17.2.717-18 · Selection, Evolution, Classification and Conservation
Examples of Artificial Selection — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions
5 min read 9 parts Lesson 13 of 36 Unit quiz
Lesson summary
17.2.7 Examples of Artificial Selection — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Describe examples of selective breeding in crops and livestock and evaluate their benefits and drawbacks.Most selective breeding aims to increase the yield of a saleable product.…
- Main examples
- Other selected traits in wheat
- Brassica oleracea (wild cabbage)
- Inbreeding
- Outbreeding
- Hybrid vigour
Section 1 of 9Aims of selective breeding in agriculture
Most selective breeding aims to increase the yield of a saleable product.
It does not consider the survival of the organism, so it can produce organisms poorly adapted to their environment.
Unless the genetic mechanism of a trait is understood, other traits may be accidentally enhanced.
Main examples
- Main examples disease resistance in wheat and rice, hybridisation in maize, milk yield in cattle.
Section 2 of 9Disease resistance in wheat and rice
Disease resistance in wheat and rice
Wheat has been selectively bred for hundreds of years.
- Fusarium fungus causes head blight, which destroys plants and reduces yield.
- A fungus-resistant allele from another wheat species is introduced, making hybrids not susceptible to infection and raising yield.
- Introducing the allele can take many generations and collaboration with researchers and breeders.
Rice is prone to bacterial and fungal diseases.
- Bacterial blight and rice blast, caused by the fungus Magnaporthe, reduce yield.
- Scientists are breeding varieties resistant to several diseases.
Other selected traits in wheat
- Other selected traits in wheat larger grains, non-shattering ears, uniform ripening, dwarf varieties and drought tolerance.
Section 3 of 9Inbreeding and hybridisation in maize
1
Maize is a staple crop grown for livestock and people.
2
Heavy inbreeding gave small, weak plants with less vigour (inbreeding depression).
- The chance of harmful recessive alleles combining and being expressed rises.
- Homozygosity increases, so alleles at each locus are identical.
- Growth and survivability decrease.
3
Outbreeding (breeding unrelated individuals) prevents this.
- Harmful recessive alleles are less likely to combine in an individual.
- Heterozygosity increases, giving hybrid vigour: taller, healthier plants with greater yield.
4
Uniformity matters because plants must ripen together and be of similar height.
- Completely random outbreeding gives too much variation within a field.
- Farmers buy sets of homozygous seed from specialist companies and cross them to give a uniform F1 generation.
5
New hybrids are continually tested for pest and disease resistance, higher yield and growth in poor conditions.
Section 4 of 9Improving milk yield in cattle
1
Milk is a global food source rich in calcium and protein.
2
Farmers select the female cows with the highest milk yield and cross them with bulls related to high-yield females.
- Over generations this gave cows with greater milk yields, a great economic benefit.
3
Bulls are assessed by progeny testing and their semen is used for artificial insemination.
4
Breeding concentrates on one or a few traits, often to an extreme, with little thought for health.
- Selected cows suffer more mastitis, milk fever and lameness than cows that bred at random.
Section 5 of 9Other examples
Other examples
Brassica oleracea (wild cabbage)
- Brassica oleracea (wild cabbage) selection of different enlarged organs gave several vegetables, all still one species.
- Cabbage — terminal bud;
- kale — leaves;
- Brussels sprouts — lateral buds;
- kohlrabi — swollen stem;
- cauliflower and broccoli — flower clusters.
Dogs
- Dogs all breeds descend from wolves and are one species, but extreme breeds have inherited problems such as breathing difficulty and hip dysplasia.
Chickens
- Chickens broilers reach market mass in about six weeks, and laying hens produce over 300 eggs a year compared with about 20 for the wild ancestor.
Section 6 of 9Evaluating selective breeding
Benefits
- Benefits higher yield, uniform products and disease resistance.
Costs
- Costs inbreeding, loss of genetic diversity, health and welfare problems and loss of traditional varieties.
Genetic markers speed up selection, and gene editing can introduce resistance directly.
Seed banks and wild relatives keep a reservoir of alleles for new threats.
In artificial selection humans apply the selection pressure for a desired outcome,
- whereas in natural selection the environment applies it without an aim.
Section 7 of 9More from the specification
More from the specification
Selective breeding and allele frequency changes
- Artificial selection involves humans choosing parents with desirable traits to breed, progressively increasing the frequency of favoured alleles across successive generations.
Inbreeding depression and loss of heterozygosity
- Prolonged selective breeding narrows the gene pool,
- increasing homozygosity for deleterious recessive alleles and reducing population resilience.
Dairy cattle and crop plant improvements
- Selective breeding has substantially enhanced milk yields in dairy cattle and drought resistance or grain size in staple cereal crops.
Selective breeding and allele frequency changes
- Artificial selection involves humans choosing parents with desirable traits to breed, progressively increasing the frequency of favoured alleles across successive generations.
Inbreeding depression and loss of heterozygosity
- Prolonged selective breeding narrows the gene pool,
- increasing homozygosity for deleterious recessive alleles and reducing population resilience.
Dairy cattle and crop plant improvements
- Selective breeding has substantially enhanced milk yields in dairy cattle and drought resistance or grain size in staple cereal crops.
Genetic drift
- Change in allele frequency due to chance, most important in small populations.
Founder effect
- Loss of genetic variation when a few individuals found a new population.
Bottleneck effect
- Loss of genetic variation after a sudden fall in population size.
Directional selection
- Selection favouring one extreme phenotype.
Hardy-Weinberg principle
- Equations predicting allele and genotype frequencies in a non-evolving population.
Stabilising selection
- Selection favouring the average phenotype.
Disruptive selection
- Selection favouring both extreme phenotypes.
Population
- A group of organisms of one species in the same area at the same time.
Frequency
- Proportion of sampling units in which a species is present.
Phenotype
- The observable characteristic.
Antibiotic
- A drug that kills bacteria or stops their growth and does not affect viruses.
Dominant
- Expressed when present in the genotype.
Allele frequency
- The proportion of all alleles of a gene that are of one form.
p + q = 1
What the symbols mean: p = frequency of the dominant allele · q = frequency of the recessive allele · p2 = homozygous dominant · 2pq = heterozygous · q2 = homozygous recessive
Section 8 of 9Key terms
| Term | Meaning |
|---|---|
| Selective breeding | Breeding of chosen individuals to increase the expression of a desired trait. |
| Inbreeding | Breeding of closely related individuals, raising homozygosity. |
| Outbreeding | Breeding of individuals that are not closely related. |
| Hybrid vigour | Greater growth and yield in heterozygous offspring. |
| F1 generation | First generation of offspring from a cross of two parent lines. |
| Mastitis | Inflammation of the udder in dairy cows. |
Section 9 of 9Exam technique
1
Saying selective breeding improves the organism's survival — it improves yield for humans, and survival may fall.
2
Confusing inbreeding and outbreeding — inbreeding raises homozygosity, outbreeding raises heterozygosity and vigour.
3
Saying outbreeding gives uniform plants — random outbreeding gives variation, so F1 seed from homozygous parents is used.
4
Treating the selection pressure in artificial selection as environmental — it is applied by humans.
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