"Artificial selection" is the first evolution term most students meet, because it is the one Darwin used to open On the Origin of Species: breeders had been changing animals and plants for thousands of years by choosing which ones reproduced, and he argued that nature does the same thing without a breeder. This page gives the definition, the four-step method, twenty-five examples (several of which surprise students, like the one plant behind six vegetables), the problems the method causes, and the comparison table with natural selection and genetic modification that every exam asks for. The printable has the method, the examples and the table on two pages.
Download the artificial selection sheet (PDF) (the definition, the four-step method, 25 examples with the trait selected, the problems, the comparison table with natural selection and genetic modification, and ten practice questions with answers; two pages, free).
The definition
Artificial selection (selective breeding) is the process by which humans choose which individuals of a species reproduce, so that the traits they want become more common over generations. The word artificial means "made by people"; the selecting agent is a human rather than the environment. Darwin coined the term in 1859 to contrast it with natural selection, in which the environment does the choosing, and he used the pigeon fanciers of London, who had bred a single wild rock dove into dozens of extraordinary varieties, as his proof that selection can reshape a species.
The method works because of two facts that are true of every species: individuals vary (some cows give more milk than others), and much of that variation is inherited (high-milk cows tend to have high-milk daughters). Choose the parents, and the next generation shifts.
How selective breeding works
- Decide the trait: milk yield, size, sweetness, tameness, colour, disease resistance, speed.
- Choose the parents that show the trait most strongly, and breed only from them.
- Select among the offspring the ones that show the trait most strongly, and breed only from them.
- Repeat for many generations. Each round moves the average a little; over decades or centuries the change is enormous.
A breeder may cross two varieties to combine traits (a disease-resistant wheat with a high-yielding one), then select among the offspring for both. The whole process uses only the variation already in the species, which is why it is slow and why it eventually runs out of variation to select.
Twenty-five examples
| Organism | Bred from | Trait selected | Notes |
|---|---|---|---|
| Dogs (every breed) | the grey wolf, from about 15,000 years ago | tameness first, then size, coat, shape, behaviour (herding, retrieving, guarding) | the most varied species on Earth; a Chihuahua and a Great Dane are the same species |
| Broccoli, cabbage, cauliflower, kale, kohlrabi, Brussels sprouts | one wild mustard, Brassica oleracea | flower buds (broccoli, cauliflower), leaves (cabbage, kale), stem (kohlrabi), side buds (sprouts) | six vegetables from one plant: the classic exam example |
| Maize (corn) | teosinte, a Mexican grass with five to twelve hard kernels | kernel number, size, softness, a single cob | about 9,000 years of selection |
| Wheat | wild einkorn and emmer | grains that stay on the stalk, larger seeds, then dwarf stems in the 1960s | the dwarf varieties of the Green Revolution doubled yields |
| Rice | wild rice of Asia | yield, grain size, non-shattering | IR8 "miracle rice", 1966 |
| Bananas | wild bananas full of hard seeds | seedless, sweet fruit | the Cavendish is sterile and grown from cuttings: every one is a clone |
| Seedless grapes and watermelons | seeded ancestors | seedlessness | watermelons are made seedless by breeding triploid plants |
| Strawberries | small wild strawberries | size, sweetness | the garden strawberry is a cross of a Chilean and a North American species, 1750s |
| Apples | Central Asian wild apples | size, sweetness, storage | every named apple is grafted, a clone of one original tree |
| Tomatoes | a pea-sized Andean wild tomato | size, flesh, colour | |
| Carrots | a thin, pale, bitter wild root | orange colour (the Netherlands, 1600s), size, sweetness | |
| Sugar beet | a leafy beet of 4 percent sugar | sugar content, now about 20 percent | 1700s to 1800s, one of the fastest documented changes |
| Dairy cows (Holstein) | aurochs, via early domestic cattle | milk yield: about 10,000 litres a year, ten times a wild cow's | |
| Belgian Blue cattle | European cattle | a mutation that doubles muscle | calves often need caesarean delivery |
| Broiler chickens | the red junglefowl of Asia | growth rate: market weight in six weeks, four times faster than in 1950 | leg and heart problems follow |
| Laying hens | the same junglefowl | eggs: 300 a year against the junglefowl's 10 to 15 | |
| Thoroughbred horses | Arabian and English horses, 1700s | speed over a mile or two | nearly all descend from three stallions |
| Sheep (Merino) | wild mouflon | fine, continuous wool | wild sheep shed hair; wool that never stops growing is bred in |
| Pigs | wild boar | fast growth, lean meat, litter size | |
| Silkworms | a wild moth, in China about 5,000 years ago | silk yield; the adults can no longer fly | fully dependent on humans |
| Belyaev's silver foxes | farmed wild foxes, Siberia, from 1959 | tameness only | within 40 generations the foxes wagged tails, had floppy ears and piebald coats: domestication in one lifetime |
| Fancy goldfish | the Prussian carp | colour, double tails, bubble eyes | |
| Laboratory mice and rats | wild house mice | uniformity, docility, specific diseases | |
| Roses, tulips, orchids | wild species | colour, form, scent, double flowers | the Dutch tulip craze of the 1630s was about selected varieties |
| Honeybees | wild Apis mellifera | gentleness, honey yield, disease resistance |
The problems
- Inbreeding: the best individuals are often relatives, and breeding relatives doubles up harmful recessive genes. Pedigree dogs show it most: hip dysplasia in German shepherds, heart disease in Cavalier King Charles spaniels, breathing disorders in pugs and bulldogs, deafness in Dalmatians.
