Selective Breeding

How humans shape the characteristics of living organisms through controlled reproduction.

VTR Think–Puzzle–Explore

Before we begin — let's surface what you already know and where your curiosity lies.

What do you THINK you know about selective breeding?

What PUZZLES you about it?

How could we EXPLORE these puzzles?

Prior Knowledge Retrieval

Before we learn new content, let's check what you remember from earlier topics.

Retrieval Practice

Organisms of the same species show differences in their characteristics. What is the term for these differences?

Retrieval Practice

What is natural selection?

What is Selective Breeding?

Selective breeding is the breeding of only those individuals with desirable features or characteristics. It is sometimes called 'artificial selection' because human choice, rather than environmental factors, provides the selection pressure.

The History

About 12,000 years ago, human life changed dramatically. Humans began to grow plants and keep animals for milk and meat — becoming farmers rather than hunters. This first took place in the Middle East, with similar changes in the Americas and Far East.

Ever since the cultivation of the first wheat and barley, humans have tried to obtain bigger yields. They cross-bred different plants to obtain strains that produced more grain. They bred sheep and goats to give more milk and meat. Selective breeding had begun.

Today, modern gene technology makes it possible to create a new strain of plant within weeks, rather than years.

Dual Coding — Compare the Processes

Quick Check

What is the key difference between natural selection and selective breeding?

Quick Check

Which of the following is another term for selective breeding?

Selective Breeding in Plants

5.10

Crop plants are bred to produce strains that: give higher yields, are resistant to certain diseases, are resistant to insect pest damage, are hardier (survive in harsher climates or are productive for longer), and have a better balance of nutrients.

Case Study: Wheat

The production of modern bread wheat is one of the earliest examples of selective breeding. About 11,000 years ago, two strains of wild wheat (Triticum monococcum and an unknown wild wheat) were cultivated. Initially, cross-breeding produced only sterile offspring.

About 8,000 years ago, a fertile hybrid called Triticum turgidum (emmer wheat) appeared — with a much higher yield. This was then cross-bred with another wild relative to produce Triticum aestivum — modern bread wheat, with an even bigger yield and much easier to process into flour.

Did you know? Each original wild wheat species had 14 chromosomes per cell. The wild emmer hybrid had 28 chromosomes per cell. Modern bread wheat has 42 chromosomes per cell.

Case Study: Brassica

Brassica is a genus of cabbage-like plants. One species of wild brassica (Brassica oleracea) was selectively bred to give several strains, each with specific enhanced features:

🥬

Cabbage
Terminal bud enlarged

🥦

Broccoli
Flowers and stems

🌿

Kale
Large leaves

🥗

Cauliflower
Large flower cluster

🟢

Brussels Sprouts
Lateral buds enlarged

🟩

Kohlrabi
Swollen stem

All of these vegetables came from the same original wild plant. The only difference is which feature humans selected for when breeding.

VTR See–Think–Wonder

Look at the Brassica varieties above. All came from one wild plant species.

What do you SEE?

What do you THINK is going on?

What does it make you WONDER?

Progress Check — Plants

Complete the sentences using the word bank below:

desirable yields resistant selective breeding cross-bred humans

Farmers have used for thousands of years to produce crops with higher . Plants with features are together. The selection pressure comes from , not the environment. Some crops have been bred to be to diseases or pests.

Answer in full sentences:

1. Describe the process by which modern bread wheat (Triticum aestivum) was developed from wild wheat species.

Two wild wheat species (Triticum monococcum and an unknown wild wheat) were cross-bred about 11,000 years ago. Initially this produced only sterile offspring. About 8,000 years ago, a fertile hybrid called emmer wheat (Triticum turgidum) appeared with a much higher yield. This emmer wheat was then cross-bred with another wild relative to produce Triticum aestivum — modern bread wheat — which had an even higher yield and was easier to process into flour.

2. Explain why all Brassica vegetables (cabbage, broccoli, kale, etc.) are considered the result of selective breeding from one species.

