Evolution is not only driven by survival of the fittest. Much of what happens to genes over time is the result of chance, random fluctuations in which individuals happen to survive and reproduce in any given generation. This process is called genetic drift, and it can be just as powerful as natural selection in shaping the course of evolution, especially in small populations.
Understanding genetic drift means understanding that evolution is not always adaptive. Some genes spread because they are beneficial. Others spread (or disappear) for no reason at all.
What Is Genetic Drift?
In any population, individuals carry different versions of genes (called alleles). Natural selection favors alleles that confer survival or reproductive advantages. But not every difference between individuals translates into a survival difference. And even when it does, chance still plays a role: a beneficial allele can be lost simply because its bearer happened to be eaten by a predator, or failed to find a mate, or was struck by lightning before it could reproduce.
Genetic drift is the random change in allele frequencies from one generation to the next due to chance sampling. It is analogous to flipping a coin: if you flip a fair coin 1,000 times, you expect about 500 heads, and the result will be close to that. But if you flip it only 10 times, you might easily get 7 or 8 heads just by chance. Small samples produce large random deviations from expectation. Small populations are like small coin flips: random effects dominate.
The mathematical framework for genetic drift was developed by Sewall Wright in the 1930s, building on earlier work by Ronald Fisher. Wright’s concept of effective population size (the number of individuals in a population that contributes to the next generation’s gene pool) became one of the most important parameters in population genetics.
Why Small Population Size Matters
In a large population, allele frequencies change slowly. A rare mutation has millions of copies of the alternative allele competing against it. The law of large numbers pushes outcomes toward their expected values, and selection has the opportunity to act. A beneficial allele will spread; a harmful one will be weeded out.
In a small population, a single individual’s fate can dramatically shift allele frequencies. If an allele is carried by only two individuals in a population of twenty, and both happen to die childless in the same season, that allele is gone forever, regardless of whether it was beneficial, neutral, or harmful.
This is the essence of genetic drift: in small populations, alleles can become fixed (reach 100% frequency) or lost (reach 0%) purely by chance, without any selective advantage or disadvantage.
The rate of genetic drift is inversely proportional to population size. In a population of N individuals, the probability that any given neutral allele ultimately becomes fixed is 1/2N (for diploid organisms), and the average time to fixation is approximately 4N generations. In a population of 10 individuals, fixation can happen within 40 generations. In a population of 1 million, it takes roughly 4 million generations.
The Founder Effect
When a small number of individuals from a larger population colonize a new environment (a remote island, a mountain valley, a new habitat), they carry only a subset of the original population’s genetic diversity. The allele frequencies in this founding group may differ substantially from the source population simply by chance. This is the founder effect.
The founding group’s genetics then become the genetic starting point for the new population, regardless of how representative they are of the original. Over subsequent generations, genetic drift continues to act on the now-isolated population. Some alleles that were rare in the source population may become common; others that were common may disappear.
The founder effect has been documented extensively in human populations. The Amish of Pennsylvania descended from a small number of 18th-century Swiss-German founders. As a result, certain rare genetic conditions (including Ellis-van Creveld syndrome (a form of dwarfism) and polydactyly) are far more common among the Amish than in the broader population. This is not because these alleles are advantageous; it is because they happened to be present in the founders.
Similarly, genetic studies of Pacific Islander populations show strong signatures of founder effects, consistent with the historical record of small groups voyaging to new islands and establishing new populations from very limited founding groups.
The Bottleneck Effect
The bottleneck effect occurs when a population is drastically reduced in size (by disease, disaster, overhunting, or any other cause) and then recovers from the surviving remnant. Like the founder effect, a bottleneck strips away genetic diversity at random. The surviving population carries only the alleles that happened to be present in the survivors.
The northern elephant seal provides a textbook example. Hunted nearly to extinction in the 19th century, the species was reduced to perhaps 20–30 individuals by the 1890s. The population has since recovered to over 200,000. But genetic analysis reveals that northern elephant seals have extremely low genetic diversity (among the lowest of any mammal species) because all living individuals are descended from that tiny bottleneck population. Southern elephant seals, which were hunted less severely, retained far more genetic diversity.
Humans show genetic evidence of a bottleneck. Mitochondrial DNA and Y-chromosome analyses suggest that all living humans descend from a relatively small ancestral population, possibly as small as several thousand effective individuals, at some point in the past (though the timing and cause are debated). Some researchers link this to a migration event or catastrophic population reduction, others to the statistical effects of ancestry accumulation.
Genetic Drift vs. Natural Selection
The interplay between genetic drift and natural selection is one of the central themes of population genetics. They operate on different principles and dominate under different conditions:
Natural selection is directional: it systematically favors alleles that increase fitness and eliminates those that decrease it. Given enough time and population size, selection can predictably spread beneficial alleles and purge harmful ones.
Genetic drift is random: it favors no allele over another. It acts equally on beneficial, neutral, and harmful alleles. In small populations, it can overwhelm selection, even causing harmful alleles to become fixed or beneficial alleles to be lost.
The threshold is approximately defined by the product Ns (population size times selection coefficient). If Ns >> 1, selection dominates. If Ns << 1, drift dominates. In a large population, even a tiny selective advantage (s = 0.001) will eventually spread an allele. In a population of 100, an allele needs a much larger selective advantage to reliably spread against the noise of drift.
