# Natural selection

> IB Biology SL · Theme D: Continuity and Change
> Source: https://www.owlsprep.com/study/ib-biology-sl-u4-natural-selection/

This sub-topic covers the core mechanism of adaptive evolution: natural selection. You will learn the key observations, inferences, and applications of natural selection, and how it drives evolutionary change in populations.

**Prerequisites:** [Understanding of genetic variation and Mendelian inheritance](https://www.owlsprep.com/study/ib-biology-sl-u3-inheritance/)

## Learning objectives

- Explain the key observations and inferences that underpin natural selection
- Distinguish between variation, adaptation, and fitness
- Describe how natural selection leads to evolution of populations
- Apply natural selection principles to real-world examples

## Key Observations and Inferences

**Natural Selection** — The non-random process where differential survival and reproduction of individuals leads to changes in the heritable traits of a population over generations, as advantageous traits are passed to offspring more frequently.

Darwin and Wallace independently developed the theory of evolution by natural selection based on a set of observable facts that lead to testable inferences:

- **Observations**: Populations produce more offspring than resources can support; individuals vary in heritable traits; populations stay stable in size overall
- **Inferences**: There is a struggle for existence; individuals with advantageous traits survive and reproduce more; advantageous traits accumulate over generations

**Worked example:** Peppered moths in industrial UK shifted from mostly light-colored to mostly dark-colored over 100 years. Explain this change via natural selection.

1. 1. Pre-existing heritable variation: Light and dark wing color are genetic traits present in the original population.
2. 2. Environmental change: Coal pollution darkened tree bark, removing light-colored lichen that camouflaged light moths.
3. 3. Differential survival: Dark moths were less likely to be eaten by bird predators, so more survived to reproduce.
4. 4. Over generations: The allele for dark wing color was passed on more frequently, increasing its frequency in the population.

## Variation, Adaptation, and Fitness

Variation is the raw material for natural selection. Without heritable variation between individuals, no trait can be selected for or against, so no evolutionary change can occur.

**Adaptation** — An adaptation is a heritable trait that increases the relative fitness of an individual compared to individuals without the trait. Adaptations are context-dependent, only advantageous in a specific environment.

Fitness is defined by reproductive success, not just survival. A trait that increases offspring production, even if it reduces lifespan, is still considered to increase fitness.

**Worked example:** Male peacocks have large, elaborate tails that make them more visible to predators and require lots of energy. Why did this trait evolve via natural selection?

1. 1. Heritable variation exists: Tail size varies among males, and the trait is genetic.
2. 2. Female peahens prefer to mate with males with larger, more elaborate tails, giving these males more mating opportunities.
3. 3. Even though large tails reduce survival probability, they significantly increase the number of offspring the male produces, increasing his overall fitness.
4. 4. Over generations, alleles for large tails are passed on more frequently, making the trait common in the population.

## Natural Selection as the Mechanism of Evolution

> **info**
>
> A common misconception: Populations evolve, not individual organisms. Natural selection acts on individual phenotypes, but the evolutionary change occurs in the population's trait frequencies over generations.

Evolution is defined as the change in heritable characteristics of a population over successive generations. Natural selection is the only mechanism that consistently produces adaptive evolution, unlike random processes like genetic drift.

**Worked example:** Explain how antibiotic resistance evolves in a population of bacteria exposed to antibiotics.

1. 1. Random mutation creates a heritable antibiotic resistance gene in a small number of individuals in the population, before any exposure to antibiotics.
2. 2. When exposed to antibiotics, all non-resistant bacteria are killed, while resistant individuals survive.
3. 3. Surviving resistant bacteria reproduce rapidly, passing the resistance gene to all offspring, and spreading it to other bacteria via horizontal gene transfer.
4. 4. Over time, almost all individuals in the population carry the resistance gene, so the population has evolved antibiotic resistance.

## Exam Command Terms for Natural Selection Questions

**Exam command terms**

- **Outline** — Give a brief summary of key points, no extended explanation *(Outline the process of natural selection (3-4 marks))*

- **Explain** — Connect all steps of the process to the outcome in detail *(Explain how natural selection leads to evolution (6-8 marks))*

- **Distinguish** — Highlight clear differences between two concepts *(Distinguish between adaptation and variation (4 marks))*

**Check your understanding**

1. Which statement about natural selection is correct?

   - Individuals evolve adaptations during their lifetime
   - Populations evolve over generations via changes in trait frequencies
   - Natural selection always produces the largest strongest individuals
   - Environments create new variation when needed

   *Answer:* Populations evolve over generations via changes in trait frequencies

   *Why:* Correct: Natural selection changes trait frequencies in populations over time, individuals do not evolve.

2. What is fitness in the context of natural selection?

   - How strong and healthy an individual is
   - How long an individual lives
   - How many viable offspring an individual produces
   - How well an individual survives harsh conditions

   *Answer:* How many viable offspring an individual produces

   *Why:* Correct: Fitness is a relative measure of reproductive success.

## Common pitfalls

- **Wrong:** Claiming individuals evolve adaptations during their lifetime to fit their environment.
  - Why it fails: Evolution is change in the population over generations, not change within an individual. Acquired traits are not heritable for SL Biology.
  - Correct: State that populations evolve over generations, with pre-existing heritable traits becoming more common if they increase fitness.
- **Wrong:** Claiming the environment causes mutations to create needed variation, e.g. antibiotics cause bacteria to mutate.
  - Why it fails: Mutations are random, variation exists in the population before environmental change. The environment selects for existing variation, it does not create it.
  - Correct: State that random mutations create resistance variation before exposure, and antibiotics select for existing resistant individuals.
- **Wrong:** Claiming natural selection produces perfect adaptations.
  - Why it fails: Natural selection can only act on existing variation, environments change, and adaptations are often compromises between competing selection pressures.
  - Correct: State that natural selection produces adaptations that are 'good enough' for the current environment, not perfect.
- **Wrong:** Claiming fitness means being the largest or strongest individual in the population.
  - Why it fails: Fitness is defined by reproductive success, not size, strength, or even just survival.
  - Correct: State that fitness is the relative number of viable offspring an individual produces compared to other individuals in the population.

## Cheatsheet

| Key Concept | Core Definition | Key Exam Point |
| --- | --- | --- |
| Natural Selection | Non-random differential survival/reproduction | Acts on individuals, changes populations |
| Variation | Heritable phenotypic differences between individuals | Raw material for selection, arises randomly |
| Adaptation | Heritable trait that increases fitness | Only advantageous in specific environments |
| Fitness | Relative reproductive success | Counts offspring, not just size/survival |
| Evolution | Change in heritable traits over generations | Natural selection causes adaptive evolution |

## What's next

Natural selection is the core mechanism of adaptive evolution, and it connects to nearly all other topics in IB Biology SL. It explains the origin of biodiversity, the emergence of public health threats like antibiotic resistance, and how species adapt to changing climates. Mastery of natural selection is required to understand speciation, evolutionary relationships, and ecological dynamics. Next, you will build on this foundation to explore how new species form, what evidence supports evolution, and how natural selection shapes interactions in ecosystems.

- [Adaptation and speciation](https://www.owlsprep.com/study/ib-biology-sl-u4-adaptation-and-speciation/)
- [Evolutionary history and biodiversity](https://www.owlsprep.com/study/ib-biology-sl-u4-evolutionary-history-and-biodiversity/)

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