All these vegetables originated from the same wild species, Brassica oleracea. Farmers selectively bred plants with specific enhanced features over many generations. Those with larger terminal buds became cabbage, those with larger flower clusters became cauliflower, those with enlarged lateral buds became Brussels sprouts, and so on. Each variety was produced by choosing plants with the desired feature and breeding them together repeatedly.

Extended response — no scaffolding:

A plant breeder wants to develop a new variety of tomato that is both disease-resistant AND produces larger fruit. The breeder has two existing varieties: one that is disease-resistant but produces small fruit, and one that produces large fruit but is easily infected by disease.

Describe in detail how the breeder could use selective breeding to develop the desired variety. Explain why this process would take several generations.

The breeder would cross-breed the disease-resistant small-fruited plant with the large-fruited disease-susceptible plant. The offspring (F1 generation) would show a mix of characteristics. The breeder would then select offspring that show both desirable traits — some disease resistance AND larger-than-normal fruit. These selected individuals would be bred together. This process would be repeated over many generations, each time selecting the offspring that best combine both traits. It takes several generations because: (1) the desired combination of alleles may not appear in every offspring due to random assortment during meiosis, (2) the traits may be controlled by multiple genes, (3) each generation must be grown and assessed before selection can occur, and (4) the breeder needs to establish a stable breeding line where the traits breed true.

Selective Breeding in Animals

5.11

Farmers have bred stock animals to produce increased: meat, milk or egg production; fur quality; offspring numbers; and disease resistance. All modern dog breeds originated from the domesticated wolf.

How It Works

For many thousands of years, the only way to improve livestock was to mate a male and a female with the features desired in the offspring. For example, high-yielding cows would be bred with bulls from other high-yielding cows.

Since about 1950, artificial insemination (AI)A technique where semen is collected from a male with desirable features, diluted, frozen, and stored. It can then be used to inseminate many females — meaning one prize bull can father thousands of calves. has become widely available. Bulls with many desirable features are kept and semen is obtained, diluted, frozen and stored. This makes it possible for one prize bull to fertilise many thousands of cows.

Case Study: Dogs from Wolves

All modern dog breeds — from Great Danes to Chihuahuas — originated from the domesticated wolf. In domesticating the wolf, humans gained an animal capable of herding stock. The sheepdog has all the same instincts as the wolf except the instinct to kill — this has been selectively 'bred out'.

The enormous variety of dog breeds today demonstrates the power of selective breeding to dramatically change an organism's characteristics over many generations.

Progress Check — Animals

Match the animal breeding goal with the correct description:

Which of these is a reason farmers selectively breed cattle?

What best describes artificial insemination?

💡 Hint: Think about what "artificial" means (done by humans) and "insemination" means (introducing sperm).

Answer in full sentences:

1. Explain the advantage of artificial insemination over traditional breeding methods in a selective breeding programme.

Artificial insemination allows semen from one prize male with many desirable characteristics to be collected, diluted, frozen and stored. This semen can then be used to fertilise many thousands of females, rather than the male being limited to mating with only a few females naturally. This means desirable genes can be spread much more quickly through a population, speeding up the selective breeding process and allowing farmers to improve their herds more efficiently.

2. Explain how modern dog breeds demonstrate the effects of selective breeding.

All modern dogs originated from one ancestral type — the domesticated wolf. The enormous range of dog breeds today (from tiny Chihuahuas to large Great Danes, with many different coat types, body shapes, and temperaments) demonstrates that selective breeding can produce dramatic changes in an organism's characteristics over many generations. Different breeds were produced by selecting for different traits — speed in greyhounds, herding ability in sheepdogs, companionship in labradors, etc.

Extended analysis — no scaffolding:

A diagram shows features of a cow that might be used in a breeding programme: total body mass, milk yield, growth rate, meat:fat ratio, and feed to meat/milk conversion rate.

a) Which features would you consider important in a breeding programme for dairy cattle? Justify your choices.

b) Assume you had all the techniques of modern selective breeding available. Describe how you would set about producing a herd of high-yielding beef cattle.

a) For dairy cattle, the most important features would be: milk yield (the primary product), feed to milk conversion rate (efficiency reduces costs), and total body mass (larger cows may produce more milk but also require more feed, so there's an optimum). Growth rate and meat:fat ratio are less important for dairy cattle as the focus is on milk production, not meat.

b) To produce high-yielding beef cattle: (1) Identify bulls and cows with the best combination of high growth rate, high meat:fat ratio, high total body mass, and efficient feed to meat conversion. (2) Use artificial insemination from the best bulls to fertilise many cows. (3) Monitor the offspring for these desired characteristics. (4) Select the best offspring and breed them together. (5) Continue this process over several generations, always selecting for the combination of desired traits. (6) Maintain genetic records to avoid inbreeding, which could reduce the health of the herd.

