# Artificial selection

> CIE A-Level Biology · 9700 Unit 18: Selection and Evolution
> Source: https://www.owlsprep.com/study/cie-9700-u18-artificial-selection/

This sub-topic explains how intentional human selection of desirable traits changes populations of domesticated plants and animals. You will compare artificial selection to natural selection, explore core examples, and identify key risks of selective breeding.

**Prerequisites:** [Natural selection](https://www.owlsprep.com/study/cie-9700-u18-natural-selection/); [Genetic variation and inheritance](https://www.owlsprep.com/study/cie-9700-u16-inheritance/)

## Learning objectives

- Distinguish between artificial selection and natural selection
- Describe the process of artificial selection in crop and animal breeding
- Explain the genetic consequences of artificial selection
- Evaluate the benefits and risks of artificial selection

## Key Definitions and Comparison to Natural Selection

**Artificial selection** — The process by which humans intentionally select organisms with desirable phenotypic traits to reproduce, producing offspring with improved or desired characteristics.

*Notation:* also called selective breeding

*Example:* Breeders selecting cows that produce the highest volume of milk for mating.

Artificial selection shares the same fundamental basis as natural selection: both act on existing genetic variation in a population, favouring individuals with advantageous heritable traits. The key difference is in the selective agent and the goal of selection.

| Feature | Natural Selection | Artificial Selection |
| --- | --- | --- |
| Selective agent | Natural environmental pressures | Humans intentionally selecting traits |
| Goal | Increase survival/reproduction in the wild | Produce traits desirable for human use |
| Rate of change | Slow, over thousands of generations | Fast, dramatic changes in hundreds of generations |

**Check your understanding**

Test your understanding:

1. Which of the following is a defining feature only of artificial selection?

   - Selection acts on genetic variation
   - Desirable traits are heritable
   - Humans intentionally choose which individuals breed
   - Trait frequencies change over generations

   *Why:* All options except C are true of both processes. Only artificial selection is driven by intentional human choice.

**Worked example:** A student claims natural selection and artificial selection are completely different processes. Evaluate this claim.

1. Step 1: Identify similarities between the two processes
2. Both rely on existing genetic variation, only act on heritable traits, and change allele frequencies in the population over time.
3. Step 2: Identify key differences
4. Natural selection is driven by environmental pressures that improve organism survival/reproduction, while artificial selection is driven by human choice of traits beneficial to humans.
5. Step 3: Form a conclusion
6. The claim is incorrect. Both processes share the same underlying mechanism of selection, but differ in the agent and goal of selection.

> **Exam tip:** In CIE exams, when asked to compare artificial and natural selection, always include both similarities and differences for full marks.

## Artificial Selection in Crop Plants

Modern crop plants are the result of thousands of years of artificial selection from wild ancestors. Two core examples commonly tested in CIE are bread wheat and brassica crops.

**Inbreeding** — Breeding between closely related individuals, used to 'fix' desirable traits in a population by making strains homozygous for the trait of interest.

**Worked example:** Describe how modern bread wheat (*Triticum aestivum*) was produced by artificial selection.

1. Step 1: Select initial desirable traits from wild populations
2. Early farmers selected wild wheat plants with larger grains and non-shattering heads (that do not drop grains when ripe) for easier harvesting.
3. Step 2: Repeat selection over generations
4. Only seeds from selected plants were planted, so average grain size increased gradually over hundreds of generations.
5. Step 3: Select for hybrid polyploid plants
6. Natural hybridisation between wheat species produced polyploid plants with even larger grains, which farmers selected for further breeding to produce modern hexaploid bread wheat.

Another well-documented example comes from wild *Brassica oleracea*. Artificial selection for different traits produced multiple distinct crops: kale (large leaves), cabbage (large terminal buds), broccoli (large flower buds), and cauliflower (large clustered flower heads).

> **Exam tip:** Be prepared to describe at least one plant example of artificial selection for 3-4 mark questions.

## Artificial Selection in Domestic Animals

Domestic animals are selectively bred for traits desirable for agriculture, companionship, or working purposes. Two common examples tested are dairy cattle and purebred domestic dogs.

**Worked example:** Explain how modern high-yield dairy cattle are produced by artificial selection.

1. Step 1: Select individuals with desirable traits
2. Female cows with the highest milk yield are selected. Bulls are selected based on the milk yield of their female offspring, since bulls cannot produce milk themselves.
3. Step 2: Controlled breeding
4. Selected cows and bulls are mated, usually via artificial insemination to spread high-quality bull genetics across many farms.
5. Step 3: Repeat selection over generations
6. Over many generations of continuous selection, average milk yield per cow has increased by more than four times compared to original 19th century cattle populations.

All modern domestic dog breeds are another extreme example of artificial selection. All breeds are descended from wild grey wolves, selected over 10,000 years for traits ranging from size and coat colour to temperament and working ability.

