Genetic Drift

Natural selection is not the only way in which evolution occurs. Organisms that are well adapted to their environment might die by chance during a natural disaster or other extreme events. This results in the loss of the advantageous traits these organisms possessed from the general population. Here we will discuss genetic drift and its evolutionary significance.

Genetic Drift Definition

Any population can be subjected to genetic drift, but its effects are stronger in small populations. The dramatic reduction of a beneficial allele or genotype can decrease the overall fitness of a small population because there are few individuals with these alleles to begin with. It is less likely that a large population would lose a significant percentage of these beneficial alleles or genotypes. Genetic drift can reduce the genetic variation within a population (through the removal of alleles or genes) and the changes that this drift produces are generally non-adaptive.

Another effect of genetic drift occurs when species are divided into several different populations. In this situation, as the allele frequencies within one population shift due to genetic drift, the genetic differences between this population and the other ones can increase.

Usually, populations of the same species already differ in some traits as they adapt to local conditions. But since they are still from the same species, they share many of the same traits and genes. If one population loses a gene or allele that was shared with other populations, it now differs more from the other populations. If the population continues to diverge and isolate from the other ones, this can eventually lead to speciation.

Genetic Drift vs. Natural Selection

Natural selection and genetic drift are both mechanisms that can drive evolution, meaning that both can cause changes to the genetic composition within populations. However, there are important differences between them. When evolution is driven by natural selection it means that the individuals better suited to a particular environment are more likely to survive and will contribute more offspring with the same traits.

Genetic drift, on the other hand, means that a random event happens and the surviving individuals are not necessarily better suited to that particular environment, as better suited individuals may have died by chance. In this case, the surviving less suited individuals will contribute more to the next generations, thus the population will evolve with less adaptation to the environment.

Therefore, evolution driven by natural selection leads to adaptive changes (that increase the survival and reproductive probabilities), while changes caused by genetic drift are usually non-adaptive.

Types of Genetic Drift

Bottleneck

Think of a bottle filled with candy balls. The bottle originally had 5 different colors of candy, but only three colors passed through the bottleneck by chance (technically called a sampling error). These candy balls represent the individuals from a population, and the colors are alleles. The population went through a bottleneck event (such as a wildfire) and now the few survivors only carry 3 of the 5 original alleles the population had for that gene (see Fig. 1).

In conclusion, the individuals who survived a bottleneck event did so by chance, unrelated to their traits.

Figure 1. A bottleneck event is a type of genetic drift where there is a sudden decrease in the size of a population, causing a loss in alleles in the population's gene pool.

Genetic Drift Founder effect

The results of a founder effect are similar to those of a bottleneck. In summary, the new population is significantly smaller, with different allele frequencies and probably lower genetic variation, compared to the original population (Fig. 2). However, a bottleneck is caused by a random, usually adverse environmental event, while a founder effect is mostly caused by the geographical separation of part of the population. With the founder effect, the original population usually persists.

Figure 2. Genetic drift can also be caused by a founder event, where a small fraction of a population gets physically separated from the main population or colonizes a new area.

The impact of genetic drift can be strong and long-term. A common consequence is that individuals breed with other very genetically similar individuals, resulting in what is called inbreeding. This increases the chances of an individual inheriting two harmful recessive alleles (from both parents) that were low in frequency in the general population before the drift event. This is the way genetic drift can eventually lead to complete homozygosis in small populations and magnify the negative effects of harmful recessive alleles.

Let’s look at another example of genetic drift. Wild populations of cheetahs have depleted genetic diversity. Although great efforts have been made in cheetah recovery and conservation programs for the past 4 decades, they are still being subjected to the long-term effects of previous genetic drift events that have hindered their ability to adapt to changes in their environment.

1. Alicia Abadía-Cardoso et al., Molecular Population Genetics of the Northern Elephant Seal Mirounga angustirostris, Journal of Heredity, 2017.

2. Laurie Marker et al., A Brief History of Cheetah Conservation, 2020.

3. Pavel Dobrynin et al., Genomic legacy of the African cheetah, Acinonyx jubatus, Genome Biology, 2014.

https://cheetah.org/resource-library/

4. Campbell and Reece, Biology 7th edition, 2005.