Evolution is the change in the heritable characteristics of a population over generations. It occurs in populations, not in individual organisms, and the changes must be genetically heritable.
This definition is required at both IB Biology SL and HL in subtopic A4.1, Evolution and speciation. A characteristic acquired during an organism’s lifetime is not evolution unless it results from heritable genetic variation and can be passed to offspring.
How Evolution Occurs
- Mutation produces new alleles, while sexual reproduction produces new combinations of alleles.
- Individuals within a population therefore differ in their heritable characteristics.
- More offspring may be produced than the environment’s limited resources can support, causing competition.
- Selection pressures, such as temperature, predators or resource availability, affect individuals differently.
- Individuals with advantageous heritable characteristics tend to survive and reproduce more successfully. This is greater fitness.
- Their offspring inherit the advantageous alleles, so the population’s characteristics change over generations through natural selection.
| Is evolution occurring? | Reason |
|---|---|
| A bacterial population becomes increasingly antibiotic-resistant | Resistant bacteria survive, reproduce and pass resistance alleles to offspring. |
| A person develops larger muscles through exercise | The acquired muscle growth is not inherited, and the population has not changed. |
| A bird population gradually develops a higher frequency of advantageous beak forms | Differential reproduction changes heritable characteristics across generations. |
A common misconception is that individual organisms evolve because they “need” to adapt. Individuals may show phenotypic plasticity, but evolution is a population-level process and is not goal-directed. At HL, this can be expressed genetically as a change in allele frequency within the population’s gene pool.
Exam Technique
For the command term define, state: “Evolution is change in the heritable characteristics of a population.” For explain, add variation, selection pressure, differential survival and reproduction, inheritance, and change over generations. Avoid defining evolution simply as “change” or “survival of the fittest,” because both are incomplete.