Selective Breeding of Plants and Animals

Wheat has been bred to produce more and bigger grains on each plant. Modern varieties also have short stems, so the plants do not fall over in the wind and rain and more of the energy goes into the grain.

Breeders cross crops with plants that resist a disease and select the offspring that do not become infected, such as wheat that resists rust fungus or potatoes that resist blight.

Fruits have been bred to be larger, sweeter, seedless or longer lasting on the shelf, and crops are bred to have a uniform size so that they are easy to harvest.

All of cabbage, broccoli, cauliflower, kale and kohlrabi come from one wild plant, the wild cabbage, because people selected different features: large leaves, flower heads, buds or swollen stems.

Cows that give a large volume of milk are bred together. The average milk yield of dairy cows has more than doubled in 60 years in some countries.

Animals with more muscle and fast growth are chosen for meat, and sheep are chosen for the quality of their wool.

Hens that lay many eggs have been bred for egg production, and different breeds have been selected for fast growth to produce meat.

Breeders also choose for resistance to disease, for fertility and for a calm temperament, because these reduce the cost of keeping the animals.

This is the breeding of closely related individuals. In selective breeding a breeder often uses the same few best parents again and again, so the animals or plants in the population become closely related.

The population ends up with fewer different alleles, so the individuals are very similar genetically. If the environment changes or a new disease arrives, there may be no individual with an allele that gives resistance.

Inbreeding makes it more likely that offspring inherit two copies of a harmful recessive allele, so inherited disorders become common. Many pedigree dogs suffer from joint, heart or breathing problems for this reason.

A single variety of a crop grown over a large area is all equally open to the same pest or disease. Potato blight in Ireland in the 1840s destroyed most of the crop, because most farmers grew one type of potato from tubers, so almost all of the plants were equally open to the disease.

A higher yield of better quality food from the same land, faster growth, fewer losses to disease, and cheaper food, so it contributes to food security.

It takes many generations and can only use alleles that are already present. A loss of variation makes the whole crop or breed vulnerable, and some features that were chosen are harmful to the animal.

Fast growth in chickens can leave their legs unable to carry their weight, and very high milk yields can cause painful udder infections and lameness in cows.

Farmers grow fewer varieties and breeds, and the older varieties that carry useful alleles may be lost.