Genetic drift is a mechanism of evolution that changes the frequency of gene variants, called alleles, in a population over generations. These changes are not due to any environmental pressures or advantages the genes might offer, but are the result of random chance. Imagine a jar filled with an equal number of red and blue marbles, where each color represents a different allele. If you were to blindly draw a handful of marbles to start a new collection in a separate jar, the proportions of red and blue would likely be different from the original, simply by chance. This random shift is analogous to how allele frequencies can change from one generation to the next.
How Chance Affects Allele Frequencies
The mechanism driving genetic drift is sampling error, as each new generation is a random sample of the previous generation’s genes. In a large population, this sample is more likely to accurately reflect the allele frequencies of the parent population. In small populations, however, chance events can have a large impact on the genetic makeup of the next generation. This is similar to flipping a coin; while you expect a 50/50 outcome over a thousand flips, you wouldn’t be surprised to get 7 heads and 3 tails in just ten flips.
The same principle applies to populations. In a large population with thousands of individuals, the random loss of a few individuals is unlikely to alter the overall allele frequencies significantly. In a population of only ten individuals, the accidental death of just two before they reproduce could eliminate certain alleles from the gene pool entirely, especially if they were the only carriers of those specific gene variants.
Population size is a determining factor in the strength of genetic drift. Small populations are much more susceptible to rapid changes in allele frequencies due to random events. Over time, this process can lead to some alleles becoming “fixed” (reaching a frequency of 100%) and others are lost completely, purely by chance.
The Bottleneck and Founder Effects
Two scenarios where genetic drift has a strong effect are the bottleneck effect and the founder effect. A population bottleneck occurs when a population’s size is drastically reduced for at least one generation due to a random environmental event, like a natural disaster or disease. The surviving individuals are a random sample of the original population, and their collective gene pool may not accurately represent its genetic diversity.
A well-documented example is the northern elephant seal. In the 1890s, hunting reduced the population to as few as 20 individuals. Although the population has since rebounded to over 100,000, their genetic variation is low compared to southern elephant seals that did not undergo such an intense bottleneck. This genetic uniformity is a direct consequence of the random survival of a very small number of seals and the subsequent loss of many alleles.
The founder effect is a similar phenomenon that occurs when a new population is established by a small number of individuals from a larger ancestral population. This smaller group may have, by chance, different allele frequencies than the original population. Some alleles may be overrepresented while others are missing entirely, which can lead to different evolutionary paths for the new population.
A human example of the founder effect is seen in certain Amish communities in the United States. These populations were established by a small number of German immigrants. Due to their relative isolation, rare alleles present in the founders have become more common than in the original or surrounding populations. This has led to a higher incidence of certain genetic conditions, such as Ellis-van Creveld syndrome.
Genetic Drift Versus Natural Selection
It is important to distinguish the random process of genetic drift from the non-random process of natural selection. Natural selection is adaptive; it sorts individuals based on traits that enhance survival and reproduction in a given environment. Alleles that confer a fitness advantage are more likely to be passed on, becoming more common over time. Genetic drift, conversely, is random and non-adaptive.
The difference lies in the cause of the change in allele frequencies. In natural selection, the change is driven by the relative success of different traits, while in genetic drift, the change is due to chance events. An allele’s frequency can increase or decrease through drift regardless of whether it is beneficial, neutral, or even harmful to the organism.
In small populations, the effects of drift can be so strong they can overpower natural selection. A slightly harmful allele might become fixed in a population by chance, an outcome that would be selected against in a larger population. Similarly, a new beneficial mutation could be lost by chance before it has the opportunity to spread.
Consequences of Genetic Drift
The primary long-term consequence of genetic drift is the loss of genetic variation. As some alleles become fixed at a frequency of 100% and others are eliminated, the overall diversity within the population’s gene pool decreases. This happens because drift is a random process that, over time, tends to wander toward one extreme or the other—either fixation or loss.
This reduction in genetic diversity can have serious implications for a population’s ability to adapt to future environmental changes. A diverse gene pool is like a toolkit with a variety of tools; it provides the raw material for natural selection to act upon. If the environment changes, a population with higher genetic variation is more likely to have individuals with pre-existing traits that allow them to survive and reproduce.
When genetic drift diminishes this variation, it limits a population’s adaptive potential. The population becomes more genetically uniform, which can make it more vulnerable to new diseases, climate shifts, or other environmental pressures. The loss of alleles, even those not currently beneficial, removes options for future adaptation, potentially jeopardizing the long-term survival of the population.