desired feature” means in that particular context.
Two separate lines can also be crossed for different desirable features, with offspring then selected for showing the preferred combination of both — the underlying cycle does not change, only the number of starting traits being combined.
What selective breeding is not
Selective breeding is not genetic engineering. It works entirely by controlling reproduction — deciding which existing organisms breed with which — and it never directly inserts or edits DNA. Intensive selection can, as a side effect, reduce genetic diversity or increase the frequency of inherited problems in a bred population; that is real and biologically valid, but it sits outside what this syllabus point explicitly requires you to state.
Natural selection vs artificial selection — Supplement
Both processes depend on heritable variation and on some individuals contributing more offspring than others — but they differ in who or what is doing the selecting, and in which traits get favoured as a result.
| Feature | Natural selection | Artificial selection |
|---|---|---|
| Selector | Environmental selection pressures | Humans |
| Criterion | Features that improve survival or reproductive success in that environment | Features humans want |
| Choice of parents | Not deliberately controlled by humans | Deliberately chosen by humans |
| Goal | No predetermined human breeding target | A human-selected outcome |
| Shared mechanism | Heritable variation and differential reproduction | Heritable variation and differential reproduction |
What both processes have in common is worth stating just as clearly as what separates them: variation among individuals, at least some of it heritable, unequal contribution to the next generation, repetition across many generations, and a resulting change in how common certain traits become in the population. The difference is entirely about the source of the selection pressure, not about whether a genuine biological mechanism of selection and inheritance is at work.
One boundary worth being careful with: do not default to “artificial selection is always fast, natural selection is always slow” as the defining contrast. Rates vary considerably in both, and the current syllabus does not require speed as the distinguishing feature — who is doing the selecting is. It is also worth remembering that a trait humans favour does not have to be the trait that would actually give the greatest advantage in the wild — a decorative feature or an unusually large fruit can be exactly what a breeder wants, while offering the organism itself no survival benefit at all.
For the full step-by-step mechanism of how the environment does the selecting instead of a human breeder, see natural selection, which also covers antibiotic resistance as a worked example of the same underlying logic running without any human decision involved.