Selective Breeding and Artificial Selection
CSIRO spent 20 years selectively breeding disease-resistant Merino sheep, by 2010 producing strains that cut chemical treatments by 90% on Australian farms.
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Q1 · How do you think farmers and breeders have changed the appearance of plants and animals over thousands of years?
Think about how different dog breeds or crop varieties came to look so different from their wild ancestors.
Q2 · What is the difference between selective breeding and genetic modification, in your own words?
Consider whether selective breeding can create traits that have never existed in a species before.
● Know
- That selective breeding is the oldest form of genetic technology
- Examples of selective breeding in crops, livestock and companion animals
- The difference between artificial selection and natural selection
● Understand
- How choosing parents with desired traits changes populations over generations
- Why selective breeding works only on heritable traits
- The advantages and limitations of selective breeding compared to other genetic technologies
● Can do
- Identify selective breeding in Australian agricultural contexts
- Compare and contrast selective breeding with natural selection
- Evaluate when selective breeding is appropriate and when it is not
Stand a Chihuahua next to a Great Dane, both descended from wolves, and you are looking at 15,000 years of humans choosing which dogs breed. Selective breeding, also called artificial selection, is the process by which humans deliberately choose which individuals reproduce in order to increase the frequency of desired traits. It is the same mechanism as natural selection, differential survival and reproduction based on heritable traits, but the selecting agent is human preference rather than environmental pressure.
Every domesticated plant and animal has been shaped by thousands of years of selective breeding. Wolves became dogs, wild grasses became wheat, and wild cabbage became broccoli, cauliflower and kale. In each case, humans noticed natural variation, selected individuals with favoured traits, and bred them together. Over many generations, the population changed so much that it barely resembled its wild ancestors.
All domestic dogs belong to the same species, Canis lupus familiaris, and share a common ancestor with the grey wolf. Through selective breeding, humans have created breeds ranging from the tiny chihuahua to the massive Great Dane, a size difference of over 100-fold. This incredible diversity was achieved in just a few thousand years by repeatedly selecting for size, shape, coat and behaviour.
Australian agriculture: The Australian Merino sheep was developed through two centuries of selective breeding for fine, soft wool. Early settlers crossed Spanish Merinos with British breeds, then selected offspring with the finest fibres. Today, Australian Merino wool is prized worldwide, and the industry continues to use modern genetic tools to accelerate selection for wool quality and parasite resistance.
Some students think selective breeding creates entirely new genes. It does not. Selective breeding only changes the frequency of alleles already present in the population. If a trait does not exist in the gene pool, breeding cannot create it. Mutations or genetic engineering are needed to introduce genuinely new genetic information.
Click each stage of the selective breeding loop.
Choose parents
Humans identify individuals with the most desirable traits (size, speed, yield, temperament).
Breed selected pair
The chosen individuals reproduce, passing their genes to the next generation.
Evaluate offspring
Humans assess which offspring show the desired traits most strongly.
Repeat over generations
The cycle continues, gradually shifting the population's genetic makeup toward the chosen ideal.
Australian wheat is one of the world's great selective breeding success stories. When European settlers first brought wheat to Australia in the late 1700s, the crops struggled with dry soils, heat and diseases like rust. Over two centuries, Australian plant breeders, including scientists at CSIRO and state agriculture departments, systematically crossed wheat varieties that survived best in Australian conditions. Today, Australian wheat varieties such as Mace and Scepter are exported globally and are bred specifically for drought tolerance, disease resistance and high protein content. This is selective breeding solving real agricultural problems.
Selective breeding has transformed human civilisation. Without it, there would be no bread, no dairy industry, no cotton clothing and no guard dogs. Modern agriculture relies on crop varieties bred for high yield, disease resistance and tolerance to drought or salinity. Livestock breeding has produced cows that produce far more milk than their wild ancestors, and chickens that grow to market weight in six weeks instead of six months.
However, selective breeding carries risks. When breeders focus intensely on a few traits, they often reduce genetic diversitythe total range of alleles in the population. Low diversity makes populations vulnerable to new diseases or environmental changes. Inbred populations may also suffer from inbreeding depression, where harmful recessive alleles become more common because relatives share many of the same genes.