This framework, developed by Motoo Kimura in the 1960s and 70s, gave rise to the neutral theory of molecular evolution: the idea that the majority of genetic changes at the molecular level are not driven by natural selection but by the random fixation of neutral mutations through genetic drift. Kimura’s theory was initially controversial but is now a cornerstone of molecular evolutionary biology.
Genetic Drift in Human Populations
Human genetic diversity reflects a complex history of drift, selection, migration, and bottlenecks. Populations that have lived in isolation for long periods (geographically or culturally) often show strong signatures of genetic drift:
ABO blood type distribution: The frequency of A, B, and O blood types varies substantially among human populations. Much of this variation reflects genetic drift rather than differential selection, since blood type confers only modest fitness differences in most environments.
Founder effects in isolated populations: Geographically or religiously isolated populations (Hutterites, Pingelap islanders, Finnish isolates) often have elevated frequencies of certain rare diseases. The BRCA1 breast cancer mutation common in Ashkenazi Jewish populations, certain Finnish disease-specific mutations, and the achromatopsia (complete color blindness) found on Pingelap atoll are all examples where a mutation that was present in a small founding group spread through drift and close intermarriage.
Mitochondrial DNA and Y-chromosome lineages: Because mitochondrial DNA is maternally inherited and the Y chromosome is paternally inherited, both can be tracked as discrete lineages through generations. Genetic drift causes lineages to go extinct and others to spread, completely independently of selection. This is why all living humans share a mitochondrial common ancestor (“Mitochondrial Eve”) and a Y-chromosome common ancestor (“Y-chromosomal Adam”): not because only one individual lived at those times, but because all other lineages went extinct through drift.
What is genetic drift?
Genetic drift is the random change in allele frequencies in a population from one generation to the next, caused by chance sampling rather than natural selection. It occurs because reproduction is not perfectly predictable: some individuals reproduce more than expected by chance, others less. Over time, random fluctuations accumulate. In small populations, genetic drift can cause alleles to become fixed (present in all individuals) or lost entirely, independent of whether they are beneficial or harmful.
How is genetic drift different from natural selection?
Natural selection is directional: it systematically favors alleles that increase an organism’s ability to survive and reproduce. Genetic drift is random: it has no preference for beneficial or harmful alleles. Selection depends on fitness differences between alleles; drift depends only on chance. In large populations, selection dominates. In small populations, drift can overpower selection, causing harmful alleles to spread or beneficial ones to disappear purely by chance.
What is the founder effect?
The founder effect occurs when a small number of individuals from a larger population establish a new population in a new location. Because the founders carry only a random sample of the original population’s alleles, the new population starts with limited and potentially unrepresentative genetic diversity. Over time, genetic drift further reduces diversity in this small isolated group. The founder effect explains why certain rare genetic conditions are unusually common in populations descended from small historical groups.
What is the bottleneck effect?
The bottleneck effect occurs when a population is dramatically reduced in size by a catastrophic event (disease, hunting, habitat loss) and then recovers from the surviving remnant. The survivors carry only a fraction of the original genetic diversity, and allele frequencies in the recovered population reflect the random genetics of the survivors rather than the original population. The northern elephant seal, reduced to ~30 individuals in the 1890s, is a classic example of an extreme population bottleneck.
Why does genetic drift matter for conservation?
In conservation biology, genetic drift is a major concern for endangered species with small populations. When a population falls below a critical size, drift begins to eliminate genetic diversity faster than mutation and selection can restore it. This reduces the population’s ability to adapt to environmental changes and increases the risk of inbreeding depression (harmful effects from mating between close relatives). Maintaining genetically diverse populations (through habitat protection, connectivity between populations, and captive breeding programs) is a core strategy in conservation genetics.
What is the neutral theory of evolution?
The neutral theory, developed by Motoo Kimura in the 1960s and 1970s, proposes that the majority of genetic differences between individuals and between species at the molecular level are not driven by natural selection but by the random fixation of neutral mutations through genetic drift. Neutral mutations neither help nor harm the organism but spread or disappear by chance. The neutral theory does not deny selection’s importance; it focuses on the molecular level, where most mutations have negligible effects on fitness. It is one of the foundational frameworks of modern molecular evolutionary biology.
Sources
Wright, S. (1931). Evolution in Mendelian populations. Genetics, 16(2), 97–159. doi:10.1093/genetics/16.2.97
Kimura, M. (1968). Evolutionary rate at the molecular level. Nature, 217(5129), 624–626. doi:10.1038/217624a0
Kimura, M. (1983). The Neutral Theory of Molecular Evolution. Cambridge University Press.
Hedrick, P.W. (2011). Genetics of Populations (4th ed.). Jones & Bartlett.
Nei, M., Maruyama, T., & Chakraborty, R. (1975). The bottleneck effect and genetic variability in populations. Evolution, 29(1), 1–10. doi:10.1111/j.1558-5646.1975.tb00807.x
Hoelzel, A.R. et al. (1993). Elephant seal genetic variation and the use of simulation models to investigate historical population bottlenecks. Journal of Heredity, 84(6), 443–449. doi:10.1093/jhered/84.6.443