What causes a strong genetic drift

Genetic drift

The evolutionary factor genetic drift changes the genetic structure of populations. We explain how this works and what the terms founder effect and bottleneck effect mean using examples.

What is genetic drift?

Genetic drift or genetic drift is that random Change in the frequency of certain gene variants (Alleles) in the gene pool of a population. All genes of a certain population “swim” in the gene pool, so to speak. This could be a species of bird on an island, for example. Each gene occurs in certain variants - the alleles. For example, there are two alleles of a gene that determines a bird's feather color: One allele takes care of the expression greener Feathers, the other for expression yellower Feathers.

The respective alleles occur in the gene pool in different frequencies. This is what you call allele frequency or allele frequency. Genetic drift therefore either leads to a decrease or increase in certain alleles in the gene pool.

It is important that you remember that random events such as a volcanic eruption or a flood are responsible for the genetic drift. Especially in small populations, drift has a major impact because certain alleles can "disappear" from the pool more quickly.

The terms are often used in relation to genetic drift Bottle neck effect and Founder Effect. Both effects lead to a decreased genetic variability (Diversity) in a population.

Genetic drift (also: allele drift / Sewall-Wright effect) is referred to in population genetics as the change in the allele frequency within a gene pool of a population due to random events.

Bottle neck effect

An important variant of the genetic drift is Bottle neck effect. Here the size of a certain population shrinks drastically. Under a population you generally understand a group of individuals of a species that have the same habitat and can reproduce with one another.

For the sudden downsizing of the population are Disasters such as epidemics, volcanism, floods, persistent drought or cold. But we too People can contribute to the shrinking of certain populations, e.g. through air or water pollution.

For example, out of our bird population with the three different colors of feathers survive - yellow, green and red - mainly due to a volcanic eruption Red and green Birds. The yellow Birds were rare before and have now almost "disappeared".

Due to the bottleneck effect, only a small part of the original population happens to survive, a large part disappears, so to speak. Of course, this also has an effect on the allele frequency in the gene pool: Certain alleles are retained and others can even disappear from the gene pool due to the death of the individual. Of course, rare alleles are more likely not to be included in the new gene pool.

The genetic drift thus leads to an "impoverishment" of the genetic variability. You also call this the genetic bottleneck. That can be certain Problems As a result, we have increased inbreeding (reproduction among closely related organisms) or lower resistance to diseases when beneficial alleles "disappear". Especially in the case of threatened species with low populations, the bottleneck effect can lead to the extinction of these species.

Bottle neck effect model:

You can compare the bottle neck effect to a model of a bottle with different colored balls.

1. Let's say you are in the bottle blueness and yellow Bullets. They occur in roughly the same ratio and stand for two alleles of a population (= original population).

2. We turn the bottle and at the same time randomly roll out a few balls. Since the neck of the bottle is very narrow, only a small part of the balls will fit through. That stands for a random environmental event like a tsunami that greatly reduces the population.

3. We only have a lot now littleyellow, but for that lotsblueness Bullets. The allele frequency of the surviving population thus deviates from the original one (= genetic drift).

4. After overcoming the catastrophe, the population grows again. Your allele frequencies (little Alleles for yellow, lots Alleles for blue) are now usually reflected in the surviving population.

Founder Effect

We have a similar situation with the founder effect. Here, however, the initial population does not decrease drastically, rather a few individuals in a population colonize a new habitat. So you start a new population.


  • a storm blows some birds of one kind to another island ("Darwin finches")
  • Humans bring certain species e.g. by ship to another continent on which they were not previously native (e.g. species of fruit flies from Europe to Chile; raccoons from America to Germany)
  • Ocean currents ensure that aquatic animals find new habitats

In the "new" smaller population - the Founder population - So now only a fraction of the alleles of the starting population are present. Which these are is decided by chance. The allele frequencies between the starting population (P1) and the founder population (P2) therefore differ (= gene drift).

The isolation also means that gene flow (= mutual exchange of genes) between the populations is no longer possible. This leads to a lower genetic diversity because, for example, some alleles of the original population can no longer be found in the new founder population.

Let us state once again: The prerequisite for the founder effect is one isolation between the starting population and the founder population, for example by means of a spatial separation ("separation"). New species can also arise from this, which you call allopatric speciation.

Gene drift as an evolutionary factor

By evolution factors you understand forces that can change the genetic structure of a population.

The genetic drift is one accidental and undirected Factor that changes the allele frequency of a population. As selection ("Auslese"), on the other hand, you describe the targeted sorting out of certain individuals based on their adaptation to the environment.