AP® Biology review sheet from Aim for Five (aimforfive.com/bio/units/7/7-3)
Unit 7 · Topic 7.3
7.3 Artificial Selection
In artificial selection, humans choose which plants or animals breed, so humans become the selective pressure. Selective breeding has produced dog breeds, crops and livestock, and shows how much and how fast selection can change a population when variation is available.
Key terms
- artificial selection
- selective breeding
- domestication
- variation
How artificial selection works
Artificial selection (selective breeding) works like natural selection, except that people decide who reproduces. Breeders pick individuals with the traits they want, such as bigger fruit, gentler behavior or more milk, and breed only those. If the trait is heritable, each generation shifts toward it. Over many generations the population can look very different from its ancestors.
Darwin used artificial selection as a model for natural selection: if breeders could create such big changes in a few centuries, natural selection could do even more over millions of years.
Famous examples
- Wild cabbage (Brassica oleracea, called wild mustard in some textbooks): one wild plant species was bred into many vegetables by selecting different parts. Selecting for large terminal buds gave cabbage; for big flower clusters, broccoli and cauliflower; for leaves, kale; for side buds, Brussels sprouts; for a swollen stem, kohlrabi. They're all still the same species.
- Dogs: domesticated from wolves at least 15,000 years ago, then bred for herding, guarding, hunting and looks. Breeds range from Chihuahuas to Great Danes but can still interbreed.
- Corn (maize): domesticated from a wild grass called teosinte in Mexico about 9,000 years ago. Teosinte has a few small, hard kernels; selection produced large cobs with hundreds of soft kernels.
- Livestock and crops: dairy cows that produce far more milk, chickens that grow faster, and wheat with larger seed heads.
Effects on variation
Humans affect the variation in other species. Artificial selection usually pushes a trait strongly in one direction and narrows the gene pool, because breeders use only a few chosen individuals. That gives uniform crops and breeds, but it can also lower genetic diversity, making a crop vulnerable to a single disease (see 7.11) and concentrating harmful recessive alleles in some dog breeds.
It also shows the limits of selection: you can only select among variation that exists. Once all individuals carry the same alleles for a trait, further selection does nothing until new mutations arise.
Artificial selection is also a tool for experiments. Scientists breed lab populations of fast-reproducing organisms, such as fruit flies or fast-growing plants, choosing parents with the most extreme value of a trait each generation, and compare them with a control line bred from randomly chosen parents. If the selected line shifts and the control doesn't, the trait must be heritable, and the size of the shift shows how much usable genetic variation the population had.
Natural vs. artificial selection
| Feature | Natural selection | Artificial selection |
|---|---|---|
| Who 'chooses' | The environment (predators, climate, disease, mates) | Humans |
| Traits favored | Those that increase survival and reproduction in that environment | Those people want, even if they lower fitness in the wild |
| Needs heritable variation? | Yes | Yes |
| Speed | Often slower | Can be very fast, since breeding is tightly controlled |
Worked examples
Try each one yourself first, then open the solution.
- Example 1Calculator allowed
Describing selection data
Students breed fast-growing radish plants. In generation 1, mean plant mass is 2.0 g. Each generation they breed only the heaviest 10% of plants. By generation 5, mean mass is 2.6 g. A control line bred from randomly chosen plants stays at about 2.0 g. Calculate the percent change in the selected line and explain the result.
Show the solutionHide the solution
- Step 1: Percent change = (new − old) ÷ old × 100 = (2.6 − 2.0) ÷ 2.0 × 100 = 30%.
- Step 2: Describe: mean mass rose 30% over four rounds of selection in the selected line but stayed about the same in the control line.
- Step 3: Explain: plant mass varies and is partly heritable. Breeding only the heaviest plants increased the frequency of alleles linked to high mass.
- Step 4: The control line rules out a general cause like better growing conditions, since it was raised the same way without selection.
Answer: A 30% increase. Selecting the heaviest plants each generation shifted the population toward higher mass because the trait is heritable; the unchanged control shows selection, not growing conditions, caused the change.
Common mistakes
- Thinking broccoli, kale and cabbage are different species. They're varieties of one species produced by artificial selection.
- Saying artificial selection creates new alleles. It only changes the frequency of alleles already present; new alleles come from mutation.
- Assuming artificially selected traits help the organism survive in the wild. Many don't; humans pick traits useful to humans.
On the exam
- Data questions often show a trait changing over generations in a selected line versus a control. Describe the trend with numbers and identify the role of the control.
- Be ready to explain how artificial selection reduces genetic diversity and why that can make a population vulnerable.
Connected topics
Videos
Check yourself
4 questions on 7.3 Artificial Selection. Pick an answer to see if you got it, and why.
Broccoli, cabbage, kale and cauliflower all descend from a single species of wild mustard. Which process best explains the evolution of these different crops?
| Generation | Mean oil content, high-oil line (%) | Mean oil content, low-oil line (%) |
|---|---|---|
| 0 | 4.7 | 4.7 |
| 10 | 7.2 | 3.0 |
| 30 | 11.0 | 1.6 |
| 50 | 15.4 | 1.0 |
Experimental data: starting from one corn variety, breeders planted only the kernels with the highest oil content each generation in one line, and only the kernels with the lowest oil content in another line.
Which of the following best explains why the two lines became so different?
What was the average change in oil content per generation in the high-oil line from generation 0 to generation 10?
Students count the hairs on the leaves of 200 fast-growing plants. They cross-pollinate only the 20 hairiest plants, and the offspring have more hairs on average than the original 200. Which addition to the experiment would best show that the increase is due to selection on heritable variation?
0 of 4 answered