- Loss of genetic diversity: a crop that is all one variety has no individuals that happen to resist a new disease. The Irish potato famine (1845 to 1849) struck the single Lumper variety and a million people died; the Gros Michel banana was wiped out by Panama disease in the 1950s and its replacement, the Cavendish, is a clone now threatened by a new strain.
- Traits that harm the organism: udders too large to walk comfortably, chickens whose legs cannot carry their bodies, Belgian Blues that cannot calve unaided, silkworms that cannot fly or feed themselves.
- Slowness and limits: selection can only shuffle variation that already exists, so after many generations the gains stop unless new variation (mutation, or a cross with another variety) arrives.
Artificial selection, natural selection, genetic modification
| Artificial selection | Natural selection | Genetic modification | |
|---|---|---|---|
| Who or what selects | humans | the environment (predators, climate, food, disease, mates) | humans, directly on the DNA |
| What is favoured | traits humans want | traits that help survival and reproduction in the wild | whatever gene is inserted or edited |
| Speed | generations to centuries | thousands to millions of years (or faster under strong pressure: antibiotic resistance in years) | one generation |
| Source of variation | existing variation in the species, plus crosses | mutation and existing variation | genes from any species, or edits (CRISPR) |
| Examples | dog breeds, crops, livestock | peppered moths, finch beaks, antibiotic-resistant bacteria | Bt cotton, golden rice, insulin-making bacteria |
| Result | domesticated varieties, often unfit for the wild | adaptation to the environment | a specific new trait |
Both selections work the same way (differential reproduction changes the population); the difference is who chooses and why. Darwin's argument was exactly that: if breeders can make a wolf into a dachshund by choosing, nature choosing over millions of years can make a species into another.
Where it sits in school
Artificial selection is grade 7 and 8 life science in the United States (NGSS MS-LS4-5, "the influence of humans on the inheritance of desired traits") and high-school biology (HS-LS4), GCSE Biology (selective breeding, with the Brassica and dog examples, and the inbreeding problems), Year 10 Science in Australia (evolution and the Belyaev foxes), and CBSE Class 9 (Improvement in Food Resources: crop and animal breeding) and Class 10 (Heredity and Evolution, where Brassica is the textbook figure). The exam asks for the method in steps, two examples, two problems, and the comparison with natural selection.
Practice: ten questions
- Define artificial selection in one sentence.
- Give the four steps of selective breeding for a faster racehorse.
- Name six vegetables bred from one wild mustard plant.
- What is the wild ancestor of maize, and what was selected?
- Why are most pedigree dog breeds prone to inherited diseases?
- Why is the Cavendish banana at risk from a single disease?
- State two differences between artificial and natural selection.
- Is the Belyaev fox experiment artificial or natural selection? What was selected?
- How does genetic modification differ from selective breeding in speed and in the source of the new trait?
- Darwin opened his book with pigeon breeding. Why?
Answers: 1) Humans choosing which organisms reproduce so that wanted traits become more common. 2) Decide on speed; breed from the fastest horses; select the fastest offspring; repeat over generations. 3) Broccoli, cabbage, cauliflower, kale, kohlrabi, Brussels sprouts. 4) Teosinte; kernel number, size and softness. 5) Inbreeding from a small number of ancestors doubles up harmful recessive genes. 6) It is a clone with no genetic variation, so no plant can resist a disease the others cannot. 7) Humans against the environment selecting; traits for human use against traits for survival (also speed, and fitness for the wild). 8) Artificial; tameness. 9) GM is one generation against many; the trait can come from another species or an edit, not only from existing variation. 10) To show that selection alone, with no new mechanism, can produce huge change in a species, which is the engine of his theory.
Download the artificial selection sheet (PDF)
Where to get help
A 1-on-1 online biology class with a tutor who holds a master's degree in science, for grades 7 to 12, GCSE and A level, the Australian years and CBSE Class 9 and 10: selection worked through real examples on the shared screen, the comparison tables built rather than memorised, then the exam questions marked to the scheme. The recording and notes after every class, the same tutor every week, US$10 to $13 an hour. The biology page has the courses; the free 30-minute first class is a real lesson, usually on the homework the student brings.
Questions parents ask
1What is artificial selection?
2What is the difference between artificial selection and natural selection?
3What are examples of artificial selection?
4What are the disadvantages of artificial selection?
5Is artificial selection the same as genetic modification?
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