Drive across Saskatchewan in July and you will see fields of bright yellow canola stretching to the horizon. Every one of those plants exists because of choices people made: which plants to cross, which offspring to keep, and which to throw away. Selective breeding (artificial selection) is evolution steered by humans. It has fed billions of people and built much of Canada's farm economy. But each generation of selection also narrows a population's gene pool, and a crop or herd with little genetic variation can be wiped out by a single disease. This lesson looks at how breeding works, what it has achieved, and why its successes come with real risks.
How selective breeding works
Selective breeding needs the same three ingredients as natural selection: variation in a trait, inheritance of that variation, and selection of who reproduces. The difference is that people, not the environment, decide which individuals are "fit". Because the trait must be inherited, breeders choose parents whose advantage comes from their genes, not just from good feeding or a lucky growing season. Over generations, alleles for the chosen traits become more common, and the population changes, just as allele frequencies change in nature. Breeders use two main strategies. Inbreeding crosses closely related individuals to make a line uniform and predictable. Hybridization (crossbreeding) crosses two different lines to combine their best traits; first-generation hybrids are often more vigorous than either parent, an effect called hybrid vigour, which is why most corn planted in Ontario is hybrid seed.
- Set a goal: decide which trait to improve, such as yield, early ripening, disease resistance or milk production.
- Find variation: collect individuals or varieties that differ in that trait, including wild relatives or heritage breeds.
- Select parents with the best expression of the trait, and cross them.
- Test the offspring: grow or raise them, measure the trait, and keep only the best performers.
- Repeat for many generations, then release the new variety or use the best animals widely, often by artificial insemination.
Canadian success stories
| Product | Started from | Trait selected | Result |
|---|---|---|---|
| Marquis wheat (released about 1909) | A cross of Red Fife with Hard Red Calcutta wheat | Earlier ripening and good baking quality | Ripened before the early Prairie frosts; became the main wheat on the Prairies |
| Canola (1970s) | Rapeseed, whose oil and meal contained harmful compounds | Low erucic acid and low glucosinolates | A healthy cooking oil and livestock feed; now one of Canada's most valuable crops |
| Holstein dairy cattle | Black-and-white cattle from the Netherlands and Germany | Milk yield, udder health, longevity | About 9 in 10 Canadian dairy cows are Holsteins; milk per cow has more than doubled since the 1960s |
Marquis wheat was developed by Charles Saunders at Canada's Central Experimental Farm in Ottawa. The Prairie growing season is short, and Red Fife often froze before harvest; Marquis ripened several days earlier and opened vast areas of the West to wheat farming. Canola came from plant breeders Baldur Stefansson at the University of Manitoba and Keith Downey at Agriculture Canada in Saskatoon. Ordinary rapeseed oil was high in erucic acid, a concern for heart health, and its meal was high in glucosinolates, which made it bitter and harmful to livestock. By screening thousands of seeds and selecting rare low-content plants, they bred "double-low" varieties, and the name canola (from Canada and oil) was registered in 1978. Holstein breeders measure every cow's milk and use artificial insemination so that one top bull can father tens of thousands of calves, which spreads valuable alleles very quickly, and as you will see, harmful ones too.
The trade-offs
Selection removes variation; that is its whole purpose. But variation is a population's insurance against change. When breeders use only a few top individuals as parents, the population becomes more inbred. Close relatives are likely to carry the same harmful recessive alleles, so their offspring are more often homozygous for them, and the hidden defects appear. The result is inbreeding depression: lower fertility, weaker immune systems and inherited disorders. Many dog breeds show it, from hip dysplasia in large breeds to breathing problems in flat-faced breeds selected for appearance. Holsteins have had inherited defects traced back to a few widely used bulls, so breeders now track inbreeding and genetically test bulls. A second risk is monoculture: planting huge areas with one genetically uniform variety. If a new pathogen can attack one plant, it can attack them all. In 1916 a stem rust epidemic caused heavy losses across Prairie wheat fields, spurring Canadian research into rust-resistant varieties.
- Higher yields feed more people from the same land.
- Disease-resistant and drought-tolerant varieties reduce pesticide use and crop losses.
- Healthier foods, such as canola oil low in erucic acid.
- Large economic gains for farmers and exports.
- Loss of genetic diversity within crops and breeds.
- Inbreeding depression and inherited disorders.
- Monocultures vulnerable to one new pest or pathogen.
- Traits that help humans can harm the animal's welfare, such as extreme body shapes.
To protect the variation that breeders will need in the future, countries store seeds of old varieties and wild relatives in seed banks. Canada's national seed genebank, Plant Gene Resources of Canada, is in Saskatoon, and backup copies of seeds from around the world are frozen in the Svalbard Global Seed Vault in Arctic Norway. When a new disease appears, breeders search these collections for a resistance allele that was bred out of modern varieties. Heritage livestock breeds, such as the Canadienne dairy cow, play the same role for animals.
Selective breeding vs genetic modification
| Selective breeding | Genetic modification (GM) | |
|---|---|---|
| What changes | Whole genomes are mixed; many genes change at once | One or a few specific genes are added or edited in the lab |
| Source of genes | The same species or a close relative that can interbreed | Can come from any organism, even a bacterium |
| Speed | Many generations, often 10 or more years | One generation to insert the gene, plus years of testing |
| Precision | Unwanted genes can be carried along with wanted ones | Targets one known gene |
| Example | Marquis wheat, original canola, Holstein cattle | Herbicide-tolerant canola; Bt corn that makes an insect-killing protein |