Selection

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).

---

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.

---

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.

---

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)

---

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.

---

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.

---

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.

---

Exam-Style Questions

Practice these original exam-style questions to test your understanding. Each question mirrors the style, structure, and mark allocation of real Cambridge 0610 Theory papers.

Exam-Style Question 1 — Short Answer [5 marks]

Question:

Some species of bacteria are becoming increasingly resistant to antibiotics.

(a) Define the term antibiotic. [1]

(b) Describe how natural selection can lead to the development of antibiotic-resistant bacteria. [4]

Worked Solution:

(a)

1. An antibiotic is a chemical or drug that kills or inhibits the growth of bacteria. [Definition of antibiotic]

How to earn full marks:

• State that an antibiotic either kills bacteria OR inhibits their growth, not both.
• Mention that it affects bacteria, not viruses or other microorganisms.

(b)

1. There is genetic variation in the bacteria population, some bacteria have alleles that make them resistant to the antibiotic. [Explains that some bacteria are naturally resistant due to genetic variation.]

2. When antibiotics are used, most non-resistant bacteria are killed. [Explains that the antibiotic kills non-resistant bacteria.]

3. Resistant bacteria survive and reproduce, passing on the alleles for resistance to their offspring. [Explains that resistant bacteria reproduce and pass on their alleles.]

4. Over time, the proportion of resistant bacteria in the population increases. [Explains how the population shifts toward resistance.]

How to earn full marks:

• Clearly state there is genetic variation in the population.
• Mention that the antibiotic kills the non-resistant bacteria.
• Explain that resistant bacteria are able to survive and reproduce.
• Show that the proportion of resistant bacteria increases over time.

Common Pitfall: Remember to specify that bacteria inherit alleles, not genes, from their parents. Also, be sure to mention that the antibiotic kills the non-resistant bacteria, allowing the resistant ones to thrive.

Exam-Style Question 2 — Short Answer [6 marks]

Question:

Farmers use selective breeding to improve the characteristics of their crops and livestock.

(a) Define the term selective breeding. [1]

(b) Describe how a farmer might use selective breeding to increase meat production in chickens. [5]

Worked Solution:

(a)

1. Selective breeding is the process of humans selecting individuals with desirable features to breed together. [Definition of selective breeding]

How to earn full marks:

• Clearly state that humans are selecting individuals.
• Mention that individuals have desirable features.
• Show that the selected individuals are bred together.

(b)

1. The farmer selects chickens that have the largest muscle mass (most meat). [States that the farmer chooses high-meat-yield chickens.]

2. These chickens are then bred together. [Explains how the selected chickens are bred.]

3. The offspring of these chickens are evaluated for muscle mass. [States that the offspring are assessed.]

4. The chickens from the offspring that have the largest muscle mass are selected and bred again. [Explains how the process is repeated with the offspring.]

5. This process is repeated over many generations to increase the average meat production of the flock. [Shows how repeated selection leads to improvement over time.]

How to earn full marks:

• Mention the selection of high-meat-producing chickens.
• Explain the breeding process.
• State that the offspring are evaluated.
• Show that the process is repeated over many generations.
• Clearly explain that the goal is to increase average meat production.

Common Pitfall: Make sure to emphasize that selective breeding is a repeated process over many generations. Also, remember that humans are the driving force behind this selection, choosing individuals with desirable traits.

Exam-Style Question 3 — Extended Response [8 marks]

Question:

A population of beetles lives on an island. Some beetles are green, and others are brown. Birds are predators of the beetles.

(a) Explain how natural selection could lead to a higher proportion of green beetles in a forest with green foliage. [6]

(b) Compare and contrast natural selection and artificial selection. [2]

Worked Solution:

(a)

1. There is genetic variation in the beetle population, with some beetles having alleles for green coloration and others having alleles for brown coloration. [States genetic variation exists.]

2. In a forest with green foliage, green beetles are better camouflaged than brown beetles. [Explains the advantage of green beetles in the environment.]

