Evolution is often described as a process in which organisms become better suited to their environments. That is true for natural selection, but not every evolutionary change is an adaptation.
Sometimes a population changes simply because of chance.
This process is called genetic drift. It occurs when random events cause some genetic variants to become more common or less common in a population from one generation to the next. Drift can reduce genetic diversity, make populations genetically different from one another, and even cause particular variants to disappear entirely.
The key idea is simple: genetic drift changes allele frequencies because of random sampling, not because a variant is necessarily helpful or harmful.
What genetic drift means
An allele is one version of a gene. A population may contain several alleles for the same gene, with each allele occurring at a particular frequency.
Suppose a population contains 100 individuals and two alleles, A and B, are equally common. If reproduction were a perfectly representative process, the next generation would contain roughly equal proportions of the two alleles.
Real populations do not work that way. Individuals leave different numbers of offspring, some offspring survive while others do not, and chance affects which individuals reproduce. As a result, the next generation may happen to contain more copies of A than B—or more B than A.
That random change in allele frequency is genetic drift.
Importantly, drift does not require an allele to have an effect on survival or reproduction. A neutral allele can become common or rare simply because of chance.
Why population size matters
Genetic drift is strongest in small populations.
Imagine drawing 10 marbles from a bag containing equal numbers of red and blue marbles. The result could easily be six red and four blue. If you instead draw 10,000 marbles, the proportions are much more likely to remain close to the original ratio.
Reproduction creates a similar sampling effect. In a small population, chance events can substantially alter which alleles are passed to the next generation. In a large population, random fluctuations tend to have a smaller effect because the larger number of individuals provides more opportunity for the original allele frequencies to be represented.
This is why population size is central to understanding drift. The smaller the population, the larger the potential effect of random changes in allele frequencies.
A simple example of genetic drift
Consider a small population of beetles containing two alleles for a particular gene:
- 50% carry allele A
- 50% carry allele B
Suppose a storm randomly kills many beetles before they reproduce. The storm does not preferentially kill A or B; it simply happens to kill more beetles carrying B.
Among the survivors, A might now make up 70% of the relevant alleles and B only 30%.
If those survivors produce the next generation, the population begins with a different genetic composition. Nothing about allele A necessarily made it more useful. The change occurred because of a random event.
A second random event could push the frequencies in the opposite direction.
Over many generations, drift can eventually cause one allele to reach fixation, meaning it becomes the only allele present at that genetic location in the population. The alternative allele has then been lost from that population.
Genetic drift versus natural selection
Genetic drift and natural selection both change allele frequencies, but they do so for fundamentally different reasons.
Natural selection is nonrandom with respect to inherited traits and reproductive success. If an inherited characteristic increases an individual’s chances of surviving or reproducing in a particular environment, the associated alleles can become more common over generations.
Genetic drift is random with respect to the allele’s effect on fitness. A variant can increase in frequency even if it provides no advantage, or decrease even if it would have been beneficial.
The distinction matters because evolutionary change does not automatically mean adaptation.
A population can become genetically different without becoming better adapted to its environment. Drift can even remove a beneficial allele from a small population before natural selection has much opportunity to increase its frequency.
In real populations, drift and natural selection can occur simultaneously. Which force has the larger effect depends partly on population size, the strength of selection, and the particular genetic variant involved.
The founder effect: when a few individuals start a population
One important form of genetic drift is the founder effect.
It occurs when a new population is established by a small number of individuals from a larger population. Because the founders carry only a sample of the original population’s genetic variation, the new population may have allele frequencies that differ substantially from those of the source population.
For example, imagine a large population containing several alleles at a gene. If a small group becomes isolated and establishes a new population elsewhere, the founders may happen to carry unusually high frequencies of one allele and very low frequencies of another.
The new population begins with that genetic imbalance. Subsequent generations can amplify the difference through additional drift.
The founder effect is therefore not a separate evolutionary force from genetic drift. It is genetic drift associated with population establishment by a small group of founders.
The bottleneck effect: when a population is sharply reduced
Another major form of drift is the population bottleneck.
A bottleneck occurs when a population undergoes a substantial reduction in size. The cause might be an environmental disaster, habitat destruction, disease, or another event that leaves only a small number of survivors.
The survivors carry only some of the genetic variation that existed in the original population. If particular alleles were uncommon among the survivors, those alleles may become rare or disappear entirely even if they were common before the population crash.
When the population later grows, its size may return to something much larger, but its genetic composition may not return to what it was before the bottleneck. Lost alleles cannot simply reappear through reproduction unless they are reintroduced from another population or arise again through mutation.
This makes population bottlenecks important not only because they change allele frequencies, but also because they can reduce genetic diversity.
What genetic drift does to genetic diversity
A major consequence of drift is the loss of genetic variation within populations.
When an allele disappears from a population through drift, that population no longer contains that particular variant. Repeated random losses can leave a small population with fewer alleles than it had originally.
At the same time, drift can increase genetic differences between populations.
Imagine two populations that begin with similar allele frequencies but become isolated. Because chance operates independently in each population, one population may gradually become dominated by one allele while the other becomes dominated by another. Over time, their genetic compositions can diverge even if they experience similar environments.
This is one reason why geographically separated populations can become genetically distinct.
Can genetic drift create new traits?
Genetic drift does not create new alleles. Mutation is the ultimate source of new genetic variants, while genetic drift changes the frequencies of variants that are already present.
However, drift can strongly influence which variants remain in a population.
A new mutation may initially occur in only one individual or a small number of individuals. In a finite population, chance can determine whether that variant is passed on repeatedly or disappears. If the variant is neutral, its fate may be largely governed by drift.
Thus, mutation supplies new variation, while evolutionary processes such as drift and natural selection influence what happens to that variation.
Genetic drift and human populations
Genetic drift also occurs in human populations. Its effects are particularly important when populations are small, isolated, or founded by relatively few individuals.
Historical population reductions, geographic isolation, and the establishment of new populations can all alter the frequencies of genetic variants through drift.
This does not mean that a genetic variant common in one human population is necessarily beneficial there. Differences in allele frequencies can arise through several evolutionary processes, including random drift, natural selection, migration, and mutation.
Human genetic variation is therefore best understood as the product of multiple interacting forces rather than as a simple record of adaptation.
Why genetic drift matters in evolution
Genetic drift helps explain several patterns that natural selection alone cannot adequately account for.
It explains how neutral variants can become common, how alleles can disappear without being harmful, and why isolated populations can diverge genetically. It also helps explain why populations that have passed through severe reductions in size may retain less genetic diversity than their ancestors.
Most importantly, drift illustrates a fundamental feature of evolution: evolution is a change in the genetic composition of populations over generations, and that change does not always have a purposeful or adaptive direction.
Chance can alter which alleles survive and reproduce. In small populations, those chance effects can be powerful enough to reshape the population’s genetic makeup.
