Domestication of plants and animals has a long history. Plant breeding began about 8000 B.C. when instead of just collecting seeds from wild plants, people started to sow seeds. Surviving plants that gave many seeds were the ones that got their seed re-sown, and by that, natural selection turned into selection guided by human interests. In animals, genetically transmitted changes in behaviour were an important part of domestication. The first animal to be domesticated was the dog, some 10 000 years ago. When humans developed from nomads to village inhabitants who cultivated plants, hunting in the vicinity of the villages eroded wild animal populations and motivated people to explore the husbandry of mammals and poultry. Selection for production and reproduction traits resulted in dairy cows, laying hens and other farm animals. Today’s farm animals and cultivated crops have come a long way from their wild relatives.
During the 19th century, Darwin explained evolution and Mendel discovered the basic rules of inheritance. With an increased knowledge about the heritability of traits, and the discovery of chromosomes and genes, we have gone from unintentional selection to advanced breeding programmes where more and more knowledge of the mechanisms behind different traits is used to tailor the sources of our food. Thanks to breeding we have access to healthier livestock and crops, producing milk, meat and grain at levels our ancestors could only have dreamt of. However, the evolution of insect pests and disease-causing microbes continues, and this means breeding for resistant crops is a never-ending project. Certain unfavourable genetic correlations between production traits and other traits have meant that breeding for increased production results in health problems in farm animals, and these problems also have to be handled by improved breeding programmes.
The negative impacts of agriculture on the environment have to be reduced. Agriculture accounts for almost 20% of the greenhouse gas emissions in Sweden, and for around 40% of the Swedish nitrogen and phosphorous load entering the Baltic and the North Sea. On top of this, as we live longer, more people have to be fed in the world, while fossil fuels need to be replaced and food production has to adapt to climate change. New genotypes and production systems have to be developed in response to the changing climatic conditions, not least to the new pests and diseases that can be expected.
Like evolution, breeding is dependent on genetic diversity and the recombination of genes. But the genetic variation, or “gene pool”, can be more or less restricted. If all individuals belong to the same family there is limited variation. Also, a desirable trait, such as growth rate, may be closely linked to undesirable traits, and therefore selection for one trait can result in negative changes in others. This explains why different ways to increase genetic diversity have been explored – by increasing the mutation rate, bringing about crossings that would hardly occur in nature, and even by merging cells from different species. Growing biological knowledge and technological development now enables us to turn genes on or off, or to move genes between individuals. And our knowledge of gene functions and traits in different species, in combination with statistics, also gives us the ability to select individuals on the basis not only of specific genes, but entire individual genetic set up. The toolbox used for modern breeding includes a large variety of tools, such as tissue culture, artificial insemination, genomic selection and genetic modification (GM).