Bananas are a cautionary tale. Most commercial bananas are clones of a single variety called Cavendish. Because they are genetically identical, a new fungal disease called Tropical Race 4 can wipe out entire plantations. The banana industry is now racing to breed or engineer new varieties with resistance before the fungus spreads globally.
Australian viticulture: Australian winemakers have developed grape varieties suited to hot, dry climates through selective breeding and grafting. The CSIRO has released new grape varieties such as Tyrian and Arra that produce quality wine in warmer conditions, helping the industry adapt to climate change while maintaining genetic diversity in vineyard rootstocks.
- Higher yield
- Reduced diversity
- Inbreeding depression
- Faster growth rate
- Harmful recessive alleles become more common
- Animals reach market size in less time
- More grain, milk or meat per animal or hectare
- Fewer alleles in the population, increasing disease risk
Charles Darwin developed his theory of natural selection partly by observing artificial selection. He noticed that pigeon fanciers could create astonishing variety by selective breeding, and he reasoned that nature must do something similar, but over vastly longer timescales and without conscious intent. In both cases, the mechanism is the same: some individuals reproduce more than others, and their traits become more common.
The difference lies in what determines reproductive success. In natural selection, success depends on survival in the wild, finding food, avoiding predators, resisting disease, attracting mates. In artificial selection, success depends on human preference, taste, size, colour, docility, or wool quality. Both processes shape evolution, but natural selection has no goal or endpoint. It simply favours whatever works in the current environment.
Consider fruit size in tomatoes. Wild tomatoes are marble-sized berries. Through centuries of artificial selection, farmers favoured plants with larger fruit, and today we have beefsteak tomatoes weighing hundreds of grams. If humans stopped selecting for size, natural selection would gradually favour smaller, more prolific fruit again, because smaller fruit requires less energy to produce and is easier for birds to disperse.
Australian native plants: Aboriginal Australians practised a form of selective breeding by propagating plants with desirable traits, such as yams with larger tubers or grasses with more seeds. This traditional ecological knowledge, accumulated over tens of thousands of years, represents one of the world's longest-running agricultural experiments and has shaped the Australian landscape.
Thoroughbred racehorses are perhaps the most valuable selectively bred animals on Earth. Every thoroughbred alive today can trace its ancestry back to just three founding stallions imported to England in the 16th and 17th centuries. In Australia, the Melbourne Cup"the race that stops a nation", showcases the result of centuries of selective breeding for speed, stamina and heart size. The legendary Phar Lap (1926–1932) had a heart nearly twice the average size for a horse, a heritable trait that contributed to his extraordinary endurance. Modern breeders use pedigree analysis combined with genetic testing to predict racing potential before a horse ever sees a track.
| Feature | Natural Selection | Selective Breeding (Artificial Selection) |
|---|---|---|
| Who selects? | The environment | Humans |
| Goal? | No goal, survival and reproduction | Specific human-desired trait |
| Speed | Slow, usually thousands of generations | Faster, can see results in dozens of generations |
| New alleles? | Can arise through mutation | Only works with existing variation |
| Survival in wild? | Individuals are adapted to their environment | Some bred organisms struggle without human care |
| Example | Peppered moths changing colour during the Industrial Revolution | Dairy cows producing 9,000 L of milk per year |
Wrong: "Selective breeding creates new genes."
Right: Selective breeding changes the frequency of existing alleles in a population. It does not create new alleles, new alleles arise only through mutation.
Wrong: Selective breeding only changes the frequency of existing alleles in the gene pool. It does not create new alleles. New alleles arise through mutation, which is rare and random.
Right: Selective breeding only alters how common existing alleles are. New alleles are created by random mutation, not by breeding choices.
The Banana Problem
The Cavendish banana, which accounts for nearly all bananas sold in Australian supermarkets, is effectively a single clone, every plant is genetically identical because they are reproduced vegetatively (not from seeds). When a disease called Panama disease TR4 emerged, it threatened the entire global crop because there was almost no genetic variation to resist it. This is a powerful reminder that low genetic diversity is dangerous, even when a crop has been highly successful through selective breeding.