Progress Check 1

Retrieval practice on selective breeding content. Try to answer without looking back.

Progress Check

1. Which of the following statements is/are correct?
  1. Antibiotics are made by bacteria
  2. Antibiotics kill bacteria
  3. Antibiotics do not work on viruses

Progress Check

2. Which of the following best describes the meaning of biological 'fitness'?

Progress Check

3. What is the source of genetic variation?

Progress Check

4. Why is natural selection easy to observe in bacteria?

How confident are you so far?

Interleaved Practice

Questions from earlier topics mixed with selective breeding — this helps build stronger long-term memory.

Interleaved — Cell Structure

Which structure is found in plant cells but NOT in animal cells?

Back to Selective Breeding

A farmer wants to breed sheep that produce more wool. Which is the correct process?

Interleaved — Biological Molecules

Starch is a storage molecule found in plants. What is starch made of?

Interleaved — Movement of Substances

Which process moves molecules from a region of higher concentration to a region of lower concentration without requiring energy?

Interleaved — Respiration

What is the word equation for aerobic respiration?

Interleaved — Transport

Which type of blood vessel carries blood away from the heart?

Back to Selective Breeding

Warfarin is a pesticide developed to kill rats. When first used in 1950, it was very effective. Some rats had a mutant allele that made them resistant. Nowadays warfarin is much less effective. Which best explains this?

Interleaved — Gas Exchange

Which feature of alveoli helps maximise the rate of gas exchange?

Progress Check 2 — Tiered

Select your level. Foundation has scaffolding. Core is guided. Challenge is independent.

Fill in the blanks to describe the selective breeding process:

desirable breed offspring selected generations artificial

Step 1: Choose individuals with characteristics.

Step 2: these individuals together.

Step 3: From the , choose those with the best characteristics.

Step 4: These are and bred again.

Step 5: Repeat over many .

This process is also called selection.

VTR Claim–Support–Question

Consider this claim: "Selective breeding is always beneficial for the species being bred."

Make a CLAIM — do you agree or disagree? State your position.

Provide SUPPORT — what evidence supports your claim?

Raise a QUESTION — what challenge or uncertainty remains?

Key thinking points:
• Selective breeding can reduce genetic diversity within a population, making organisms more vulnerable to disease.
• Inbreeding (breeding closely related individuals) can lead to health problems — many pedigree dog breeds suffer from genetic disorders.
• While humans benefit from higher yields, the organisms themselves may not benefit (e.g., broiler chickens bred for such rapid growth that their legs cannot support their body weight).
• Selective breeding has provided food security for billions of people.
• The process is irreversible — once genetic diversity is lost, it cannot be recovered.

Read the scenario and answer all parts:

Natural selection happens when a selection pressure favours individuals with particular characteristics, so that they have a selective advantage.

Some plants growing in areas contaminated by waste from mines have developed a tolerance to toxic metals such as lead and copper. They are able to grow on polluted soil, while non-tolerant plants are killed by the metals in the soil.

a) How did the new tolerant varieties of plants arise?

b) With reference to this example, explain the terms:

i. selection pressure

ii. selective advantage

iii. natural selection

c) When metal-tolerant plants are grown on uncontaminated soil, they are out-competed by non-tolerant plants. Suggest a reason for this.

a) Random mutations in some plants gave them alleles that allowed them to tolerate toxic metals. These plants survived on contaminated soil while others died, and they reproduced and passed on the tolerance alleles.

b) i. Selection pressure: The toxic metals in the soil act as the selection pressure — they kill non-tolerant plants, meaning only tolerant plants survive to reproduce.
ii. Selective advantage: Metal-tolerant plants have a selective advantage on contaminated soil because they can survive and reproduce where other plants cannot.
iii. Natural selection: The process by which the tolerant plants survive and reproduce more than non-tolerant plants on contaminated soil, leading to an increase in the frequency of the tolerance allele in the population over generations.

c) Metal tolerance likely comes with an energy cost or reduced growth rate. On uncontaminated soil, there is no advantage to being tolerant, so non-tolerant plants that don't bear this cost can grow faster and out-compete the tolerant plants for resources like light, water, and minerals.

