AP Syllabus focus:
‘Genetic drift, including bottleneck and founder effects, changes allele frequencies in small populations through chance events.’
Genetic drift explains evolutionary change that occurs without organisms being “better adapted.” In small populations especially, random sampling during reproduction can shift allele frequencies across generations, sometimes rapidly and unpredictably.
Genetic drift: evolution by chance
What genetic drift is (and when it matters most)
Genetic drift: a change in allele frequencies from one generation to the next due to random chance events (sampling error), strongest in small populations.
Drift occurs because populations pass genes forward through a limited sample of gametes and offspring, so chance can overrepresent some alleles and underrepresent others.
How drift changes allele frequencies
Random sampling of alleles: not all individuals contribute equally to the next generation, even if they have similar survival and reproductive ability.
Generation-to-generation fluctuation: allele frequencies can “zig-zag” unpredictably, rather than moving in a consistent direction.
Multiple simulated populations start with the same allele frequency but diverge randomly over generations due to sampling error. Some trajectories drift upward to fixation (frequency = 1), while others drift downward to loss (frequency = 0), even without any selective advantage. The spread of paths illustrates why drift produces different outcomes in different small populations. Source
Fixation or loss by chance:
An allele can become fixed (frequency reaches 1.0) or be lost (frequency reaches 0) without providing any advantage.
This plot shows the expected Hardy–Weinberg genotype frequencies as the allele frequency varies from 0 to 1. The heterozygote frequency (2pq) peaks at intermediate allele frequencies, while fixation at or corresponds to a single homozygous genotype reaching frequency 1. In drift, allele frequency can move toward these endpoints by chance, producing the same “all one homozygote” outcome without selection. Source
Stronger effects in small populations:
Each birth/death has a larger proportional impact, so chance outcomes are amplified.
Genetic consequences of drift
Reduced genetic variation within a population
Rare alleles are especially vulnerable to loss during random sampling.
Increased differences between populations
Separate small populations can drift in different directions, creating divergence even in similar environments.
Drift is not goal-directed
The alleles that increase in frequency are not necessarily “better”; they may simply be the ones that, by chance, left more descendants.
Bottleneck effect
Population crashes and nonrepresentative survivors
Bottleneck effect: a form of genetic drift in which a sudden, severe reduction in population size causes allele frequencies to change because the surviving individuals are a chance sample of the original population.
After a bottleneck, even if the population size rebounds, the gene pool may reflect only what happened to survive the crash.
Key features of bottlenecks
Cause: events such as fires, storms, disease outbreaks, habitat destruction, or overhunting that abruptly reduce population size.
Chance sampling: survivors may not represent the original allele frequencies.
Lasting genetic impact:
Lower diversity can persist for many generations because lost alleles can’t be recovered without new input from outside (not assumed here).
Founder effect
New populations started by a few individuals
A founder effect is drift associated with colonisation, not catastrophe.
Founder effect: a form of genetic drift in which a new population is established by a small number of individuals, so allele frequencies in the new population differ from the source population by chance.
Founder effects are common on islands, in isolated habitats, or whenever a few individuals become separated and begin reproducing independently.
Key features of founder effects
Small starting gene pool
Some alleles may be missing entirely; others may become unusually common.
Rapid shifts in allele frequency
Early generations can show strong drift because each founder’s genetic contribution is large.
Potential for unusual trait frequencies
If founders carry a rare allele, it can become relatively frequent in the descendant population simply due to the initial sampling.
Distinguishing drift from “adaptive” change (conceptually)
Drift produces random allele frequency changes, especially noticeable in small populations.
Bottleneck and founder effects are classic cases where chance events (survival through a crash or membership in the founding group) reshape allele frequencies.
Across populations, drift can yield different outcomes even under similar conditions, because the underlying changes are driven by random sampling.
FAQ
Effective population size is the number of individuals effectively contributing genes to the next generation, which can be smaller than the census size.
It’s lowered by unequal sex ratios, high variation in reproductive success, and fluctuating population size, all of which intensify drift.
They use replication and uncertainty estimates.
Common approaches include:
repeat sampling across time points and sites
confidence intervals for allele frequency estimates
checking whether patterns are consistent across independent loci
Fixation time is highly variable, but smaller populations tend to reach fixation faster because random fluctuations are larger.
Expected times depend on population structure and starting allele frequency, so two populations of the same size can differ substantially.
If one or more founders happen to carry a rare allele, the allele’s starting frequency can be much higher than in the source population.
Subsequent isolation and small population size can maintain or increase that frequency through continued drift.
Strategies focus on increasing genetic representation:
maximise the number of breeding individuals
equalise family sizes (avoid a few individuals producing most offspring)
mix individuals from separate remnants when appropriate to broaden the gene pool
Practice Questions
Explain how genetic drift can change allele frequencies in a small population. (2 marks)
States that allele frequencies change due to chance/random sampling of alleles in reproduction (1)
Explains that the effect is stronger in small populations because each individual’s contribution has a larger impact (1)
Describe the bottleneck effect and the founder effect, and explain two genetic consequences they can have for the affected population. (6 marks)
Defines bottleneck effect as a sharp reduction in population size causing chance changes in allele frequencies (1)
Explains survivors are a nonrepresentative sample of the original gene pool (1)
Defines founder effect as a new population started by few individuals causing chance differences from the source population (1)
Explains founders carry only a subset of alleles from the source population (1)
States a genetic consequence: reduced genetic variation (e.g., loss of alleles/heterozygosity) (1)
States a second genetic consequence: increased divergence in allele frequencies compared with the original or other populations (1)