Selective Breeding Scenarios
1 A wheat farmer in Wagga Wagga only keeps seeds from plants that survived a severe drought and replants them the next season.
2 A dog breeder mates two Labrador retrievers with excellent temperaments to produce puppies for guide-dog training.
3 An Australian Angus cattle stud uses performance records to choose bulls with the highest meat quality scores.
Selective Breeding vs Natural Selection
1 Explain why a dairy cow bred for high milk production might struggle to survive in the wild.
2 A student claims that selective breeding is "just faster natural selection." Is this claim accurate? Provide two reasons for your answer.
3 Describe one situation where selective breeding is the best approach, and one situation where it cannot achieve the desired outcome. Justify each choice.
Copy Into Your Book
▼Core Definitions
- Selective breeding = choosing parents with desired traits
- Artificial selection = same as selective breeding
- Natural selection = environment decides who survives and reproduces
- Heritable trait = controlled by genes, can be passed on
- Gene pool = all alleles in a population
How It Works
- Identify goal trait
- Select parents with best expression
- Let them reproduce
- Evaluate offspring and repeat
- Desirable alleles increase in frequency
Australian Examples
- Merino sheep, ultra-fine wool
- Angus cattle, meat quality
- Australian wheat, drought resistance
- Thoroughbreds, speed and stamina
Advantages vs Limitations
- Adv: safe, proven, accepted
- Lim: slow, limited variation, inbreeding risk
- Does NOT create new alleles
- Key diff from natural selection: humans are the selecting agent
At the start of this lesson you were asked to think about how humans turned wolves into over 400 dog breeds, from Chihuahuas to Great Danes, in around 15,000 years simply by choosing which animals breed. That extraordinary story was designed to make you think about the power of applying genetic principles deliberately over many generations.
Now that you understand the mechanisms of selective breeding, artificial selection and its limitations, explain in your own words how that transformation was possible, and what it tells us about the relationship between heritable variation, selection pressure and change over generations.
Q1. Define selective breeding and explain why it is considered a form of genetic technology even though no DNA is edited in a laboratory. 3 MARKS
Q2. Compare selective breeding and natural selection using two similarities and two differences. Use an Australian example to illustrate your answer. 4 MARKS
Q3. A cattle breeder wants to create a new variety of beef cattle that is resistant to a newly discovered viral disease. No cattle in the current herd show any resistance. Explain why selective breeding alone cannot solve this problem, and suggest what other genetic technology might be needed. 5 MARKS
Revisit Your Initial Thinking
Go back to your Think First responses at the top of the lesson.
- Did you correctly identify that humans have changed species by choosing which individuals reproduce?
- Did you recognise that this process relies on heritable traits and pre-existing variation?
- Write one sentence explaining the most important difference between selective breeding and natural selection.
Model answers (click to reveal)
Comprehensive Answers
▼Activity 1, Selective Breeding Scenarios
1. Wheat farmer in Wagga Wagga: Trait: drought tolerance [1 mark]. How applied: The farmer is selecting plants that survived drought (already had drought-resistant alleles) and using their seeds for the next crop, increasing the frequency of drought-resistant alleles [1 mark]. Limitation: If no plant in the population had any drought resistance, this method would fail, selective breeding cannot create new alleles [1 mark].
2. Dog breeder: Trait: calm temperament / trainability [1 mark]. How applied: Only dogs with excellent temperaments are bred, so offspring are more likely to inherit calm-behaviour alleles [1 mark]. Limitation: Inbreeding among a small population of breeding dogs can increase the risk of inherited health problems like hip dysplasia [1 mark].
3. Angus cattle stud: Trait: meat quality / marbling [1 mark]. How applied: Bulls with the highest meat quality scores are chosen as sires, passing on alleles for better marbling [1 mark]. Limitation: Selecting heavily for one trait may reduce genetic diversity or accidentally select for unwanted linked traits [1 mark].