3. Birds are more likely to see and eat the brown beetles because they are more visible against the green foliage. [Explains the predation pressure.]

4. Green beetles are more likely to survive and reproduce because they are less likely to be eaten by birds. [Explains the survival and reproduction advantage.]

5. Green beetles pass on their alleles for green coloration to their offspring. [Explains the inheritance of traits.]

6. Over time, the proportion of green beetles in the population increases because they are better adapted to the environment. [Explains the shift in population composition.]

How to earn full marks:

• Clearly state that there is genetic variation in the population.
• Explain that green coloration provides better camouflage.
• Explain that brown beetles are more likely to be eaten.
• Show that green beetles are more likely to survive and reproduce.
• Mention the inheritance of color alleles.
• Explain that the proportion of green beetles increases over time.

(b)

1. Both natural and artificial selection involve the passing on of alleles to the next generation. [States a similarity: both involve inheritance.]

2. In natural selection, the environment selects which individuals survive and reproduce, while in artificial selection, humans select which individuals breed. [States a key difference: the selective agent.]

How to earn full marks:

• Must state at least one similarity.
• Must state at least one difference.

Common Pitfall: When comparing natural and artificial selection, remember that both involve the inheritance of alleles. The key difference lies in who or what is doing the selecting – the environment in natural selection, and humans in artificial selection.

Exam-Style Question 4 — Extended Response [9 marks]

Question:

A farmer is growing tomatoes. Some of the tomato plants produce small fruits, while others produce large fruits. The farmer wants to produce a variety of tomato that is both high-yielding (many fruits per plant) and produces large fruits.

(a) Describe how the farmer could use selective breeding to achieve this goal. [5]

(b) Explain how adaptation is related to natural selection. [2]

(c) Suggest two potential disadvantages of using selective breeding to improve crop plants. [2]

Worked Solution:

(a)

1. The farmer selects tomato plants that are both high-yielding (produce many fruits) and produce large fruits. [States that the farmer selects for both traits.]

2. The selected plants are then cross-pollinated, ensuring that the pollen from one selected plant fertilizes the ovules of another selected plant. This can be done by hand or by using insects. [Explains the cross-pollination process.]

3. The seeds from the cross-pollinated plants are collected and planted. [States that seeds are collected and planted.]

4. The resulting offspring are evaluated for both yield (number of fruits) and fruit size. [States that offspring are evaluated.]

5. Only the plants that are both high-yielding and produce large fruits are selected for further breeding; this process is repeated over many generations. [Shows that the selection and breeding cycle is repeated.]

How to earn full marks:

• Mention selection based on both high yield and large fruit size.
• Explain the process of cross-pollination.
• State that the offspring are evaluated for both traits.
• Show that the selection and breeding cycle is repeated over many generations.

(b)

1. Adaptation is the process, resulting from natural selection, by which populations become more suited to their environment over many generations. [Defines adaptation.]

2. Natural selection provides the mechanism for adaptation; individuals with advantageous alleles are more likely to survive, reproduce, and pass on those alleles, leading to a population better suited to its environment. [Explains the relationship between natural selection and adaptation.]

How to earn full marks:

• Must define adaptation.
• Must explain how natural selection provides the mechanism for adaptation.

(c)

1. Selective breeding can reduce genetic diversity in crop plants, making them more susceptible to pests or diseases. [States a disadvantage: reduced genetic diversity.]

2. Selective breeding can lead to a loss of other desirable traits that were not specifically selected for, such as resilience to drought or extreme temperatures. [States a disadvantage: loss of other desirable traits.]

How to earn full marks:

• Must state at least one potential disadvantage.
• Each disadvantage must be different and biologically plausible.

Common Pitfall: Remember that adaptation is a process driven by natural selection. Also, be aware that selective breeding, while beneficial, can inadvertently reduce genetic diversity, making crops more vulnerable to unforeseen challenges.