Selective BreedingEdexcel IGCSE Biology: Revision notes
Section 1
What is selective breeding?
Selective breeding (also called artificial selection) is the process by which humans choose organisms with desirable characteristics and breed them together, over many generations, to produce offspring with those characteristics. This differs from natural selection, where the environment (not humans) determines which organisms survive and reproduce.
The basic process for both plants and animals follows the same pattern:
- Choose parents that show the desired characteristic most strongly
- Breed those parents together
- Select the offspring that best show the desired characteristic
- Breed the selected offspring together
- Repeat this process over many generations until all offspring consistently show the desired characteristic
Examiners want the full repeated-generations cycle, not just 'pick the best ones and breed them' — always mention that the process is repeated over many generations.
Section 2
How does selective breeding develop plants with desired characteristics?
Selective breeding is used to develop crop plants with characteristics that are useful to farmers or consumers, such as:
- Higher crop yield (more grain, fruit, or vegetable produced per plant)
- Resistance to disease or pests
- Faster growth or earlier ripening
- Improved taste, size, or nutritional content
- Tolerance to drought, cold, or poor soil
A plant breeder identifies individual plants already showing the desired trait (for example, the highest-yielding wheat plants in a field), collects seed from only those plants, and grows the next generation from that seed. The best plants from that new generation are again selected and bred. Over many generations, the desired characteristic becomes stronger and more consistent across the whole population.
Modern wheat varieties have been selectively bred over many generations for shorter stems (so more energy goes into producing grain rather than stem growth) and higher grain yield per plant.
Section 3
How does selective breeding develop animals with desired characteristics?
The same principle applies to livestock. Farmers select breeding animals that show the desired characteristic and breed them together, repeating this over generations. Characteristics commonly selected for include:
- High milk yield in dairy cattle
- Fast growth rate and high meat yield in beef cattle, pigs, or chickens
- High egg production in hens
- Docile (calm) temperament, making animals easier and safer to handle
- Disease resistance
For fish farming specifically, selective breeding is used alongside other management methods (maintaining water quality, controlling predation and disease, removing waste, and controlling feeding) to increase the yield of farmed fish such as salmon or trout.
Section 4
What are the advantages and disadvantages of selective breeding?
| Advantages | Disadvantages |
|---|---|
| Produces higher yields, improving food security | Reduces genetic variation in the population |
| Can improve disease resistance or hardiness | Reduced variation makes the population more vulnerable to a new disease or environmental change, since fewer individuals carry resistant alleles |
| Improves consistency and quality of produce | Can lead to inbreeding, increasing the chance of inherited defects appearing |
| Can produce animals/plants better suited to farming conditions | Selectively bred organisms may become dependent on human care (e.g. selective breeding for high yield can reduce natural hardiness) |
Students often only give advantages of selective breeding and forget the key disadvantage: reduced genetic variation, which increases vulnerability to disease. Always mention both sides if the question asks for advantages AND disadvantages.
Must Know
- Selective breeding = humans choose organisms with desired characteristics and breed them together over many generations
- The cycle (select parents → breed → select best offspring → breed again) is repeated many times
- Used in plants to improve yield, disease resistance, growth rate, and quality
- Used in animals to improve milk/meat/egg yield, temperament, and disease resistance
- Selective breeding in fish farming works alongside water quality, disease, predation, and feeding control
- Main disadvantage: reduces genetic variation, increasing vulnerability to disease and increasing risk of inherited defects
That's the notes covered.
Carry on to the next subtopic.