AP Syllabus focus:
‘Evolution is also driven by random occurrences that affect the genetic makeup of populations.’
Randomness matters in evolution because populations are finite and real environments are unpredictable. Chance alone can change which alleles are passed on, sometimes producing evolutionary change even when no trait has a consistent advantage.
Core idea: evolution can be stochastic
Evolutionary change is often taught through natural selection, but populations can also evolve through random occurrences that alter their genetic makeup. In AP Biology, this emphasizes that evolution is not always “adaptive” or directed.
Random processes are most visible when:
Population size is small (chance effects are stronger)
Events are sudden or unpredictable (fires, storms, disease outbreaks)
Reproduction involves limited sampling (only some individuals reproduce)
These changes can shift allele frequencies from one generation to the next, producing measurable evolutionary change.
Simulated allele-frequency trajectories under genetic drift across multiple replicate populations. In small populations, allele frequencies show large random swings and often reach fixation (frequency 1) or loss (frequency 0) quickly, whereas larger populations fluctuate less from generation to generation. This illustrates how sampling error alone can produce divergence among populations even without selection. Source
Essential terms for this subtopic
Evolution: A change in allele frequencies in a population across generations.
Evolution in this context is population-level and generation-to-generation; individuals do not “evolve” during their lifetimes.
Allele frequency: The proportion of all copies of a gene in a population that are a particular allele.
Allele frequency is the key quantity that random occurrences can alter, even if the environment does not consistently favor one phenotype.
Stochastic process: A process influenced by random chance, so outcomes vary among trials even under the same conditions.
Stochasticity helps explain why two similar populations can diverge genetically over time simply due to different chance outcomes.
How random occurrences alter a population’s genetic makeup
Random occurrences influence evolution by changing which alleles make it into the next generation. Common pathways include:
Random survival and reproduction
Even if individuals are similar in phenotype, chance differences in survival or reproductive output can occur:
An organism may reproduce more because it encounters mates first
A random injury may prevent reproduction
Some offspring may survive due to luck (e.g., avoiding predators)
The key point is that the genetic contribution to the next generation can be uneven for reasons unrelated to “better” traits.
Random sampling of gametes and offspring
Reproduction is inherently a sampling process:
Only a subset of adults produce offspring
Only a subset of gametes form zygotes
Not all zygotes survive to reproduce
In finite populations, this sampling can cause allele frequencies to fluctuate between generations without any consistent selective pattern.
Random demographic and environmental events
Some events change population composition abruptly:
Natural disasters reduce numbers unpredictably
Patchy resource availability causes uneven reproductive success
Local die-offs remove individuals regardless of genotype
Because these events can remove (or spare) individuals by chance, the population’s allele frequencies after the event can differ from before, creating evolutionary change.
A population bottleneck reduces population size sharply, so only a chance sample of the original genetic variation persists. The surviving individuals carry a shifted set of allele frequencies, and some alleles may be lost entirely. Even if the population later grows, the post-bottleneck gene pool reflects that random sampling event rather than adaptation. Source
What random-driven evolution looks like in data
Random processes tend to produce patterns distinct from consistently directional change:
Unpredictability: allele frequency may rise in one population but fall in another under similar conditions
Non-directionality over short intervals: changes may “zig-zag” across generations
Stronger effects in small populations: chance outcomes are less likely to average out
Loss of variation can occur: some alleles may disappear simply because they were not passed on
These ideas align with the syllabus focus that evolution can be driven by random occurrences affecting the genetic makeup of populations over time.
FAQ
They often use replicate populations, long time-series data, or statistical models to ask whether observed changes fall within expected random variation.
Large, consistent shifts in the same direction across replicates are less consistent with chance alone.
In small populations, each individual represents a larger fraction of the gene pool.
So random differences in who survives or reproduces can disproportionately change allele frequencies.
Yes. A chance increase of an allele can make it common even without a consistent advantage.
Later, it may appear well-fitted simply because it is widespread, not because it was favoured initially.
Effective population size is the number of individuals effectively contributing genes to the next generation.
It can be smaller than the census size if reproduction is uneven, strengthening chance-driven allele frequency change.
Sampling error arises because you genotype only some individuals from a population.
Measurement error comes from lab or scoring mistakes; careful controls and repeat genotyping reduce it, but sampling error remains even with perfect technique.
Practice Questions
State two reasons why random occurrences can cause evolutionary change in a small population. (2 marks)
States that chance can change which individuals survive and reproduce, altering allele frequencies (1)
States that in small populations, sampling effects are stronger so allele frequencies fluctuate more between generations (1)
A storm randomly kills 70% of individuals in a coastal lizard population, regardless of phenotype. After the storm, the frequency of allele differs from its pre-storm value. Explain how this change can represent evolution, and why the direction of change is difficult to predict. (6 marks)
Defines/links evolution to a change in allele frequencies across generations (1)
Identifies that deaths occurred regardless of phenotype, so the change can be due to chance rather than consistent advantage (1)
Explains that random survival changes which alleles remain in the breeding population (1)
Explains that subsequent reproduction samples alleles from the reduced set of survivors, shifting allele frequencies (1)
States that with random processes, outcomes vary among populations/events, so direction is unpredictable (1)
Notes that the effect is amplified by reduced population size (chance effects less likely to average out) (1)
