CIE AS/A Level Biology -17.2 Natural and artificial selection- Study Notes- New Syllabus
CIE AS/A Level Biology -17.2 Natural and artificial selection- Study Notes- New Syllabus
Ace A level Biology Exam with CIE AS/A Level Biology -17.2 Natural and artificial selection- Study Notes- New Syllabus
Key Concepts:
- explain that natural selection occurs because populations have the capacity to produce many offspring that compete for resources; in the ‘struggle for existence’, individuals that are best adapted are most likely to survive to reproduce and pass on their alleles to the next generation
- explain how environmental factors can act as stabilising, disruptive and directional forces of natural selection
- explain how selection, the founder effect and genetic drift, including the bottleneck effect, may affect allele frequencies in populations
- outline how bacteria become resistant to antibiotics as an example of natural selection
- use the Hardy–Weinberg principle to calculate allele and genotype frequencies in populations and state the conditions when this principle can be applied (the two equations for the Hardy–Weinberg principle will be provided, as shown in the Mathematical requirements)
- describe the principles of selective breeding (artificial selection)
- outline the following examples of selective breeding:
- the introduction of disease resistance to varieties of wheat and rice
- inbreeding and hybridisation to produce vigorous, uniform varieties of maize
- improving the milk yield of dairy cattle
Natural Selection
🔹 Key Concept
- Natural selection is the process by which individuals with traits best suited to their environment are more likely to survive and reproduce.
- Over time, this leads to changes in allele frequencies in a population.
🌿 Process of Natural Selection
- Overproduction of offspring: Populations can produce more offspring than the environment can support. Leads to competition for limited resources (food, space, mates).
- Variation within the population: Individuals show genetic differences (due to mutations, gene shuffling). Some traits make individuals better adapted to their environment.
- Struggle for existence: Not all individuals survive due to competition. Only the fittest individuals survive and reproduce.
- Survival and reproduction of the fittest: Individuals with advantageous traits pass on their alleles to the next generation. Over generations, the population evolves with more individuals having advantageous traits.
📌 Example
- Peppered moths during the Industrial Revolution:
- Dark moths survived better in polluted areas → increased frequency of dark-colored allele.
- Light moths survived better in clean areas → increased frequency of light-colored allele.
✅ Summary:
Natural selection depends on variation, competition, and differential survival.
Best-adapted individuals contribute more to the next generation, shaping the population over time.
Types of Natural Selection by Environmental Factors
1. Stabilising Selection
- Definition: Environmental conditions favour average phenotypes and select against extremes.
- Effect: Reduces variation; population stays around the mean.
- Example: Human birth weight: very low or very high birth weights have higher mortality → average birth weight favoured.
2. Directional Selection
- Definition: Environmental factors favour one extreme phenotype over the other.
- Effect: Shifts the population’s traits toward that extreme over generations.
- Example: Peppered moths during Industrial Revolution: dark-coloured moths favoured → population shifted toward darker phenotype.
3. Disruptive (Diversifying) Selection
- Definition: Environmental factors favour both extreme phenotypes and select against the average.
- Effect: Can lead to two or more distinct phenotypes in the population.
- Example: Beak size in birds: very large or very small beaks are favoured for accessing different seed types; medium beaks disadvantaged.
📌 Key Points
- Stabilising: favours average → reduces extremes
- Directional: favours one extreme → shifts population
- Disruptive: favours both extremes → increases variation or creates distinct groups
Factors Affecting Allele Frequencies in Populations
1. Natural Selection
- Definition: Certain alleles confer a survival or reproductive advantage.
- Effect: Advantageous alleles increase in frequency over generations; disadvantageous alleles decrease in frequency.
- Example: Peppered moths → dark allele increased in polluted areas.
2. Founder Effect
- Definition: When a small group of individuals forms a new population, the allele frequencies may differ from the original population.
- Effect: Can lead to high frequency of rare alleles or loss of alleles.
- Example: Certain genetic disorders are more common in isolated human populations (e.g., Ellis-van Creveld syndrome in the Amish).
3. Genetic Drift
- Definition: Random changes in allele frequencies in small populations due to chance events.
- Effect: Can lead to loss or fixation of alleles over time; more pronounced in small populations.
4. Bottleneck Effect
- Definition: Sudden drastic reduction in population size due to a catastrophic event (natural disaster, disease).
- Effect: Reduces genetic diversity; surviving population may have allele frequencies different from the original population.
- Example: Northern elephant seal population was reduced → low genetic diversity today.
📌 Key Points
| Factor | Mechanism | Effect on Allele Frequency | Example |
|---|---|---|---|
| Natural Selection | Advantageous alleles increase survival | Increase of beneficial alleles | Peppered moths |
| Founder Effect | Small new population | Random allele frequencies | Amish population disorders |
| Genetic Drift | Random chance in small populations | Loss or fixation of alleles | Small isolated island populations |
| Bottleneck Effect | Sudden population reduction | Reduced diversity, altered frequencies | Northern elephant seals |
Antibiotic Resistance in Bacteria – Example of Natural Selection
🌱 Key Concept
- Antibiotic resistance is a classic example of natural selection in action.
