Horses: Selectively bred for specific traits such as speed (e.g., Thoroughbreds for racing), strength (e.g., Clydesdales for heavy work), and endurance (e.g., Arabians for long-distance riding).
Cattle:
Dairy Cattle: Bred for higher milk production (e.g., Holsteins).
Beef Cattle: Selected for increased muscle mass and meat quality (e.g., Angus).
Pigs: Bred for traits like faster growth rates, higher meat yield, and specific body compositions.
1.2 Crop Improvement
Maize (Corn): Enhanced for traits such as higher yields, drought resistance, pest resistance, and improved nutritional content.
Wheat (Triticum aestivum): Selectively bred for disease resistance (e.g., against Fusarium), gluten content variations, and increased yield.
Rice (Oryza sativa): Improved for higher yield, disease resistance (e.g., to bacterial blight and rice blast), and adaptability to different climatic conditions.
Vegetables and Fruits: Enhanced for size, flavor, shelf-life, and resistance to pests and diseases (e.g., tomatoes, apples).
2. Steps in the Artificial Selection Process
Artificial selection follows a systematic process to ensure the desired traits become prevalent in the population. The key steps include:
2.1 Variation in Population
Genetic Variation: Start with a population that exhibits variation in the traits of interest. This variation arises from genetic differences among individuals.
Example: In a wheat population, there may be variation in disease resistance levels.
2.2 Selection of Parents with Desired Traits
Choosing Parent Individuals: Select individuals that exhibit the target trait(s) at a superior level.
Example: Select a wheat plant that shows high resistance to a specific fungal disease.
2.3 Selection of a Second Parent
Complementary Traits: Choose a second parent that either has the same desirable trait or another beneficial trait to introduce diversity.
Example: Select a second wheat plant that not only is disease-resistant but also has a high yield.
2.4 Breeding of Selected Parents
Crossbreeding: Mate the selected parents to combine their desirable traits in the offspring.
Example: Cross a disease-resistant wheat plant with a high-yield wheat plant to produce offspring that may exhibit both traits.
2.5 Selection in Offspring
Evaluating Offspring: Grow the offspring to maturity and assess them for the desired traits.
Example: Expose offspring wheat plants to the fungal disease and select those that show the highest resistance and yield.
2.6 Repetition Over Multiple Generations
Consistent Selection: Repeat the selection and breeding process over several generations to stabilize the desired traits within the population.
Goal: Develop a population where all individuals consistently express the targeted traits.
3. Outcomes of Artificial Selection
3.1 Homozygosity
Genetic Uniformity: Through repeated selection, individuals become homozygous at gene loci controlling the desired traits. This ensures that these traits are reliably passed on to future generations.
3.2 Phenotypic Selection
Observable Traits: Breeders often select based on observable characteristics (phenotypes) without necessarily understanding the underlying genetic makeup (genotypes). This approach is effective even for complex traits controlled by multiple genes.
3.3 Stable Populations
Consistent Traits: After several generations, the population stabilizes with individuals uniformly expressing the desired traits, making them reliable for specific human needs.
4. Variations in Breeding Strategy
4.1 Cross-Breeding Within Generations
Introducing Diversity: Breeders may cross offspring from one generation with each other or with individuals from different varieties to introduce additional desirable traits.
Example: Crossing a disease-resistant variety with a high-yield variety to combine both traits.
4.2 Introduction of New Traits
Expanding Trait Repertoire: Breeders may introduce new traits into a population by incorporating individuals from other varieties or species that possess the desired characteristics.
Example: Introducing a drought-resistant gene from one wheat variety into another to enhance overall resilience.
5. Case Studies: Introduction of Disease Resistance
5.1 Selective Breeding in Wheat (Triticum aestivum)
Species: Most modern wheat varieties belong to Triticum aestivum.
Selective Breeding Goals:
Gluten Content:
High Gluten: Desired for bread flour, providing elasticity and chewiness.
Low Gluten: Preferred for pastries and other baked goods requiring tenderness.
Disease Resistance:
Focus on resistance to fungal diseases like head blight caused by Fusarium, which can significantly reduce yield.
Breeding Challenges:
Introducing resistance alleles from wheat varieties adapted to different climates requires extensive breeding over multiple generations to ensure compatibility and effectiveness.
Wheat Genetic Improvement Network:
Established: 2003 in the UK.
Objectives: Connect researchers and commercial breeders to screen wheat seed collections for traits like disease resistance and climate resilience.
Process: Identified plants with desirable traits are propagated and distributed to commercial breeders for further development.
5.2 Selective Breeding in Rice (Oryza sativa)
Species: Oryza sativa (rice).
Selective Breeding Initiatives:
International Rice Research Institute (IRRI) in the Philippines:
Role: Maintains the rice gene bank and leads the Global Rice Science Partnership.
Focus: Improving rice varieties to meet the demands of increasing global populations.
Disease Resistance Goals:
Bacterial Diseases:
Bacterial Blight: Targeted for resistance to prevent yield loss.
Fungal Diseases:
Rice Blast: Caused by Magnaporthe fungus, a major yield-reducing disease.
Sheath Rot: Caused by Sarocladium oryzae.
Other fungal diseases include spots and smuts.
Ongoing Research:
Developing rice varieties with broad-spectrum resistance to multiple pathogens to ensure stable and reliable yields across diverse environments.
6. Genetic Concepts in Artificial Selection
6.1 Inbreeding and Inbreeding Depression
Inbreeding:
Definition: Breeding between closely related or genetically similar individuals.
Effect: Increases homozygosity, making recessive alleles more likely to be expressed.
