Biology SL · Theme D: Continuity and Change · 45 min read · Updated 2026-05-11
Natural selection — IB Biology SL
IB Biology SL · Theme D: Continuity and Change · 45 min read
1. Key Observations and Inferences★★☆☆☆⏱ 10 min
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
2. Variation, Adaptation, and Fitness★★★☆☆⏱ 15 min
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.
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.
3. Natural Selection as the Mechanism of Evolution★★★☆☆⏱ 12 min
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.
4. Exam Command Terms for Natural Selection Questions★★☆☆☆⏱ 8 min
Common Pitfalls
Why: Evolution is change in the population over generations, not change within an individual. Acquired traits are not heritable for SL Biology.
Why: Mutations are random, variation exists in the population before environmental change. The environment selects for existing variation, it does not create it.
Why: Natural selection can only act on existing variation, environments change, and adaptations are often compromises between competing selection pressures.
Why: Fitness is defined by reproductive success, not size, strength, or even just survival.