Natural selection drives evolution because individuals with advantageous inherited characteristics tend to survive and reproduce more successfully. Over generations, these characteristics become more common in the population, producing evolutionary change.
The Mechanism
In IB ESS Subtopic 3.1, evolution is the change in the inherited characteristics of a population over generations. Natural selection produces this change through the following sequence:
- Variation: Individuals within a population have different characteristics. Some of this variation is heritable.
- Overproduction: Populations produce more offspring than the environment can support.
- Competition: Limited resources such as food, territory and mates create a struggle for survival and reproduction.
- Differential survival and reproduction: Individuals with characteristics better suited to the environmental conditions are more likely to survive and reproduce. These characteristics are described as adaptations.
- Inheritance: Successful individuals pass their advantageous characteristics to offspring.
- Change over generations: The advantageous characteristics increase in frequency, so the population becomes better adapted to its environment.
A named example is the medium ground finch on Daphne Major in the Galápagos Islands. During the 1977 drought, small, soft seeds became scarce. Finches with larger, deeper beaks were better able to open the remaining hard seeds, so they survived at higher rates and produced offspring. The average beak depth increased in the next generation.
A common misconception is that individual organisms evolve because they “need” to adapt. Individuals do not evolve, and natural selection is not purposeful. The environment selects among existing inherited differences, while populations evolve across generations.
Exam Technique
For an explain question, present the mechanism as a linked causal sequence: variation, competition, differential reproduction, inheritance and population change. Use a named example and state explicitly that natural selection acts on individuals, but evolution occurs in populations. This content is required at both SL and HL; HL students can additionally connect human activity to altered selective pressures.