18.3 Selection: IGCSE Biology Revision Notes

1. Overview

Selection is the process by which certain heritable traits become more or less common in a population. It is the fundamental mechanism of evolution, explaining how species adapt to their environments over millions of years (natural selection) and how humans have modified plants and animals to suit our needs (artificial selection).

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Key Definitions

• Natural Selection: The process by which individuals with favorable phenotypes are more likely to survive and reproduce, passing their alleles to the next generation.
• Selective Breeding (Artificial Selection): The process by which humans choose specific individuals with desirable features to cross-breed and produce offspring.
• Adaptation: The process, resulting from natural selection, by which populations become more suited to their environment over many generations.
• Variation: Differences between individuals of the same species.
• Allele: An alternative form of a gene.
• Fitness: The probability of an organism surviving and reproducing in the environment in which it is found.

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Core Content

Natural Selection

Natural selection occurs because of the environmental "pressures" placed on a population. It follows a specific logical sequence:

1. Genetic Variation: Within any population, there is variation between individuals caused by different alleles (resulting from mutations).
2. Production of Many Offspring: Most populations produce more offspring than the environment can support.
3. Struggle for Survival: Overproduction leads to competition for limited resources (food, water, space, mates).
4. Survival of the Fittest: Individuals with traits that make them better adapted to their environment are more likely to survive.
5. Reproduction: These survivors reproduce and pass on their advantageous alleles to the next generation.
6. Increase in Allele Frequency: Over time, the frequency of these "favorable" alleles increases in the population.

Selective Breeding (Artificial Selection)

This is selection carried out by humans for economic or aesthetic reasons.

The Process:

1. Selection: Humans identify individuals with desirable features (e.g., high milk yield in cows, large seeds in wheat).
2. Crossing: These individuals are bred together.
3. Selection of Offspring: From the resulting offspring, those showing the best combination of the desired features are selected.
4. Repetition: This process is repeated over many generations until the trait is "fixed" in the population.

Common Applications:

• Crop Plants: Improved yield, resistance to pests/disease, or better nutritional value.
• Domesticated Animals: Cows with higher milk yield, sheep with better quality wool, or dogs with specific temperaments.

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Extended Content (Extended Curriculum Only)

Adaptation

Adaptation is the long-term result of natural selection. It describes how a population’s features change over time to match the demands of its environment.

• Structural Adaptation: Physical features (e.g., a polar bear's thick layer of blubber).
• Behavioral Adaptation: The way an organism acts (e.g., bird migration).

Development of Antibiotic Resistance in Bacteria

The evolution of "superbugs" like MRSA is a modern example of natural selection happening rapidly:

1. Variation: In a population of bacteria, a random mutation creates an allele for antibiotic resistance.
2. Selection Pressure: When a person takes an antibiotic, the drug kills the "normal" (susceptible) bacteria.
3. Survival: The resistant bacterium survives the treatment.
4. Reproduction: With no competition from other bacteria, the resistant individual divides rapidly (binary fission).
5. Inheritance: The resistance allele is passed to all offspring. Soon, the entire strain is resistant to that antibiotic.

Differences Between Natural and Artificial Selection

Feature	Natural Selection	Artificial Selection (Selective Breeding)
Selection Agent	The environment (nature)	Humans
Goal	Survival and reproduction	Desirable traits for human use
Speed	Very slow (thousands/millions of years)	Relatively fast (years/decades)
Genetic Diversity	Usually maintains diversity	Decreases genetic diversity (inbreeding)

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Key Equations

While there are few specific formulas for selection, you may be asked to calculate the percentage change in a population:

Percentage Increase/Decrease Calculation: $$\text{Percentage Change} = \frac{\text{New Value} - \text{Original Value}}{\text{Original Value}} \times 100$$

• Positive result: Percentage increase.
• Negative result: Percentage decrease.

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Common Mistakes to Avoid

• ❌ Wrong: "The giraffe stretched its neck to reach leaves, so its babies were born with long necks." (Lamarckian evolution).
• ✓ Right: Long-necked giraffes were better able to survive and reproduce, passing on the alleles for long necks to their offspring.
• ❌ Wrong: "Bacteria become immune to antibiotics."
• ✓ Right: Bacteria become resistant (immunity involves antibodies/immune systems; bacteria are single-celled and do not have immune systems).
• ❌ Wrong: "The organism chose to change to survive."
• ✓ Right: Selection is a passive process; individuals with "bad" traits die, while those with "good" traits happen to survive.

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Exam Tips

• Command Words: Look out for "Explain." In selection questions, an "explanation" usually requires the 5-step logic (Variation $\rightarrow$ Competition $\rightarrow$ Survival $\rightarrow$ Reproduction $\rightarrow$ Inheritance).
• Keywords: Always use the word "Alleles" instead of just "genes" or "traits" when discussing what is passed to the next generation.
• Contexts: Expect questions about the Peppered Moth (industrial melanism) or Antibiotic Resistance. The steps are always the same, just change the species name and the selection pressure.
• Numerical Data: You may be given tables showing the number of individuals of two varieties over time. If the numbers of one variety are increasing, identify the "selection pressure" (e.g., a predator or a climate change) causing it.