- Bacteria with mutations that confer resistance survive antibiotic treatment, while susceptible bacteria die.
How Resistance Develops
- Variation exists: Random mutations in bacterial DNA create genetic variation. Some mutations may confer resistance to a specific antibiotic.
- Selection pressure: Use of antibiotics kills susceptible bacteria, leaving resistant bacteria alive.
- Survival of the fittest: Resistant bacteria survive and reproduce, passing on the resistance allele to offspring.
- Increase in allele frequency: Over generations, the resistance allele becomes more common in the population.
📌 Example
- Staphylococcus aureus → some strains became methicillin-resistant (MRSA) due to natural selection under antibiotic pressure.
✅ Summary:
- Random mutations → some confer antibiotic resistance
- Antibiotics kill susceptible bacteria → resistant bacteria survive.
- Resistant alleles spread → population evolves resistance.
Hardy-Weinberg Principle
🌱 Key Concept
- The Hardy-Weinberg principle allows calculation of allele and genotype frequencies in a population.
- Shows how genetic variation remains constant in a population if certain conditions are met.
Conditions for Hardy-Weinberg Equilibrium
- Large population (no genetic drift)
- Random mating
- No mutation
- No migration (no gene flow)
- No natural selection
🔹 Allele and Genotype Frequencies
- If:
- p = frequency of dominant allele (A)
- q = frequency of recessive allele (a)
- Then:
- \( p + q = 1 \) (allele frequencies)
- \( p^2 + 2pq + q^2 = 1 \) (genotype frequencies)
- Where:
- \( p^2 \) = frequency of homozygous dominant (AA)
- \( 2pq \) = frequency of heterozygotes (Aa)
- \( q^2 \) = frequency of homozygous recessive (aa)
🔹 Example Calculation
- Suppose 16% of a population shows a recessive phenotype (aa):
- \( q^2 = 0.16 \) → \( q = \sqrt{0.16} = 0.4 \)
- \( p = 1 – q = 0.6 \)
- Genotype frequencies:
- AA = \( p^2 = 0.36 \)
- Aa = \( 2pq = 0.48 \)
- aa = \( q^2 = 0.16 \)
You can also refer the example beside of red and white flower.
📌 Key Points
- Useful for predicting allele/genotype frequencies in a population.
- Can indicate if a population is evolving (deviations from equilibrium).
Principles of Selective Breeding (Artificial Selection)
🔹 What is Selective Breeding?
- Selective breeding (artificial selection) is the process by which humans choose organisms with desirable traits to reproduce.
- Goal: Produce offspring with improved or specific characteristics.
🌿 Principles of Selective Breeding
- Variation exists in the population: Traits must show genetic variation among individuals.
Example: Some cows produce more milk than others. - Selection of desirable traits: Choose individuals that show the desired characteristic.
Example: Tall plants, disease-resistant animals, high-yield crops. - Controlled mating: Only the selected individuals are allowed to breed. Ensures the desirable alleles are passed to the next generation.
- Repeated selection over generations: Continue selecting the best offspring over many generations. Desired traits become more common in the population.
🔹 Examples
- Cattle: Breeding cows for higher milk yield.
- Wheat: Selecting plants with bigger grains.
- Dogs: Breeding for size, temperament, or coat colour.
📌 Key Points
- Artificial selection relies on existing genetic variation.
- Unlike natural selection, it is human-directed.
- Over time, it can significantly change the characteristics of a population.
Examples of Selective Breeding
🌾 1. Introducing Disease Resistance in Wheat and Rice
- Goal: Make crops resistant to diseases (fungal, bacterial, viral).
- Method:
- Identify plants with natural resistance.
- Cross these with high-yielding varieties.
- Select offspring that combine resistance and high yield.
- Result: New varieties that survive disease outbreaks and maintain good yield.
🌽 2. Inbreeding and Hybridisation in Maize
- Inbreeding: Mating closely related plants to fix desired traits. Produces uniform and predictable offspring, but may reduce vigour if done excessively.
- Hybridisation: Cross two inbred lines to produce a hybrid. Hybrid offspring are vigorous, high-yielding, and uniform (heterosis/“hybrid vigour”).
- Result: Maize varieties with better growth, yield, and consistency.
🐄 3. Improving Milk Yield in Dairy Cattle
- Goal: Increase milk production.
- Method:
- Select cows with high milk yield and bulls from high-yielding lines.
- Mate them, monitor offspring, and continue selecting the best producers.
- Result: Herds with higher average milk yield per cow over generations.
📌 Key Points
- Selective breeding enhances desirable traits in plants and animals.
- Techniques include inbreeding, hybridisation, and selection for specific traits.
- Long-term application changes the genetic makeup of populations.