Inbreeding Depression:
Consequences:
Expression of harmful recessive alleles can lead to reduced growth, smaller size, weaker plants, and lower vitality.
Example: In maize (Zea mays), homozygous plants often show reduced growth and vigor compared to heterozygous counterparts.
6.2 Outbreeding and Hybrid Vigour (Heterosis)
Outbreeding:
Definition: Breeding between genetically diverse or unrelated individuals.
Effect: Increases heterozygosity, which can enhance the overall health and vigor of offspring.
Hybrid Vigour (Heterosis):
Definition: The phenomenon where hybrid offspring exhibit superior qualities compared to their parents.
Benefits:
Healthier and taller plants.
Higher yields.
Improved resistance to pests and diseases.
Better adaptation to nutrient-poor soils and water-limited environments.
6.3 Balancing Uniformity and Genetic Diversity in Maize Production
Challenges:
Farmers require uniform crops for efficient harvesting and market consistency.
Random outbreeding can result in excessive genetic variation, complicating cultivation and harvest.
Solution: Hybrid Seed Production
Seed Companies:
Create homozygous parent lines through inbreeding.
Cross these parent lines to produce F1 hybrid seeds that are genetically identical and exhibit both uniformity and hybrid vigour.
Advantages of F1 Hybrids:
Combine desirable traits such as high yield, disease resistance, and environmental adaptability.
Ensure uniform growth and ripening times.
Seed Dependence:
Reason: F1 hybrids do not reliably pass on hybrid vigour and uniformity to F2 generations.
Implication: Farmers must purchase new seeds each year rather than saving seeds from hybrid crops.
7. Improving Milk Yield in Dairy Cattle
7.1 Selective Breeding for High Milk Yield
Objective: Enhance milk production in dairy cattle by selecting cows and bulls that exhibit high milk yields.
Process:
Selection of Cows: Choose cows that produce the highest volumes of milk.
Selection of Bulls: Select bulls from families with a history of high milk production.
Breeding Over Generations: Continuously select and breed individuals with the highest milk yields to maximize this trait in the population.
7.2 Artificial Selection vs. Natural Selection
Artificial Selection:
Focus: Targets specific traits, such as milk yield.
Consequences: May inadvertently reduce genetic diversity and compromise other traits, potentially leading to health disadvantages.
Natural Selection:
Focus: Balances multiple traits to enhance overall survival and reproduction in diverse environments.
Outcome: Maintains genetic diversity and trait balance necessary for adaptation.
7.3 Case Study: Holstein Cattle in the USA
Experiment Setup:
Selection Line: Breeding only high-yield cows with bulls from high-yield families.
Control Line: Breeding cows randomly without selection for milk yield.
Duration: 25 years, with all cattle kept in identical environments to isolate genetic effects.
Results:
Milk Yield Increase: The selection line exhibited a significant increase in milk yield over time, while the control line showed a slight decline.
Health Costs: The selection line experienced higher health costs due to ailments associated with high milk production.
7.4 Health Implications of High Milk Yield
Increased Health Issues in Selection Line:
Mastitis: Inflammation of the udder, increased by the physical strain of high milk production.
Lameness: Heavier udders can strain the legs, leading to mobility issues.
Reproductive and Metabolic Disorders:
Ketosis: Metabolic disorder caused by energy deficiency.
High milk yield may be genetically linked to increased susceptibility to these health issues, indicating a trade-off between productivity and health.
Health Costs Comparison:AilmentSelection Line (USD)Control Line (USD)
Ailment
Selection Line (USD)
Control Line (USD)
Mastitis
43
16
Ketosis and Milk Fever
22
12
Reproductive Issues
18
13
Lameness
10
6
Respiratory Problems
4
1
7.5 Key Takeaways
Trade-Offs of Selective Breeding:
Enhancing specific traits such as milk yield can lead to increased susceptibility to health issues and higher associated costs.
Sustainability Concerns:
Selective breeding for extreme traits may compromise animal health and welfare.
It is essential to balance productivity with overall animal well-being to ensure sustainable agricultural practices.
8. Key Terms and Definitions
Artificial Selection: Human-directed selection of individuals with preferred traits to survive and reproduce, enhancing desirable characteristics in future generations.
Phenotype: The observable physical and biochemical characteristics of an organism, resulting from both genetic makeup and environmental influences.
Genotype: The genetic constitution of an organism, including all of its genes.
Homozygosity: Having two identical alleles for a particular gene.
Heterozygosity: Having two different alleles for a particular gene.
Inbreeding: Breeding between closely related individuals, often leading to increased homozygosity and potential inbreeding depression.
Inbreeding Depression: Reduction in fitness and vitality due to the expression of harmful recessive alleles from increased homozygosity.
Outbreeding: Breeding between genetically diverse or unrelated individuals, increasing heterozygosity.
Hybrid Vigour (Heterosis): Enhanced growth, health, and fertility observed in hybrid offspring compared to their parents.
Selective Breeding: The process of choosing which individuals get to reproduce based on specific desired traits.
9. Figures and Illustrations
Figure A: Impact of Sheath Rot on Rice
Description: Illustrates the effects of sheath rot caused by Sarocladium oryzae on rice plants, highlighting symptoms such as wilting and yield reduction.
Importance: Emphasizes the necessity for breeding disease-resistant rice varieties to maintain stable yields.
Figure B: Controlled Breeding in Wheat
Description: Depicts a controlled breeding setup for wheat to prevent accidental cross-pollination, ensuring that only selected individuals contribute to the next generation.
Importance: Demonstrates the importance of maintaining genetic control during the artificial selection process to achieve desired traits.
