How selective breeding works

1. Selective breeding is choosing the organisms that show a desired characteristic and breeding them together over many generations so that the characteristic becomes more common in the population.
2. It is also called artificial selection, because the choice of which individuals reproduce is made by people rather than by the environment.
3. The characteristic must already be present as variation within the population, since breeding can only work with alleles that some individuals already carry.
4. The first step is to choose, from a varied population, the individuals that show the desired characteristic most strongly.
5. The second step is to breed those chosen individuals together, so that their alleles are brought together in the offspring.
6. The third step is to examine the offspring, keep only those that show the characteristic most strongly, and breed from those.
7. Those steps are repeated over many generations, and each round raises the proportion of individuals carrying the alleles for the characteristic.
8. The frequency of the useful alleles in the population therefore rises while the frequency of the alternative alleles falls, which is why the whole population changes and not just a few individuals.
9. Selective breeding is slow, because one round takes a full generation, and it cannot bring in an allele that no member of the species carries.

Selective breeding of crop plants

1. Growers select the plants with the best version of a characteristic and sow the seed from those plants to raise the next generation.
2. Higher yield is the characteristic selected most often, so cereals have been bred to produce more grains per ear and more ears per plant.
3. Disease resistance is selected by growing a crop where a disease is present and breeding only from the plants that stay healthy.
4. Drought and salt tolerance are selected so that a crop can be grown where water is scarce or where soil contains salt.
5. Larger or sweeter fruit and a longer shelf life are selected in fruit and vegetable crops so that more of the harvest reaches the shop in good condition.
6. Shorter, stiffer stems are selected in wheat so that the plant puts less of its biomass into the stalk and is less likely to be flattened by wind and rain.
7. Plants are convenient to breed selectively because one plant produces very many seeds, generations are short, and many varieties can be grown side by side and compared in one season.

Selective breeding of farm animals

1. Farmers choose the animals with the best version of a characteristic, mate them together, and then breed from the best of their offspring.
2. Milk yield is selected in dairy cattle, so modern dairy cows produce several times as much milk each year as the cattle they were bred from.
3. Meat production is selected in beef cattle, sheep and pigs, choosing animals that grow quickly and lay down more muscle for each kilogram of feed eaten.
4. Egg number is selected in hens, and wool quality in sheep, by keeping the birds and sheep that produce most and breeding from them.
5. Disease resistance and a calm temperament are selected as well, because healthy animals that are easy to handle are cheaper and safer to keep.
6. A characteristic that appears in only one sex can still be selected, and a bull is judged by the milk yields of his daughters rather than by anything that can be measured in the bull himself.
7. Animal breeding is much slower than plant breeding, because a cow produces roughly one calf a year and takes about two years to reach breeding age, so a noticeable change takes decades.

Impacts of selective breeding

1. The benefit is more food from the same land and the same number of animals, which lowers the cost of food and supports the food supply of a growing population.
2. A selectively bred crop is also uniform, so it ripens together and can be harvested by machine in one pass.
3. The cost is a loss of variation, because each generation only a few individuals are allowed to breed and the alleles carried by the rest are lost from the population.
4. The gene pool, which is all the alleles present in the population, therefore becomes smaller with every generation of selection.
5. Breeding from close relatives is called inbreeding, and it raises the chance that an offspring inherits two copies of the same harmful recessive allele, so the disorder that allele causes appears.
6. A population with little variation is also vulnerable as a whole, because if a new pathogen or a change in conditions harms one individual it is likely to harm every individual in the same way.
7. There may be no allele left anywhere in the population to breed a resistant variety from, so the breeder has to look for one in a wild relative or in a stored collection of older varieties.
8. Selecting hard for one characteristic can also harm the animal, and cattle bred for very high milk yields are more prone to lameness and udder infection, while dogs bred for a flattened face have difficulty breathing.
9. Breeders reduce these problems by keeping records of relatedness, by crossing in unrelated individuals from time to time, and by maintaining rare breeds and older varieties as a store of alleles.