Founder Effect: Definition, Examples, Significances

Populations change over time and this change is important for maintaining genetic diversity which is essential for the adaptability and survival of a species over generations. There are several mechanisms driving the evolutionary process. One such random evolutionary process is genetic drift, which refers to the random change in the gene pool in a population over time. 

The founder effect is a specific case of genetic drift. It occurs when a small group of individuals becomes isolated or separated from a larger population to form a new, isolated population.

The founder effect leads to lower genetic diversity in the new population compared to the species as a whole. This phenomenon can result from natural disasters, such as volcanic activity or sudden flooding, creating new environments for small groups.

The concept of establishing a population from a small group of founders, leading to significant genetic changes and evolutionary differentiation, was initially introduced by Ernst Mayr in his genetic revolution model. In this model, the founder event plays an important role in lowering levels of heterozygosity. 

Humans have experienced the founder effect throughout history as different colonies formed, with traits carried by the founders becoming more common in those populations. When a small group of humans inhabited a new land, they mated with one another, and as the population grew, the traits they carried with them became more and more common in their population than they were in the human species as a whole.

What is Genetic Drift?

Genetic drift is an evolutionary process marked by random changes in the occurrence of certain genes within a population over time. Genetic drift occurs due to chance events rather than natural selection. 

• This random process can lead to the complete loss of gene variants, resulting in decreased genetic diversity. It can also increase the frequency of initially uncommon alleles.
• In smaller populations, chance occurrences like genetic drift can significantly affect the genetic composition of the population due to the reduced number of individuals.
• The two main types of genetic drift are founder effect and population bottleneck. The founder effect occurs when a small group of individuals establishes a new population, and their genetic makeup influences the new population. Population bottleneck happens when a population undergoes a drastic size reduction, leading to a limited set of genetic diversity. 

What is Founder Effect?

The founder effect is an important concept in population genetics, which explains how small groups can significantly influence the genetic composition of a population.

• The founder effect occurs when a small group of individuals establishes a new colony or becomes isolated from a larger population, leading to a decrease in genomic variability within the isolated group.
• Over time, the genetic makeup of the new subpopulation resembles the initial small group, which can differ significantly from the original larger population. 
• This phenomenon can contribute to the increased prevalence of certain inherited diseases within specific population groups and may play a role in the emergence of new species.
• The founder effect happens when a small group starts a new population, either by moving away from a bigger one or because the population size drastically decreases. In both cases, certain alleles that were rare might become more common in the new, small population due to strong genetic drift. This emphasizes how the founder effect can have a lasting influence on genetic diversity, especially when the new population stays isolated for a long time.

Examples of Founder Effect

Some of the examples of the founder effect are briefly explained below which demonstrate how genetic mutations and rare disorders can become more prevalent due to the isolation and intermarriage within small populations. These examples show how the genetic characteristics introduced by the founders persist and become more common within a closed and isolated community.

Ashkenazi Jewish population

The Ashkenazi Jews provide a compelling example of the founder effect. As this population migrated and established communities, a limited number of founders carried specific genetic mutations. This led to a higher prevalence of certain inherited disorders, including Tay-Sachs disease and Gaucher disease.

Retinitis Pigmentosa

In the early 1800s, individuals migrating to the Tristan da Cunha islands formed a British colony. One of the colonists carried a recessive allele for Retinitis Pigmentosa, a rare disorder leading to the degeneration of retinal cells and loss of vision in individuals homozygous for the allele. By the 1960s, among the 240 residents in the isolated colony, four individuals had the disorder, and at least nine others were carriers. This higher occurrence contrasts with larger populations where this disease is relatively rare, explaining how the founder effect can increase the prevalence of specific genetic conditions in small, isolated communities.

Amish Community

In Eastern Pennsylvania, the Amish community provides a notable example of the founder effect. Around 200 individuals who migrated from Germany established this community. The Amish tend to marry within their group and live in isolation, causing genetic mutations to persist over generations. An example of this is Ellis-van Creveld syndrome, a rare disorder marked by extra fingers or toes (polydactyly), dwarfism, and occasionally congenital heart defects. Due to the founder effect, the prevalence of Ellis-van Creveld syndrome is significantly higher among the Amish community compared to larger populations. 

Limitations of Founder Effect

• The founder effect is difficult to study, emphasizing the complexity of natural processes like repeated immigration, bottlenecks, and other evolutionary forces.
• There is a lack of clear empirical support for the traditional founder effect model.
• Recent studies show that single founding events have a limited impact on genetic diversity and divergence, challenging assumptions proposed by the traditional founder model.
• The study of the silvereyes species shows that their natural colonization behavior may not align with traditional founder-effect models, which points to the need to use alternative explanations like large flock sizes and gradual drift.

Significances of Founder Effect

• Founder effects contribute to the genetic differentiation of populations. Over time, isolated groups may develop distinct genetic profiles compared to the original population.
• The founder effect plays a role in evolutionary processes by influencing the genetic composition of populations.
• Studying the founder effect is important in population genetics, helping to understand how genetic diversity and traits are distributed within populations.
• In human populations, the founder effect is useful to understand the prevalence of genetic disorders in certain communities.

References

1. “Genetic Drift and the Founder Effect.” PBS, Public Broadcasting Service, www.pbs.org/wgbh/evolution/library/06/3/l_063_03.html
2. Bailey, Regina. “What Is the Founder Effect?” ThoughtCo, Aug. 28, 2020, https://www.thoughtco.com/what-is-the-founder-effect-4586652.
3. Bottlenecks and founder effects – Understanding Evolution (berkeley.edu)
4. Clegg, S. M., Degnan, S. M., Kikkawa, J., Moritz, C., Estoup, A., & F. Owens, I. P. (2002). From the Cover: Genetic consequences of sequential founder events by an island-colonizing bird. Proceedings of the National Academy of Sciences of the United States of America, 99(12), 8127-8132. https://doi.org/10.1073/pnas.102583399
5. Founder Effect (genome.gov)
6. Genetic Drift (genome.gov)
7. Slatkin, M. (2004). A Population-Genetic Test of Founder Effects and Implications for Ashkenazi Jewish Diseases. American Journal of Human Genetics, 75(2), 282-293. https://doi.org/10.1086/423146
8. The Founder Effect’s Influence on Jewish Genetic Diseases (gaucherdisease.org)