Activity 2, Selective Breeding vs Natural Selection
1. Dairy cow in the wild: A dairy cow bred for high milk production uses enormous energy producing milk. In the wild, this energy would be better spent on survival and finding food. Additionally, dairy cows have been bred for docility, not predator avoidance, and their udders are prone to infection without human care [2 marks for any two valid reasons].
2. "Faster natural selection" claim: The claim is partially accurate but incomplete [1 mark]. It is accurate that both processes change allele frequencies in populations over time [1 mark]. However, it is inaccurate because the selecting agent is differenthumans vs environment, and selective breeding has a goal while natural selection does not [1 mark]. Also, natural selection can produce entirely new adaptations through mutation, while selective breeding is limited to existing variation [1 mark].
3. Best vs cannot achieve: Best approach: Improving wool quality in Merino sheep, because variation already exists and the trait is highly heritable [1 mark]. Cannot achieve: Creating a wheat variety resistant to a new disease if no wheat plant in existence carries resistance alleles [1 mark]. Justification: selective breeding cannot create new alleles, so if the desired trait does not exist in the gene pool, another technology (such as genetic modification) would be required [1 mark].
Multiple Choice
1. BThe fundamental difference is the selecting agent. Option A is wrong because selective breeding does not directly change DNA. Option C is wrong because both processes apply to all organisms. Option D is wrong because selective breeding does not create new alleles.
2. CSelective breeding is limited to existing variation. Option A is wrong because selective breeding needs no lab equipment. Option B is wrong because selective breeding does not cause mutations. Option D is wrong because selective breeding is legal and widely practised.
3. DThe breeder deliberately chose which plants reproduced, which is selective breeding. Option A describes GM. Option B confuses the breeder's selection with environmental selection, the breeder collected the seeds, not the drought. Option C describes gene editing.
4. AThis demonstrates that selective breeding can produce dramatic changes by increasing desirable allele frequencies over generations. Option B is wrong because this was not lab DNA editing. Option C is wrong because not all sheep naturally have such fine wool. Option D describes Lamarckism, which is incorrect.
5. BThe Cavendish banana is genetically uniform (a clone), so there is little variation for disease resistance. Option A is wrong because Cavendish bananas are not GM. Option C is biologically false. Option D overgeneralises, selective breeding does not always cause disease.
Short Answer Model Answers
Q6 (3 marks): Selective breeding is the process of choosing parents with desirable traits and allowing only those individuals to reproduce [1 mark]. It is considered a genetic technology because it deliberately changes the genetic makeup of a population by increasing the frequency of specific alleles [1 mark]. Even though DNA is not edited in a lab, the outcome is genetic change directed by human choice, which fits the definition of a technology that manipulates heritable characteristics [1 mark].
Q7 (4 marks): Similarity 1: Both processes change allele frequencies in a population over generations [1 mark]. Similarity 2: Both rely on heritable traits and genetic variation [1 mark]. Difference 1: In natural selection, the environment selects which individuals survive and reproduce; in selective breeding, humans make that choice [1 mark]. Difference 2: Natural selection has no predetermined goal, while selective breeding aims to increase a specific trait [1 mark]. Australian example: Australian Merino sheep were selectively bred for finer wool, whereas wild sheep (such as ancestors) were shaped by natural selection for survival in harsh environments [1 mark, bonus if included].
Q8 (5 marks): Selective breeding cannot solve this problem because it only works with genetic variation that already exists in the population [1 mark]. If no cattle carry any alleles for resistance to this new virus, there are no resistant individuals to select as parents [1 mark]. Selective breeding increases the frequency of existing alleles but cannot create new ones [1 mark]. A suitable alternative technology would be genetic modificationintroducing a resistance gene from another organism into the cattle genome [1 mark]. Another option is gene editing (CRISPR), which could potentially create or enhance resistance by making precise changes to the cattle DNA [1 mark].
Jump Through Genetics!
Climb platforms using your knowledge of selective breeding, artificial selection and natural selection. Pool: Lesson 6.