Selective breeding in plants and animals

Selective breeding is when humans choose which plants or animals are allowed to reproduce so that a desired (useful) characteristic is passed on to the next generation. Because it is people, not the environment, doing the choosing, selective breeding is also called artificial selection.

Compare it with natural selection, where the environment decides which organisms survive and reproduce. In selective breeding the human breeder is the one doing the selecting — choosing the cow that gives the most milk, or the wheat plant with the biggest grains.

A desired characteristic is simply a feature that is useful to humans, for example:

• in plants — a higher yield (more food), disease resistance, larger fruit or faster growth;
• in animals — a high milk yield in cows, more meat, more eggs in chickens, or a particular temperament in dogs.

The characteristic must be inherited (controlled by genes), because only inherited features can be passed on to the offspring.

Exam tip. A common one-mark definition is: 'selective breeding is when humans choose organisms with a desired characteristic to breed from.' The words humans/people choose are essential — they separate selective breeding from natural selection.

Selective breeding follows the same four steps whether you are breeding plants or animals. Examiners love to ask you to put these steps in order, so learn them carefully:

1. Choose the parents that show the desired characteristic (e.g. the cows that give the most milk).
2. Breed these chosen parents together.
3. From the offspring, select the best ones — those that show the desired characteristic most strongly.
4. Repeat the process over many generations, each time breeding from the best offspring.

Each generation, more of the population shows the desired characteristic, until almost all of them have it.

Exam tip. The phrase "over many generations" (or "repeated for several generations") is a frequent marking point. Do not stop after breeding once — the characteristic only becomes common in the population after the cycle is repeated.

Spec point 5.10 is about using selective breeding to develop plants with desired characteristics. Farmers and plant breeders most often select for:

• Higher yield — plants that produce more food (e.g. wheat with more or larger grains, fruit trees with bigger fruit).
• Disease resistance — crops that can survive attacks by fungi, bacteria or pests, so less of the harvest is lost.
• Other useful traits such as drought tolerance, faster growth, or sweeter taste.

For example, to breed a wheat crop with a higher yield:

1. Choose the wheat plants that produce the most grain.
2. Breed them together (cross-pollinate them).
3. From the new plants, select those with the highest grain yield.
4. Repeat over many generations, each time keeping only the highest-yielding plants.

After many generations, almost all the wheat plants give a high yield.

One reason selective breeding is often quicker and easier in plants than in animals is that many plants produce lots of offspring and can sometimes be self-pollinated or grown from cuttings, giving breeders plenty of plants to choose from.

Exam tip. When the question is about plants, give a plant example — 'wheat with higher yield' or 'a disease-resistant crop' — not a cow or dog. Matching your example to the organism in the question is an easy mark to secure.

Spec point 5.11 is about using selective breeding to develop animals with desired characteristics. Common examples are:

• Cows bred for a high milk yield (more milk per cow) or for more meat.
• Chickens bred to lay more eggs or to grow more quickly for meat.
• Dogs bred for a particular temperament, size or appearance.

For example, to breed cows with a high milk yield:

1. Choose the cow(s) that produce the most milk and a bull from a high-milk-yielding line.
2. Breed them together.
3. From the calves, select the females that go on to give the most milk.
4. Repeat over many generations.

Selective breeding is usually slower in animals than in plants, because animals have fewer offspring and take longer to mature before they can breed — so it takes more years (more generations) to see a big change.

Exam tip. Be precise about what is selected: for milk you select cows that produce the most milk (and breed them with a bull from a high-yielding line). Vague answers like 'choose the best cow' may not score — say most milk / highest milk yield.

Selective breeding is very useful, but it has important drawbacks that examiners expect you to discuss.

Advantages

• Produces plants and animals that are more useful to humans — for example more food (higher-yield crops), more milk or meat, and crops that resist disease.
• Can produce hardier crops and animals, helping to feed a growing population.

Disadvantages

• Reduced genetic variation. Because breeders keep choosing organisms with the same characteristic, the population ends up with very similar genes — there is less variation in the gene pool.
• Increased risk from disease/change. With little variation, if a new disease appears or the environment changes, few or no individuals may have the alleles needed to survive, so the whole population could be wiped out.
• Inbreeding problems. Selective breeding often involves breeding closely related organisms (inbreeding). This increases the chance that harmful recessive alleles are inherited, leading to health problems, weaker offspring, and lower fertility.

Exam tip. A 'discuss the disadvantages' question wants the mechanism, not just the word. Say selective breeding reduces genetic variation, and explain the consequence — a new disease could wipe out the population because there is not enough variation for some to survive. Linking inbreeding → harmful recessive alleles → health problems also scores well.