What Is Genetic Drift?

Genetic drift, like natural selection, is an evolutionary mechanic. But unlike natural selection, genetic drift happens purely by chance. Essentially, evolution is the passing of alleles from one generation to the next. Genetic drift causes the number of gene variations in species to fluctuate randomly—regardless of whether the genes are favourable or not.

How Does It Work?

One of the best ways to understand genetic drift is to look at an example. Below is a diagram that shows the effects of genetic drift on some rabbits' fur colour over three generations. B represents the allele for the colour brown, and b represents the allele for the colour white. Since B is the dominant allele, rabbits that are heterozygous, meaning they have both alleles, will be brown. In the first generation, there is an equal number of brown and white alleles.

Population Size Matters

The larger a population, the lower the likelihood that genetic drift will have a significant impact. For instance, if we have a population of 10000 fish, it would be far less likely for an allele to get lost, especially in such a short period, compared to the example above. If only a quarter of the 10000 fish survived to reproduce, the surviving fishes' allele frequencies would tend to reflect those in the original population more accurately.

This concept is analogous to flipping a coin. If you only flip a coin just a few times, there is a high probability that you would achieve a head-to-tails ratio different from fifty-fifty. On the other hand, if you flip a coin a hundred times, you are more likely to receive a ratio close to fifty-fifty.

Allele Benefit or Harm Does Not Matter

As previously stated, genetic drift is solely based on chance. Whether or not an expressed trait, a phenotype, is favourable does not influence genetic drift. Beneficial and harmful alleles are affected by both natural selection and genetic drift.

The Bottleneck Effect

One acute example of genetic drift is the bottleneck effect. It occurs when events, such as natural disasters and instances of overhunting, decimate a population and leave behind a small, random group of individuals. Thes allele frequencies of this group may not represent those of the population before the event, and some genes may be missing altogether.

The genetic makeup of the surviving individuals is now the genetic makeup of the entire population. However, the smaller population will be more vulnerable to genetic drift, potentially causing more alleles to get lost.

The Founder Effect

Another extreme example of genetic drift is the founder effect. It occurs when a small group of individuals separate from the main population. The new colony's allele frequencies may not represent those of the parent population, and some alleles may not be present at all.

Assuming that the new colony does not come in contact with any individuals from the parent population, the genes of the isolated subset of individuals will be the only genes that can be passed down to subsequent generations. Furthermore, the small size of the new colony means it will likely be more prone to major changes from genetic drift for generations compared to the original colony.

In the diagram above, several individuals migrate away from their parent population. Let us assume an individual's alleles for a particular gene determines its colour. Since each individual only has a single special indicator, their shape, we do not have enough information to know their genotype, composed of two alleles if they are diploid. Thus, we can assume the colours represent the individuals' phenotype, the expressed trait.

At a point after numerous generations, all of the individuals have the purple phenotype. This outcome could be because the red phenotype has been lost, or are hidden as recessive alleles.

Summary

Genetic drift is a revolutionary mechanism that changes the frequency of alleles in populations by chance. It can even result in some genes being eliminated and some reaching fixation, or 100% frequency. In contrast to natural selection, drift is not subject to how well organisms are adapted to their environments. Genetic drift affects all populations, but smaller populations experience its impacts more strongly.

Two examples of genetic drift are the bottleneck effect and the founder effect. The founder effect shares many similarities with the bottleneck effect, but it takes place through a different process. Whereas the bottleneck effect is a consequence of catastrophe, the founder effect is a consequence of colonization.

The bottleneck effect and the founder effect are scenarios where a small population emerges from a much larger one. These new populations usually have less genetic diversity than their parent population, and they may feel the effects of genetic drift profoundly until they can sufficiently replenish their population.