Struggle Zone

These questions are designed to be challenging. Productive struggle strengthens learning.

🧠 Struggle Question 1 — Maize Breeding Programme

The diagram in your textbook shows the results of a breeding programme to improve the yield of maize (sweetcorn). Pure line A × Pure line B → Hybrid E. Pure line C × Pure line D → Hybrid F. Then Hybrid E × Hybrid F → Hybrid G.

a) Describe the breeding procedure used to produce hybrid G.

b) Why might hybrid G have a higher yield than any of the pure lines?

c) How could you show that the differences between hybrid G and pure line C are genetic rather than environmental?

a) First, pure line A was cross-bred with pure line B to produce hybrid E. Separately, pure line C was cross-bred with pure line D to produce hybrid F. Then hybrid E was cross-bred with hybrid F to produce hybrid G. Each cross was between two different parental lines, bringing together different combinations of alleles.

b) Hybrid G combines genetic material from four different pure lines (A, B, C, and D). This gives it a wider range of alleles, potentially including the best yield-related alleles from each line. This is known as hybrid vigour (heterosis) — the offspring can be more productive than either parent because they benefit from the combined advantages.

c) Grow hybrid G and pure line C in exactly the same environmental conditions (same soil type, same watering regime, same temperature, same light levels, same fertiliser). If the differences in characteristics persist under identical conditions, then the differences must be genetic rather than environmental. You should also grow multiple plants of each type to ensure the results are reliable.

🧠 Struggle Question 2 — Connecting Concepts

A farmer uses selective breeding to produce a herd of high-milk-yield dairy cattle over 20 years. After 20 years, a new disease spreads through the herd and kills 90% of the cattle.

a) Using your knowledge of genetics and variation, explain why the disease was able to kill so many of the cattle.

b) Compare what happened to this herd with what might happen in a wild population of cattle facing the same disease. Explain the difference.

a) Selective breeding reduces genetic diversity because the farmer only breeds cattle with high milk yield, selecting a narrow range of alleles. Over 20 years, the herd becomes genetically very similar (homozygous for many genes). When the disease arrives, if none of the cattle carry alleles for resistance to this specific disease, the entire herd is vulnerable — they all share the same genetic weakness.

b) A wild population would have much greater genetic diversity because natural variation is maintained — no one is selecting which individuals breed. This means that within a large, genetically diverse wild population, some individuals are likely to carry alleles that provide resistance to the disease. These individuals would survive and reproduce (natural selection), and the population would recover. The selectively bred herd lacked this "genetic safety net."

Interleaved — Excretion

Which organ is primarily responsible for removing urea from the blood?

Interleaved — Coordination & Response

What is the function of a receptor in the nervous system?

Reflect on Your Learning

VTR I Used to Think… Now I Think

Reflect on how your understanding has changed during this lesson.

I used to think…

Now I think…

I still wonder…

VTR Connect–Extend–Challenge

CONNECT — How does selective breeding connect to what you already knew about natural selection and variation?

EXTEND — How has this lesson extended your thinking beyond what you knew before?

CHALLENGE — What is still challenging or unclear? What do you need to revisit?

Exit Ticket

Write a 3-sentence summary of selective breeding that includes:

✓ A definition of selective breeding
✓ One example in plants
✓ One example in animals

Final Confidence Check

Rate your confidence on each learning objective:

5.10 — I understand how selective breeding develops plants with desired characteristics

5.11 — I understand how selective breeding develops animals with desired characteristics

Lesson Complete

Evidence-informed design · Retrieval practice · Dual coding · Interleaving · Visible Thinking Routines