Artificial Selection and Selective Breeding

Artificial selection, or selective breeding, is the process by which humans choose which individuals with desired traits will reproduce, so that those traits become more common in later generations. In brief: the steps are to select parents with the desired trait, breed them, select the best offspring and repeat for many generations. Farmers have used it to produce high-yield crops, dairy cattle and hundreds of dog breeds from a common wolf-like ancestor. It works because of inherited variation, just like natural selection, but the selecting agent is human choice.

Artificial selection was one of the main arguments Charles Darwin used to explain natural selection. This guide explains the process step by step, gives well-known examples of crops and animals, compares artificial and natural selection, describes the benefits and risks such as loss of genetic diversity, works through a numerical breeding example and corrects common mistakes.

How does artificial selection work?

Artificial selection takes advantage of natural variation in a population. The breeder follows a simple cycle.

1. Decide on the desired trait, such as higher milk yield, disease resistance or larger fruit.
2. Select individuals with the best expression of this trait from the population.
3. Breed them together and allow only these individuals to reproduce.
4. Select again from the offspring those with the best expression of the trait.
5. Repeat over many generations, until the trait is at the desired level.

The method relies on the trait being inherited. If a difference in the trait is caused only by the environment, selection will have no lasting effect. The response to selection depends on the heritability of the trait, which is the proportion of the variation due to genes. Traits with high heritability respond quickly.

Related methods include inbreeding, mating close relatives to fix a trait, and hybridization (crossing different varieties or species) to combine desirable traits from each. Hybrid plants often show hybrid vigor (heterosis), as in many hybrid maize varieties.

What are examples of selective breeding?

• Brassica oleracea: a single wild plant, wild cabbage, has been bred into cabbage (selected for leaves), kale (large leaves), broccoli (flower clusters), cauliflower (flower buds), Brussels sprouts (lateral buds) and kohlrabi (stem).
• Maize: it was bred by people in Mexico from teosinte, a grass with a few small hard kernels per ear, into the large-eared plant we know. This took thousands of years.
• Wheat and rice: breeding of semi-dwarf varieties in the twentieth century, a main part of the Green Revolution, increased yields because the shorter plants do not fall over when they carry heavy heads of grain.
• Dairy cattle: in the United States, average annual milk production per cow rose from roughly 1 900 kg in 1925 to over 10 000 kg in recent years, with improved breeding, nutrition and management all contributing.
• Dogs: all breeds belong to one species, Canis familiaris or Canis lupus familiaris, and descend from wolves. Selection for different tasks, such as herding, hunting and companionship, has produced breeds of strongly different sizes and shapes.
• Chickens, sheep and horses were bred for meat, eggs, wool, speed or strength.

Artificial and natural selection compared

What are the risks of selective breeding?

• Reduced genetic diversity: breeding from a small number of individuals leaves a population vulnerable to disease and environmental change. The 1970 corn leaf blight in the United States affected a crop with a very uniform genetic background.
• Inbreeding depression: mating of close relatives increases the chance that offspring inherit two copies of harmful recessive alleles. Many pedigree dogs suffer from inherited problems such as hip dysplasia and breathing difficulties in flat-faced breeds.
• Welfare problems: extreme selection can cause animals to suffer. Fast-growing broiler chickens may have leg problems because their bodies grow faster than the skeleton can support.
• Loss of other traits: selecting for one trait, such as yield, may reduce others, such as taste, resistance or hardiness.

Worked example: selecting for milk yield

A herd of dairy cows has a mean yield of 6 000 liters per year. A farmer breeds only from the top cows, whose mean yield is 8 000 liters. The heritability of milk yield is 0.30.

1. Selection differential (S): the difference between the mean of the selected parents and the mean of the population. S = 8 000 − 6 000 = 2 000 liters.
2. Response to selection (R): the breeder’s equation is R = h² × S = 0.30 × 2 000 = 600 liters.
3. New mean of the offspring: 6 000 + 600 = 6 600 liters.
4. Second generation: suppose the mean of the new selected parents is 8 600 liters (2 000 above the new mean). Then R = 0.30 × 2 000 = 600 again, and the new mean is 7 200 liters.

After 5 generations at this rate, the mean would be 6 000 + 5 × 600 = 9 000 liters. In practice, progress slows over time as genetic variation is used up, and as biological limits are approached. The calculation shows why a trait with higher heritability (for example 0.6) would respond twice as fast, giving a gain of 1 200 liters per generation under the same selection.

How did selective breeding develop?

People began to select crops and animals when agriculture started, about 10 000 years ago. In the eighteenth century, Robert Bakewell in England systematically bred sheep and cattle for meat and recorded pedigrees. Charles Darwin kept pigeons and described in On the Origin of Species how breeders produced fancy varieties, such as fantails, from the wild rock dove. The rediscovery of Mendel’s laws in 1900 and the development of statistical genetics improved breeding methods in the twentieth century. Norman Borlaug received the Nobel Peace Prize in 1970 for the work on high-yielding wheat.

How is it used today?

Modern breeding uses DNA markers to identify animals and plants with desirable genes without waiting for the trait to appear, a method known as marker-assisted selection. Genomic selection in dairy cattle predicts the breeding value from thousands of genetic markers, so young bulls can be selected before they have any offspring. Genetic engineering and genome editing allow changes to be made faster, but conventional selective breeding remains the main method used in agriculture.

Which mistakes are common?

Mistake 1: Confusing artificial selection with genetic engineering

Selective breeding uses the natural variation and crossing of organisms. Genetic engineering directly changes or transfers genes in a laboratory.

Mistake 2: Thinking that breeders create the variation

Breeders choose among existing variation. New alleles arise by random mutation.

Mistake 3: Saying that selection changes individual animals

Selection acts on a population. Individual cows do not become better milk producers.

Mistake 4: Describing the method without mentioning repetition

Selective breeding requires many generations of choosing and breeding to achieve large changes.