How-to-approach-D4.1: Natural Selection

Welcome to the central theory of biology: Topic D4.1 Natural Selection. In the new IB Biology syllabus, the focus is on the Bio-Logic of 'Change over Time.' Natural selection is the mechanism that explains how populations adapt to their environments and how new species eventually arise. It is not about 'survival of the strongest,' but rather 'survival of the best fitted.'

This unit is a frequent subject for Paper 2 long-response questions. You must be able to explain the steps of natural selection in a logical sequence: Variation --> Overproduction --> Selection --> Differential Survival --> Inheritance. In Paper 1A (MCQs), the IBO often uses the examples of Daphne Major finches and the rapid evolution of antibiotic resistance in bacteria.

Before we look at the case studies, remember the most important rule: Natural selection acts on individuals, but evolution occurs in populations. An individual cannot evolve a new trait during its lifetime; it either has the 'winning' alleles or it doesn't. Evolution is simply the shift in allele frequencies over generations.

1. The Four Pillars of Natural Selection

For natural selection to occur, four conditions must be met:

• 1. Variation: Members of a population must have different traits (caused by mutation, meiosis, and sexual reproduction).
• 2. Overproduction: Populations produce more offspring than the environment can support, leading to a struggle for survival.
• 3. Selection: Environmental 'selective pressures' (like predators, disease, or climate) determine which individuals are disadvantaged.
• 4. Differential Survival and Reproduction: Those with favorable adaptations survive longer and pass their alleles to the next generation.

2. Adaptation: The Result of Selection

An adaptation is a characteristic that makes an individual suited to its environment.

• Adaptations can be Structural (e.g., a bird's beak shape), Behavioral (e.g., bird migration), or Physiological (e.g., a desert animal's concentrated urine).
• Adaptations develop over many generations; they are not acquired traits (like muscle growth from exercise).

3. Case Study: Antibiotic Resistance

This is the perfect example of evolution happening in 'real-time' due to human-induced selective pressure.

• 1. Variation: A few bacteria in a population have a mutation that makes them resistant to an antibiotic.
• 2. Selection: An antibiotic is applied. It kills the normal bacteria, leaving only the resistant ones.
• 3. Inheritance: The survivors reproduce rapidly, passing the resistance gene to their offspring.
• 4. Result: The entire population becomes resistant.

4. Sources of Variation in Sexual Reproduction

Sexual reproduction is essential for natural selection because it maximizes variation.

• Mutation: Creates new alleles.
• Meiosis: Shuffles alleles through crossing over (Prophase I) and independent assortment (Metaphase I).
• Random Fertilization: Any sperm can fuse with any egg, creating a unique combination of alleles in the zygote.

5. Exam Strategy: Writing the Evolution Answer

If asked to explain how a specific trait evolved (e.g., the long neck of a giraffe), follow this script:

• 1. State that there was pre-existing variation in neck length due to mutation.
• 2. Mention overproduction of offspring leads to competition for food.
• 3. Identify the selective pressure (e.g., high-up leaves).
• 4. State that those with longer necks had a selective advantage (better survival).
• 5. Conclude that they reproduced and passed the favorable alleles to the next generation.