Genetic Drift: The Sewall Wright Effect

Evolution is often understood as a process that rewards the “best adapted.” But what happens when sheer chance, not fitness, decides which genes survive into the next generation? In small, isolated populations, random events can reshape the entire genetic makeup of a group, with no regard for whether the changes help or harm. This is the phenomenon of genetic drift.

What Genetic Drift Means

Genetic drift (also called the Sewall Wright Effect, named after the geneticist who first described it mathematically) is a change in the gene and genotype frequencies of a population that occurs for purely random, unexpected, and unpredictable reasons. Two conditions make drift powerful: the population must be small, and it must be isolated from other breeding groups.

Why does size matter so much? In a large population, random fluctuations in who reproduces and who does not tend to cancel out across thousands of individuals. But when only a handful of individuals make up the breeding group, a single chance event can shift which genes get passed on in a dramatic way. The shift is not driven by whether a gene is helpful or harmful. It is driven entirely by luck.

Several types of events can set drift in motion:

• Natural calamities — Earthquakes, floods, volcanic eruptions, or epidemics can wipe out a large portion of a population, leaving a small group of survivors whose gene pool no longer mirrors the original.
• Migration — A small group may break away from a larger population and settle somewhere new, carrying only a fraction of the original genetic diversity with them.
• Reproductive failure — Some individuals may simply never reproduce, whether due to infertility, lack of mates, or bad luck. Their genes vanish from the pool entirely.
• Low offspring count — Even individuals who do reproduce may leave behind very few offspring, meaning their particular gene variants stay rare or fade out over generations.

Two Faces of Drift: Bottleneck and Founder

Genetic drift plays out through two main patterns, each triggered by a different set of circumstances.

The Bottleneck Effect: Rebuilding from the Survivors

Picture a large, genetically diverse population suddenly struck by a devastating event: a famine, a plague, an earthquake, or a major flood. A huge proportion of the group is wiped out. The survivors, a small residual population, then rebuild the group over the following generations. The problem is that this small surviving group may not carry a representative sample of the original gene pool. Some gene variants that were common before the disaster may be missing entirely, while variants that were rare may now make up a much larger share.

The population recovers in numbers, but from a narrowed genetic base. Unusual genetic conditions can end up far more common than they would ever be in a large, diverse population. Glycogen storage disease found at high frequency in certain Southeast Asian island populations is one example linked to past bottleneck events that dramatically reduced the gene pool.

The Founder Effect: Starting from a Tiny Sample

The founder effect kicks in under two scenarios. In the first, a small group migrates to previously uninhabited land and builds a new population from scratch. In the second, a group migrates to an already populated area but practises strict endogamy (marrying only within their own community), which genetically isolates them just as effectively as living on a remote island.

Either way, the founding group carries only a small, random sample of the parent population’s genetic diversity. If even one of the founders happens to carry a rare gene variant, that variant can become surprisingly common in future generations, simply because the founding pool was so small that one person’s genes had an outsized influence on the trajectory of the entire population.

Real-World Examples of Genetic Drift

Six-Fingered Dwarfism in the Old Order Amish

The Old Order Amish are a highly endogamous (marrying exclusively within the community) religious sect based in Pennsylvania, United States. Their ancestors migrated from Europe several centuries ago, and the founding group numbered only a few hundred individuals.

Among this small founding population, at least one person carried the gene for Ellis van Creveld syndrome, a condition that produces six-fingered dwarfism (extra fingers combined with short stature). Because the community has practised strict endogamy ever since, there has been very little genetic exchange with outside populations. Over generations, the frequency of this gene has stayed unusually high compared to what you would find in the general population.

Two forces drove drift here: migration (the small founding group carried a non-representative gene sample out of Europe) and inbreeding (endogamy within the community concentrated the gene further with each generation).

Retinitis Pigmentosa in Tristan da Cunha

Tristan da Cunha is a remote island located near St Helena. After Napoleon was exiled to St Helena, the British placed a small military garrison on Tristan to keep watch. When Napoleon died and the garrison was recalled, two related families chose to stay behind on the island.

These two families happened to carry a gene for retinitis pigmentosa (a condition where the retina develops spots, leading to progressive vision loss). Because the island’s entire population descended from this tiny founding group and remained isolated, with no influx of outside genes, the retinitis pigmentosa gene circulated at an unusually high frequency through the generations.

The drift here was driven by the founder effect (the population grew from just two families) combined with inbreeding (the small, isolated community had limited choice but to intermarry).

How Drift Shapes the Course of Evolution

Genetic drift is not just a curiosity that affects a few isolated groups. It has real and lasting consequences for how species evolve over long timescales.

Driving Species Apart

Drift can change the adaptive characters of a population by splitting it into sub-populations with different gene frequencies. Imagine a large population that gets divided into several small groups by geographical barriers, migration events, or catastrophes. Each small group then experiences its own random shifts in gene frequency, independent of the others. Over time, even though these groups may live in similar environments, they end up with different gene combinations.

These sub-populations, now carrying distinct genetic profiles, compete within their respective environments. The accumulating differences can eventually push the groups so far apart genetically that they can no longer interbreed. What began as random fluctuations in a handful of small populations has contributed to species divergence: the formation of separate species from a common ancestor.

Giving Disadvantageous Genes a Second Chance

In a large, well-mixed population, a gene that reduces fitness gets steadily weeded out by natural selection. Drift operates by a different logic. When a population splits and gene frequencies shift randomly, a gene that was disadvantageous in the original large population may find itself in a new combination of genes where it actually functions well. In this new genetic context, the previously harmful variant may turn out to be advantageous and gain a permanent foothold in the population.

This means drift can preserve genetic diversity that natural selection alone would have eliminated, giving evolution more raw material to work with over the long run.

When Selection and Drift Work Together

Drift does not operate in isolation from natural selection. Both forces act on a population’s gene pool at the same time, sometimes reinforcing each other and sometimes pulling in opposite directions.

A clear example comes from the distribution of ABO blood groups across the world. Livingstone (1969) proposed that infectious diseases like plague, cholera, and smallpox have acted as selective pressures favouring blood group B. Populations that historically faced heavy exposure to these diseases showed higher survival rates among individuals carrying the B allele. This explains why the frequency of the B gene rises as one moves eastward from Europe into Asia, where these diseases were historically more prevalent.

But selection is only part of the picture. The starting frequencies of blood group alleles in different populations were shaped by drift (through founder effects and bottlenecks during early human history), and selection then acted on those drift-influenced starting points. The global distribution of ABO blood groups that we see today reflects both forces working in combination.

Drift and the Spread of Small Populations

One of the most striking consequences of genetic drift connects to the origin of modern human diversity itself. If a small, isolated group develops unique gene combinations through drift, and that group later grows in size, those unique combinations have the potential to spread far and wide across the globe.

The Out of Africa model offers precisely this scenario. According to this theory, a small fraction of the African population left the continent and went on to populate the entire world over a span of roughly 50,000 to 60,000 years. The founding group that left Africa was small enough for drift to shape its genetic profile in distinctive ways. As this group expanded and diversified across different continents, the genetic signatures of that original drift event spread with it.

Much of the genetic variation seen across non-African human populations today carries the fingerprint of drift acting on a small founding group, overlaid with the effects of natural selection operating in the new environments those populations encountered.