> **Exam tip:** When describing dairy cattle selection, remember bulls do not produce milk, so their genetics are tested via their female offspring – this is a common marking point.

## Risks and Impacts of Artificial Selection

Artificial selection has delivered enormous benefits for human food security, but it also carries significant genetic and ecological risks:

- Reduced genetic diversity: only a small number of individuals with desirable traits are used for breeding, so most original genetic variation is lost
- Inbreeding depression: close breeding increases homozygosity, exposing harmful recessive alleles that cause genetic disorders
- Increased vulnerability: low genetic diversity reduces the ability of populations to adapt to new diseases or climate change
- Loss of wild genetic resources: natural habitats of crop/animal wild ancestors are often lost, erasing a valuable reservoir of genetic variation

> **warning**
>
> You may be asked to discuss ethical issues, for example the high incidence of genetic conditions in purebred dogs (such as hip dysplasia in German Shepherds or breathing problems in flat-faced breeds).

**Worked example:** Explain why purebred dogs have a higher rate of genetic disorders than mixed breed dogs.

1. Step 1: Recall how purebred dogs are maintained
2. Purebred dogs are produced by strict artificial selection to meet breed standards, requiring inbreeding between closely related individuals within the breed.
3. Step 2: Explain the genetic mechanism
4. Inbreeding increases homozygosity across the genome. Harmful recessive alleles are normally hidden by dominant wild-type alleles in heterozygotes, but are expressed much more often in homozygous inbred individuals.
5. Step 3: Conclusion
6. Intentional inbreeding to maintain breed traits leads to increased expression of harmful recessive alleles, causing higher rates of genetic disorders.

> **Exam tip:** When evaluating artificial selection, always include both benefits (higher yield, food security) and risks (reduced diversity) for full marks.

## Common pitfalls

- **Wrong:** Stating that artificial selection creates new mutations for desirable traits.
  - Why it fails: Artificial selection acts on existing genetic variation; it does not cause new mutations to arise.
  - Correct: Explain that artificial selection increases the frequency of already existing desirable alleles in a population over generations.
- **Wrong:** Only describing differences between artificial and natural selection, forgetting similarities.
  - Why it fails: CIE mark schemes almost always award marks for similarities when a comparison is requested.
  - Correct: Always include at least one similarity (both act on heritable variation, change allele frequencies) in comparison answers.
- **Wrong:** Claiming inbreeding creates new harmful mutations.
  - Why it fails: Inbreeding does not generate new harmful alleles, it just increases the chance existing harmful recessive alleles are expressed.
  - Correct: Explain that inbreeding increases homozygosity, exposing harmful recessive alleles that were previously masked in heterozygotes.
- **Wrong:** Stating all artificial selection is unethical.
  - Why it fails: CIE expects balanced evaluation, not absolute judgement. Artificial selection has enabled massive gains in global food security.
  - Correct: Discuss both benefits (higher crop yield, improved produce quality) and harms (inbreeding depression, reduced biodiversity) when evaluating artificial selection.
- **Wrong:** Forgetting to mention that bull genetics for milk yield are tested via offspring.
  - Why it fails: This is a common 1-mark marking point in questions about dairy cattle selection.
  - Correct: Always note that bulls do not produce milk, so their desirable genetics are confirmed by checking the milk yield of their female offspring.

## Cheatsheet

| Key Concept | Core Detail | Common Exam Point |
| --- | --- | --- |
| Definition | Intentional human selection of desirable heritable traits for breeding | Always compare similarities and differences to natural selection |
| Plant example: Bread wheat | Selected for large grains, non-shattering heads from wild ancestors | Mention selection over multiple generations |
| Animal example: Dairy cattle | Select cows for high yield, test bulls via offspring yield | Artificial insemination spreads desired genetics |
| Key risks | Reduced diversity, inbreeding depression, increased disease vulnerability | Inbreeding exposes harmful recessive alleles |
| Key similarity | Both act on existing heritable variation, change allele frequencies | Include this in all comparison questions |

## What's next

Artificial selection is a core application of selection and evolution principles you have learned in this unit, forming the foundation of modern agriculture and connected to modern genetic technologies. It demonstrates how quickly selection can change populations when driven by intentional choice, compared to slower natural selection. Understanding the risks of reduced genetic diversity from artificial selection also connects to conservation biology, where protecting genetic variation in wild and domestic populations is a key priority for long-term food security and ecosystem health. Build your understanding of connected topics to prepare fully for your CIE A-Level exam with the links below.

- [Natural Selection](https://www.owlsprep.com/study/cie-9700-u18-natural-selection/)
- [Speciation](https://www.owlsprep.com/study/cie-9700-u18-speciation/)
- [Isolating mechanisms](https://www.owlsprep.com/study/cie-9700-u18-isolating-mechanisms/)

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