Artificial Selection: Breeding Plants and Animals

From wolves to chihuahuas

Every dog, from a tiny chihuahua to a huge Great Dane, belongs to one species that descends from ancient wolves. Over thousands of years, people kept and bred the animals with the traits they wanted: tameness first, then herding, guarding, hunting, size, coat and look. Darwin used breeders' results as evidence that selection can produce enormous change.

Artificial selection works by the same rules as natural selection: inherited variation plus differences in who reproduces. The difference is who does the choosing. In this lesson you will see how much artificial selection has shaped the plants and animals we depend on, including major contributions from Canada and from Indigenous farmers.

How artificial selection works

1. Variation: individuals in a population differ in a trait (milk yield, fruit size, temperament).
2. Inheritance: at least part of that variation is genetic.
3. Selection by people: breeders choose individuals with the desired trait as parents and prevent others from breeding.
4. Repeat: each generation, the desired trait becomes more common or more extreme.

Because breeders can select very strongly (for example, keeping only the top 5% as parents), artificial selection can produce large changes in a short time compared with most natural selection. It can only work with variation that exists, and it often reduces genetic variation, because only a few individuals become parents.

One wild plant, six vegetables

Wild cabbage (Brassica oleracea), a member of the mustard family that grows along coasts in western Europe, gave rise to six very different-looking vegetables. Farmers selected plants for enlarged parts: leaves (kale), the end bud (cabbage), side buds (Brussels sprouts), the stem (kohlrabi), and flower clusters (broccoli and cauliflower). All of these are still one species: they can cross with each other.

Corn: a gift of Indigenous plant breeding

Corn (maize) was domesticated from a wild grass called teosinte by Indigenous farmers in what is now southern Mexico, beginning about 9000 years ago. Teosinte ears are small, with a handful of kernels each locked in a hard case. Generation after generation, farmers saved and planted seeds from the plants they preferred. The result is modern corn, with large ears of hundreds of exposed kernels, a plant that cannot even spread its own seeds without human help.

Corn spread north across the Americas. In what is now Ontario and Quebec, the Haudenosaunee and other First Nations grew corn together with beans and squash, the Three Sisters. Many Indigenous communities today continue to grow and save the seeds of heritage varieties, keeping their genetic diversity alive.

Canadian breeding successes

• Marquis wheat. In the early 1900s, Charles Saunders, working for the Dominion experimental farms, selected a wheat variety that ripened earlier than older varieties, so it could be harvested before the early frosts of the Prairies. Marquis wheat helped make Canada one of the world's great wheat producers.
• Canola. In the 1960s and 1970s, Canadian plant breeders, notably Baldur Stefansson and Keith Downey, used selection to turn rapeseed into a new crop with healthier oil (low in erucic acid) and better meal for animal feed (low in glucosinolates). It was named canola, from "Canada" and "oil." Canola is now one of Canada's most valuable crops.
• Livestock. Dairy breeders, using records of milk production and techniques like artificial insemination, have greatly increased the milk produced per cow over the last century.

Breeding methods

• Mass selection: choose the best individuals from a large population as parents. Simple and widely used.
• Inbreeding: mate closely related individuals to make a line true-breeding (offspring reliably show the trait). Downside: it reduces variation and can bring together harmful recessive alleles, causing health problems (inbreeding depression).
• Crossbreeding (hybridization): cross two different lines to combine their good traits. First-generation hybrids often show hybrid vigour: higher yields and better health. Much of the corn grown in Ontario is hybrid seed.
• Modern tools: artificial insemination and embryo transfer spread the genes of chosen animals widely; marker-assisted selection uses DNA tests to pick parents that carry desired alleles before the trait even shows. Genetic engineering, which moves specific genes directly, is a form of biotechnology covered in the next lesson and in the Genetic Processes unit.