Artificial Selection, Breeding and Agriculture

1Overview

Long before genes were discovered, farmers were already steering evolution. By saving seeds from the best plants and breeding the most useful animals, people changed wild species into crops and livestock within a few thousand years, a very short period in evolutionary terms. This lesson covers artificial selection and breeding of plants and animals, and the agricultural practices that depend on them: monoculture, polyculture and food sustainability.

2Artificial selection: evolution with a human selector

Artificial selection (also called selective breeding) is an evolutionary process in which humans deliberately favour or reject certain traits by choosing which organisms to breed, or which plants to save seeds from. It is defined as the practice of mating individuals with desired traits as a means of increasing the frequency of those traits in a population.

The underlying logic is the same as for natural selection: there must be variation, the variation must be inherited, and some individuals must reproduce more than others. The difference is who or what does the selecting. In natural selection the environment determines which traits help survival. In artificial selection a breeder determines which traits are useful to people.

Artificial selection has long been used as a model for natural selection. Because the human version can be watched in a few generations, it makes the natural version easier to believe: the step from one to the other is described as “a small step.”

3Breeding plants

Plant breeding aims to increase useful traits such as higher yield and resistance to pests and disease. The most famous example is wild mustard, Brassica oleracea. By selecting different plant parts, breeders produced cabbage, kale, broccoli, cauliflower, Brussels sprouts and kohlrabi. These vegetables look very different from each other and from the wild plant, yet they are all one species.

The method is a repeating cycle, usually written as a sequence of steps:

1. Observe variation in a population of plants (for example, size of leaves or flower heads).
2. Select the individuals with the desired trait.
3. Breed only those individuals, or save only their seeds.
4. Grow the offspring and compare them with the original population.
5. Repeat for many generations, selecting the best again each time.

Genetics explains why the cycle works. A trait like height is partly controlled by alleles that are inherited. Selecting the plants with the highest values means selecting parents that carry more of the alleles that increase height, so the offspring have, on average, more of them too. Progress slows if the trait is mostly controlled by the environment or if variation in the population is used up.

4Breeding animals

Animals have been bred for meat and milk production, for work, and for companionship. Domestic dogs descended from several wolf lineages about 20,000 to 40,000 years ago, and breeding has since produced hundreds of breeds. Dachshunds were bred to hunt small animals, and German Shepherds were bred to herd sheep. Breeds differ in traits such as size and temperament, which shows how much variation selection can shape in a relatively short time.

5The costs of breeding

Selective breeding is powerful, but it carries risks.

• Reduced genetic variation. If a few individuals are the parents of the whole population, many alleles are lost. This leaves the population less able to adapt to new conditions, such as a new disease.
• Unwanted linked traits. Rare disease alleles can be selected unknowingly along with the desired trait.
• Random mutations still occur and may be harmful.

6Agriculture and selection: monoculture and polyculture

Breeding is only one part of agriculture. How crops are arranged in the field matters just as much.

A monoculture grows one crop over a large area. It is common in industrial agriculture because it allows large-scale, efficient production and often higher yields of that single crop. A polyculture grows multiple crops in the same area.

The choice between them depends on the farmer’s goals, the local environment and market demand. Neither is automatically “right.”

The link to genetics

Monocultures often use plants that are genetically very similar, because breeders have selected for one high-performing variety. Genetic uniformity can be an advantage (all plants ripen together) and a risk: if one pest or disease can attack that genotype, nothing in the field has a different allele that would resist it. This is the same lesson as in the extinction lesson, that genetic diversity gives resilience. A polyculture, or a crop made of several varieties, places some diversity back in the field.

7Food sustainability

The human population needs food now and in the future, which is why the curriculum links agriculture to food sustainability. The Food and Agriculture Organization of the United Nations (FAO) describes sustainable food and agriculture as farming that meets the needs of today’s and future generations while staying profitable, protecting environmental health and promoting social and economic equity. It identifies three dimensions of sustainability (environmental, social and economic) and four pillars of food security (availability, access, utilization and stability).

FAO lists threats to food systems that include a growing population and rising demand, rising hunger and malnutrition, climate change, overuse of natural resources and biodiversity loss, and food loss and waste. Five principles guide sustainable agriculture: increasing productivity, employment and value addition in food systems; protecting and enhancing natural resources; improving livelihoods and promoting inclusive economic growth; strengthening resilience of people, communities and ecosystems; and adapting governance to new challenges.

Where do breeding and crop arrangement fit in? Breeding can raise yields and develop crops that resist pests and disease or tolerate drought, which can reduce losses and the need for chemical inputs. But monoculture and loss of genetic diversity can weaken resilience, and overuse of resources can harm the environment on which farming depends. Sustainable agriculture involves weighing these trade-offs.

8Investigating and evaluating

The curriculum encourages students to evaluate evidence and plan investigations. Questions that can be investigated using the ideas in this lesson include:

• How could a school experiment demonstrate artificial selection, for example by selecting seeds from the tallest plants over several generations of fast-growing plants?
• How would the results change if the trait were controlled mostly by the environment instead of genes?
• What evidence would show whether polyculture reduces pest damage compared with monoculture in a garden plot?
• Which trade-offs (yield, cost, biodiversity, resilience) should be weighed when comparing two farming approaches?

The next lesson moves from selection to direct changes in DNA: genomics, genetic engineering and GMOs. Related ideas about ecosystems are explained in What is an ecosystem? and about selection in What is natural selection?.

9Practice problems with solutions

Problem 1: predict a breeding result

A farmer wants wheat plants with shorter stems that resist lodging in wind. Each year the farmer saves seed only from the shortest 5 percent of plants. If stem length is partly inherited, average stem length decreases each generation, but the change slows as variation is used up. If the farmer then needs new variation, it can come from mutation or from crossing with other varieties.

Problem 2: calculating with a test cross

A hornless animal trait is recessive (h), and horned is dominant (H). A horned bull is mated with 6 hornless cows and all 6 calves are horned. The bull is probably HH. If the bull were Hh, each calf would have a 1/2 chance of being hornless, and the chance of 6 horned calves would be (1/2)⁶ = 1/64. This is why breeders use many offspring before deciding.

Problem 3: compare two farms

Farm A grows 100 hectares of one wheat variety. Farm B grows wheat, peas and canola in strips. A fungal disease that attacks that wheat variety arrives. Farm A may lose most of its crop because every plant is susceptible. Farm B loses part of the wheat, while the other crops are unaffected. In a year without the disease, Farm A may have lower costs and a high yield. Neither system is best in all conditions, so farmers weigh risk against efficiency.

Selection by people has side effects

Selecting for one trait can change others. Alleles are often inherited together because genes lie close together on a chromosome, so choosing a desired trait can bring along an unwanted one. If the population is small, chance can also fix harmful alleles. Breeders therefore track pedigrees, use test crosses and try to maintain enough variation. Some conservation programs for domestic breeds keep records for this reason.

Sustainability questions to consider

1. Which of the four pillars of food security (availability, access, utilization, stability) does a monoculture best support, and which does it threaten?
2. How could breeding contribute to environmental protection, for example by developing pest-resistant crops that need less spraying?
3. How might climate change alter which crops and varieties are suitable in a region?

Summary

• Artificial selection is natural selection with humans as the selector.
• Selective breeding raises the frequency of useful traits but can reduce genetic variation.
• Monoculture is efficient but vulnerable, and polyculture spreads risk and supports biodiversity.
• Sustainable agriculture balances environmental, social and economic goals.

Comparing artificial selection to GMOs

Both selective breeding and genetic engineering change the genes of crops and animals, but they work differently. Breeding selects among alleles already present in a species and takes many generations. Genetic engineering can add a specific gene, including one from another species, in a single step. Each has its own assessment rules, benefits and concerns, and the final lessons examine them.

?Quick check

Try each question first, then reveal the answer.